INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY ENVIRONMENTAL HEALTH CRITERIA 143 METHYL ETHYL KETONE This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organisation, or the World Health Organization. Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organisation, and the World Health Organization First draft prepared by Dr R.B. Williams, United States Environmental Protection Agency World Health Orgnization Geneva, 1993 The International Programme on Chemical Safety (IPCS) is a joint venture of the United Nations Environment Programme, the International Labour Organisation, and the World Health Organization. The main objective of the IPCS is to carry out and disseminate evaluations of the effects of chemicals on human health and the quality of the environment. Supporting activities include the development of epidemiological, experimental laboratory, and risk-assessment methods that could produce internationally comparable results, and the development of manpower in the field of toxicology. Other activities carried out by the IPCS include the development of know-how for coping with chemical accidents, coordination of laboratory testing and epidemiological studies, and promotion of research on the mechanisms of the biological action of chemicals. WHO Library Cataloguing in Publication Data Methyl ethyl ketone. (Environmental health criteria ; 143) 1.Butanones - adverse effects 2.Butanones - toxicity 3.Occupational exposure I.Series ISBN 92 4 157143 8 (NLM Classification: QV 633) ISSN 0250-863X The World Health Organization welcomes requests for permission to reproduce or translate its publications, in part or in full. Applications and enquiries should be addressed to the Office of Publications, World Health Organization, Geneva, Switzerland, which will be glad to provide the latest information on any changes made to the text, plans for new editions, and reprints and translations already available. (c) World Health Organization 1993 Publications of the World Health Organization enjoy copyright protection in accordance with the provisions of Protocol 2 of the Universal Copyright Convention. All rights reserved. The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or of certain manufacturers' products does not imply that they are endorsed or recommended by the World Health Organization in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. CONTENTS ENVIRONMENTAL HEALTH CRITERIA FOR METHYL ETHYL KETONE 1. SUMMARY 1.1. Properties and analytical methods 1.2. Sources of exposure and uses 1.2.1. Production and other sources 1.2.2. Uses and loss to the environment 1.3. Environmental transport and distribution 1.4. Environmental levels and human exposure 1.5. Kinetics and metabolism 1.6. Effects on experimental species 1.7. Effects on humans 1.7.1. MEK alone 1.7.2. MEK in solvent mixtures 1.8. Enhancement of the toxicity of other solvents 2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES AND ANALYTICAL METHODS 2.1. Identity 2.2. Chemical and physical properties 2.3. Conversion factors 2.4. Sampling and analytical methods 2.4.1. General considerations 2.4.2. Air 2.4.3. Water 2.4.4. Solids 2.4.5. Biological materials 3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE 3.1. Natural occurrence 3.2. Production levels, processes and uses 3.2.1. World production 3.2.2. Production processes 3.2.3. Other sources 3.2.4. Uses 3.3. Release into the environment 4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION 4.1. Transport in the environment 4.2. Bioaccumulation and biodegradation 5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE 5.1. Environmental levels 5.1.1. Air 5.1.2. Water 5.1.3. Foodstuffs 5.2. General population exposure 5.3. Occupational exposure 5.4. Peri-occupational exposure 6. KINETICS AND METABOLISM 6.1. Absorption 6.1.1. Percutaneous absorption 6.1.2. Inhalation absorption 6.1.3. Ingestion absorption 6.1.4. Intraperitoneal absorption 6.2. Distribution 6.3. Metabolic transformation 6.3.1. Animal studies 6.3.2. Human studies 6.4. Elimination and excretion 6.5. Turnover 6.6. Metabolic interactions 6.7. Mechanisms of action 7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS 7.1. Acute exposure 7.1.1. Lethal doses 7.1.2. Non-lethal doses 7.1.3. Skin and eye irritation 7.2. Repeated exposures 7.3. Neurotoxicity 7.3.1. Behavioural testing 7.3.2. Histopathology 7.4. Developmental toxicity 7.5. Mutagenicity and related end-points 7.6. Carcinogenicity 8. EFFECTS ON HUMANS 8.1. General population exposure 8.2. Effects of short-term exposure 8.3. Skin irritation and sensitization 8.4. Occupational exposure 8.4.1. MEK alone 8.4.2. MEK in solvent mixtures 8.5. Carcinogenicity 9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD 9.1. Microorganisms 9.2. Aquatic organisms 9.3. Terrestrial organisms 9.3.1. Animals 9.3.2. Plants 10. ENHANCEMENT OF THE TOXICITY OF OTHER SOLVENTS BY MEK 10.1. Hexacarbon neuropathy 10.1.1. Introduction 10.1.2. Animal studies 10.1.3. Human studies 10.1.3.1 Solvent abuse 10.1.3.2 Occupational exposure 10.2. Haloalkane solvents 10.2.1. Studies in animals 10.2.2. Potentiation of haloalkane toxicity in humans 11. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT 11.1. Human health risks 11.1.1. Non-occupational exposure 11.1.2. Occupational exposure 11.1.3. Relevant animals studies 11.2. Effects on the environment 12. RECOMMENDATIONS FOR THE PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT 12.1. Human heath protection 12.2. Environmental protection 13. FURTHER RESEARCH REFERENCES APPENDIX 1. Conversion factors for various solvents RESUME RESUMEN WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR METHYL ETHYL KETONE Members Professor E.A. Bababunmi, Postgraduate Institute for Medical Research and Training, College of Medicine, Ibadan, Nigeria Dr P.E.T. Douben, Department of Ecotoxicology, Institute for Forestry and Nature Research, Arnhem, The Netherlands Professor C.L. Galli, Toxicology Laboratory, Institute of Pharmacological Sciences, University of Milan, Milan, Italy (Chairman) Dr R.F. Hertel, Fraunhofer Institute of Toxicology and Aerosol Research, Hanover, Germany Dr H.P.A. Illing, Head of Toxicology, Health and Safety Executive, Bootle, United Kingdom Professor A. Massoud, Department of Community, Environmental & Occupational Health, Faculty of Medicine, Ain Shams University, Abbassia, Egypt (Joint Rapporteur) Dr K. Morimoto, Division of Chem-Bio Informatics, National Institute of Hygienic Sciences, Setagaya-ku, Tokyo, Japan Dr V. Riihimäki, Institute of Occupational Health, Helsinki, Finland Dr E. de Souza Nascimento, University of Sao Paulo, Sao Paulo, Brazil Dr H. Tilson, Neurotoxicology Division, Health Effects Research Laboratory, US Environmental Protection Agency, Research Triangle Park, USA Dr R.B. Williams, Office of Research and Development, US Environmental Protection Agency, Washington D.C., USA (Joint Rapporteur) Secretariat Dr P.G. Jenkins, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland Dr E. Smith, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland NOTE TO READERS OF THE CRITERIA MONOGRAPHS Every effort has been made to present information in the criteria monographs as accurately as possible without unduly delaying their publication. In the interest of all users of the Environmental Health Criteria monographs, readers are kindly requested to communicate any errors that may have occurred to the Director of the International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland, in order that they may be included in corrigenda. * * * A detailed data profile and a legal file can be obtained from the International Register of Potentially Toxic Chemicals, Palais des Nations, 1211 Geneva 10, Switzerland (Telephone No. 7988400 or 7985850). ENVIRONMENTAL HEALTH CRITERIA FOR METHYL ETHYL KETONE A WHO Task Group on Environmental Health Criteria for Methyl Ethyl Ketone (MEK) met at the World Health Organization, Geneva, from 9 to 13 September 1991. Dr E. Smith welcomed the participants on behalf of Dr M. Mercier, Director, IPCS, and on behalf of the heads of the three IPCS cooperating organizations (UNEP/ILO/WHO). The Task Group reviewed and revised the draft monograph and made an evaluation of the risks for human health and the environment from exposure to MEK. The first draft of this monograph was prepared by Dr R.B. Williams, Office of Research and Development, US Environmental Protection Agency. Dr E. Smith and Dr P.G. Jenkins, both members of the IPCS Central Unit, were responsible for the scientific content and technical editing, respectively. The efforts of all who helped in the preparation and finalization of the monograph are gratefully acknowledged. ABBREVIATIONS ALT alanine transferase BEI biological exposure index DCB dichlorobenzene DMA dimethylamine DMF dimethylformamide DNPH 2,4-dinitrophenyl hydrazine EBK ethyl n-butyl ketone ECD electron-capture detection FID flame ionization detection FT-IR Fourier transform infrared GC gas chromatography GLDH glutamate dehydrogenase GPT glutamic-pyruvic transaminase GST glutathione-S-transferase 2,5-HD 2,5-hexanedione 2,5-Hpdn 2,5-heptanedione HPLC high-performance liquid chromatography HS headspace IR infrared LC50 median lethal concentration LDQ lowest detectable quantity MAC maximum allowable concentration MBK methyl n-butyl ketone MEK methyl ethyl ketone MIBK methyl isobutyl ketone MS mass spectrometry NADPH reduced nicotinamide adenine dinucleotide phosphate OCT ornithine carbamyl transferase PID photo-ionization detection SRT simple reaction time STEL short-term exposure limit TLV threshold limit value TWA time-weighted average UV ultraviolet 1. SUMMARY 1.1 Properties and analytical methods Methyl ethyl ketone (MEK) is a clear, colourless, volatile, highly flammable liquid with an acetone-like odour. It is stable under ordinary conditions but can form peroxides on prolonged storage; these may be explosive. MEK can also form explosive mixtures with air. It is very soluble in water, miscible with many organic solvents, and forms azeotropes with water and many organic liquids. In the atmosphere MEK produces free radicals, which may lead to the formation of photochemical smog. Several analytical methods exist for the measurement of MEK at environmental levels in air, water, biological samples, waste and other materials. In the more sensitive methods, MEK is trapped and concentrated either on a solid sorbant or as a derivative of 2,4-dinitrophenylhydrazine (DNPH). Absorbed MEK and other volatile organic compounds are desorbed, separated by gas chromatography and measured with a mass spectrometer or flame ionization detector. Derivatized MEK is separated from related compounds by high performance liquid chromatography and measured by ultraviolet absorption. In media such as solid waste and biological materials, MEK must first be separated from the substrate by methods such as solvent extraction or steam distillation. High concentrations of MEK in air can be monitored continuously by infrared absorption. Detection limits are 3 µg/m3 in air, 0.05 µg/litre in drinking-water, 1.0 µg/litre in other types of water, 20 µg/litre in whole blood and 100 µg/litre in urine. 1.2 Sources of exposure and uses 1.2.1 Production and other sources Recent values for annual industrial manufacture (in thousands of tonnes) are: USA, 212 to 305; western Europe, 215; Japan, 139. In addition to its manufacture, sources of MEK in the environment are exhaust from jet and internal combustion engines, and industrial activities such as gasification of coal. It is found in substantial amounts in tobacco smoke. In the USA, production of MEK by engines is no more than 1% of its deliberate manufacture. In smog episodes, photochemical production of MEK and other carbonyls from free radicals can be far greater than direct anthropogenic emission. MEK is produced biologically and has been identified as a product of microbial metabolism. It has also been detected in a wide diversity of natural products including higher plants, insect pheromones, animal tissues, and human blood, urine and exhaled air. It is probably a minor product of normal mammalian metabolism. 1.2.2 Uses and loss to the environment The major use of MEK, application of protective coatings and adhesives, reflects its excellent characteristics as a solvent. It also is used as a chemical intermediate, as a solvent in magnetic tape production and the dewaxing of lubricating oil, and in food processing. In addition to industrial applications, it is a common ingredient in consumer products such as varnishes and glues. In most applications MEK is a component of a mixture of organic solvents. Losses to the environment are mainly to the air and result principally from solvent evaporation from coated surfaces. MEK is released into water as a component of the waste from its manufacture and from a variety of industrial operations. It has been detected in natural waters and could have originated from microbial activities and from atmospheric input, as well as from anthropogenic pollution. 1.3 Environmental transport and distribution MEK is highly mobile in the natural environment and subject to rapid turnover. It is very soluble in water and evaporates readily into the atmosphere. In air MEK is subject to rapid photochemical decomposition and is also synthesized by photochemical processes. In water containing free halogens or hypohalites, it reacts to form a haloform that is more toxic than the original compound. MEK is distributed by both air and water, but does not accumulate in any individual compartment, and does not persist long where there is microbial activity. It is rapidly metabolized by microbes and mammals. There is no evidence of bioaccumulation. MEK occurs naturally in some clover species and is produced by fungi to concentrations that affect the germination of some plants. 1.4 Environmental levels and human exposure General population exposure to low levels of MEK is widespread. In minimally polluted air, the concentration is less than 3 µg/m3 (< 1 ppb), but a level of 131 µg/m3 (44.5 ppb) has been measured under conditions of heavy air pollution. Away from industrial areas where MEK is manufactured or used, major sources may be vehicle exhaust and photochemical reactions in the atmosphere. Cigarettes and other tobacco products that are burned contribute to individual exposure (20 cigarettes contain up to 1.6 mg). Volatilization of MEK from building materials and consumer products can pollute indoor air to levels far above adjacent outdoor air. MEK concentrations in exposed natural waters are rarely above 100 µg/litre (100 ppb) and are usually below a detectable level. Trace amounts of MEK, however, have been detected widely in drinking-water (approximately 2 µg/litre) and presumably originated as solvent leached from cemented joints of plastic pipe. Although MEK is a normal component of many foods, concentrations are low and food consumption cannot be considered a significant source of population exposure. Average daily per capita intake in the USA via foodstuffs is estimated to be 1.6 mg, most coming from white bread, tomatoes and Cheddar cheese. In addition to MEK present naturally, foods may contain MEK from cheese ripening, aging of poultry meat, cooking or food processing, or by absorption from plastic packaging materials. Industrial exposure to moderate levels of MEK is widespread. However, in some regions workers in small factories (e.g., shoe factories, printing plants and painting operations) are exposed to much higher concentrations due to inadequate ventilation. In these factories, exposure is usually to a mixture of solvents including n-hexane. 1.5 Kinetics and metabolism Absorption of MEK is rapid via dermal contact, inhalation, ingestion and intraperitoneal injection. It is rapidly transferred into the blood and thence to other tissues. The solubility of MEK appears similar for all tissues. The clearance of MEK and its metabolites in mammals is essentially complete in 24 h. It is metabolized in the liver where it is mainly oxidized to 3-hydroxy-2-butanone and subsequently reduced to 2,3-butanediol. A small portion may be reduced to 2-butanol, but 2-butanol is rapidly oxidized back to MEK. The bulk of MEK taken into the mammalian body enters the general metabolism and/or is eliminated as simple compounds such as carbon dioxide and water. Excretion of MEK and its recognizable metabolites is mainly through the lungs, although small amounts are excreted via the kidneys. MEK increases microsomal cytochrome P-450 enzyme activities. This enhancement of enzymatic activity and thus of the body's potential for metabolic transformation may well be the mechanism by which MEK potentiates the toxicity of haloalkane and aliphatic hexacarbon solvents. 1.6 Effects on experimental species MEK has low to moderate acute, short-term and chronic toxicity for mammals. LD50 values for adult mice and rats are 2 to 6 g/kg body weight, death occurring within 1 to 14 days following a single oral dose. Average vapour concentrations producing lethality in rats following a single exposure are around 29 400 mg/m3 (10 000 ppm), although guinea-pigs survived a 4-h exposure to this concentration. The lowest acute oral dose modifying body structure is 1 g/kg body weight, which damaged kidney tubules in the rat. Inhalation by rats of 74 mg/m3 (25 ppm) for 6 h produced measurable changes in behaviour which persisted for several days. Repeated exposure of rats to 14 750 mg/m3 (5000 ppm) (6 h/day, 5 days/week) produced no lethality, had only minor effects on growth and structure, and there were no neuropathological changes. There was no evidence that MEK produced neuropathological changes in chickens, cats or mice exposed to 3975 mg/m3 (1500 ppm) for periods of up to 12 weeks. Transient effects on behaviour or neurophysiology were detected following repeated exposure of rats and baboons to concentrations as low as 295-590 mg/m3 (100 to 200 ppm). There is evidence for a low level of fetotoxicity without any maternal toxicity at 8825 mg/m3 (3000 ppm), but no evidence for embryotoxic or teratogenic effects at lower exposure levels. Repeated exposure of pregnant rats to 8825 mg/m3 induced in their offspring a small but significant increase in skeletal abnormalities of types that occurred at low incidences among the unexposed population. Although examined in a number of conventional mutagenicity test systems, the only evidence of mutagenicity was provided by a study on aneuploidy in the yeast Saccharomyces cerevisiae. MEK is not acutely toxic to fish or aquatic invertebrates and LC50 values range from 1382 to 8890 mg/litre. MEK has an inhibiting effect on the germination of several plant species, even at levels occurring naturally. The growth of aquatic algae is inhibited. Compared with natural background levels, relatively high concentrations of MEK have been used for fumigation under experimental conditions. It is moderately effective as a fumigant against the Caribbean fruit fly and is a very effective attractant for tsetse flies. Levels of MEK up to 20 mg/litre retard biodegradation but do not stop the process entirely. At levels of up to 100 mg/litre, MEK is biostatic to a variety of bacteria. At higher concentrations (1000 mg/litre or more) inhibition of the growth of bacteria and protozoa occurs. 1.7 Effects on humans 1.7.1 MEK alone Exposure to 590 mg/m3 (200 ppm) had no significant effect in a variety of behavioural and psychological tests. Short-term exposure to MEK alone does not appear to be a significant hazard, either occupationally or for the general public. Experimental exposure to a concentration of 794 mg/m3 (270 ppm) for 4 h/day had little or no effect on behaviour, and a 5-min contact with liquid MEK produced no more than a temporary whitening of the skin. There is only one non-occupational report of acute toxicity to MEK. This resulted from accidental ingestion and appeared to produce no lasting harm. There is no evidence that occupational MEK exposure has resulted in death. There have been two reports of chronic occupational poisoning and one questionable report of acute occupational poisoning. In one of the chronic cases, exposure to 880-1770 mg/m3 (300-600 ppm) resulted in dermatoses, numbness of fingers and arms, and various symptoms such as headache, dizziness, gastrointestinal upset, and loss of appetite and weight. This paucity of incidents of reputed poisoning by MEK alone reflects both the low toxicity of MEK and the fact that it is most commonly used not on its own but as a component of solvent mixtures. 1.7.2 MEK in solvent mixtures Exposure to solvent mixtures containing MEK has been associated with some reduction in nerve conduction velocity, memory and motor alterations, dermatoses and vomiting. In one longitudinal study, consecutive measurements of simple reaction time showed an improvement in performance in parallel with decreasing concentrations of MEK to one tenth the original values (which were up to 4000 mg/m3 for certain routine tasks). 1.8 Enhancement of the toxicity of other solvents MEK potentiates the neurotoxicity of hexacarbon compounds ( n-hexane, methyl- n-butylketone and 2,5-hexanedione) and the liver and kidney toxicity of haloalkane (carbon tetrachloride and trichloromethane) solvents. The potentiation of the neurotoxic effects of hexacarbons has been demonstrated for all three hexacarbons in animals. The peripheral neuropathies observed in humans followed changes in the formulations of solvents to which they had been exposed, either voluntarily or occupationally. The mechanism by which this potentiation occurs is unclear. Evidence for potentiation of the liver and kidney toxicity of haloalkanes comes from animal studies. MEK probably activates the haloalkane metabolism to tissue-damaging species as a result of induction of the relevant oxidation enzymes. 2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES AND ANALYTICAL METHODS 2.1 Identity H H O H | | || | Chemical structure: H - C - C - C - C - H | | | H H H Chemical formula: C4H8O Synonyms: Butanone, 2-butanone, butane-2-one, ethyl methyl ketone, MEK, MEETCO, methyl acetone, methylpropanone CAS registry number: 78-93-3 RTECS registry number: EL 6475000 UN registry number: 1193 EC registry number: 606-002-00-3 Relative molecular mass: 72.10 2.2 Chemical and physical properties Methyl ethyl ketone (MEK) is an important synthetic organic chemical. The physical properties of MEK are summarized in Table 1. It is a highly flammable, volatile, clear, colourless liquid that is stable under ordinary conditions. The vapour forms explosive mixtures with air over a range of approximately 2% to 12% (vol./vol.). The odour is acetone-like and variously described as sharp, fresh or sweet. The odour threshold appears to be around 5.9 mg/m3 (2 ppm) although a range between 0.74 and 147.5 mg/m3 has been reported (Ruth, 1986). MEK is moderately soluble in water; the solubility decreases with increasing temperature. It is miscible with organic solvents such as alcohol, ether and benzene, and forms azeotropes with water and many organic liquids. The value in Table 1 for log Po/w (logarithm of the octanol/ water partition ratio) of 0.26 is taken from Verschuren (1983). Banergee & Howard (1988) quoted a slightly higher value of 0.29. Other partition values for MEK (at 37 °C) are: water/air = 254; blood/air = 202; olive oil/air = 263; olive oil/water = 1.0; and olive oil/blood = 1.3 (Sato & Nakajima, 1979). Perbellini et al. (1984), however, determined partition values for saline solution/air and olive oil/air of 193 and 191 respectively. Table 1. Physical properties of MEK Reference Appearance colourless liquid Relative molecular mass 72.10 Papa & Sherman (1978) Specific gravity (liquid density) (at 20 °/4 °C)a 0.805 Krasavage et al. (1982) Vapour density (air = 1.00) 2.41 Verschuren (1983) Vapour pressure at 20 °C (torr) 77.5 Weast (1986) Boiling point (°C) 79.6 Weast (1986) Melting point (°C) -86 Weast (1986) Water solubility at 20 °C (g/litre) 275 Windholz (1983) Refractive index 1.3788 Krasavage et al. (1982) Flash point (closed cup) (°C) -6 Papa & Sherman (1978) Log Po/w 0.26 Verschuren (1983) 0.29 Banergee & Howard (1988) Saturation concentration in air (g/m3 at 20 °C) 301 Krasavage et al. (1982) a Specific gravity at 20 °C relative to the density of water at 4 °C The physical and chemical properties of MEK are determined largely by its carbonyl group. MEK engages in reactions typical of saturated aliphatic ketones. These include condensations with amines, aldehydes and many other organic compounds, hydrolysis (catalysed with acid or base), oxidation via concentrated oxidizing acids or acidic peroxides, and reduction with hydrogen and metal catalysts. None of these reactions is likely to be important in nature. On the other hand, MEK and other methyl ketones will react with halogens and hypohalides in aqueous solution to form a carboxylic acid and a haloform. The reaction provides a specific test for methyl ketones, and may produce chloroform in chlorinated water supplies contaminated with methyl ketones. MEK and other ketones are photochemically reactive when excited by wavelengths occurring in the atmosphere and produce free radicals which lead to the formation of photochemical smog (Grosjean et al., 1983). 2.3 Conversion factors 1 ppm = 2.95 mg/m3; 1 mg/m3 = 0.34 ppm (at 25 °C and 101.3 kPa) 2.4 Sampling and analytical methods 2.4.1 General considerations Analytical methods for MEK depend on the matrix. They are summarized in Table 2. Where MEK is present in a substantial concentration and is known to be the only or the dominant organic contaminant, simplified methodology is feasible. The occupational atmospheric exposure limits, currently in the range 295-590 mg/m3 (100-200 ppm), permit monitoring in the workplace with less sensitive procedures. The precise determination of MEK when present in the environment at low concentrations is a complex task because of the wide variety of other organic compounds that may be present and the many possibilities for error, interference and contamination. MEK, other ketones and other interfering substances are so prevalent in laboratory and industrial air that care must be taken in all determinations to minimize the possibility of contamination of samples, equipment and reagents. Care must be taken to avoid contamination in sampling since, for example, easily unnoticed sources like PVC (polyvinyl chloride) glue in collection equipment may leach a significant amount of MEK into water samples (Kent et al., 1985). Table 2. Some analytical techniques for determining MEK concentrations in environmental media and biological materialsa Methods Detection Comments Reference limits Air Trapping in solid sorbant tube (Tenax(R)); 200 µg/m3 working range is 0.2-100 mg/m3; analysis can Brown & Purnell thermal desorption: separation-detection; be automated (1979) GC-FID Trapping in DNPH; separation: HPLC 3-6 µg/m3 general method for aldehydes and ketones in Riggin (1984) (reverse phase); detection: UV air; some isomeric aldehydes and ketones are absorption not well separated Trapping in solid sorbant tube 0.15 mg per working range is 50-1500 mg/m3; acetone and US NIOSH (1984a) (Ambersorb XE-347(R)); desorption: sample isopropanol interfere CS2; separation-detection: GC-FID Absorption of specific IR wavelengths 3 mg/m3 can measure several different pollutant Persson et al. from CO2 laser; automated, computer- vapours simultaneously and continuously (1984) controlled system Trapping in DNPH; colour matching 300 mg/m3 working range is 300-1200 mg/m3; other Smith & Wood against standards aldehydes and ketones interfere; method (1972) requires no specialized equipment Water Separation from water sample by heated 0.05-1.0 water samples must be preserved from bacterial Pellizzari et al. gas purge; trapping on Tenax(R); thermal µg/litreb action with methylene chloride, and free chlorine (1985) desorption; separation-detection: GC/MS removed with thiosulfate; achieving lower limit of detection requires concentration by distillation; no interference reported Table 2 (contd.) Methods Detection Comments Reference limits Concentration on zeolite (ZSM-5); 2 µg/litre developed for drinking-water analysis; no Ogawa & Fritz elution with acetonitrile; derivatization interference reported (1985) with DNPH; separation-detection: HPLC/UV Direct injection of aqueous sample; 40 µg/litre developed for industrial waste-water analysis; Middleditch et al. separation-detection: GC/FID no interference reported (1987) Solids Solvent extraction with tetraethylene 0.5-5 µg/g tetraglyme must be purified and stabilized to Gurka et al. glycoldimethyl ether (tetraglyme); purge (wet weight) prevent peroxide formation; no interference (1984) and trap; separation-detection: GC/MS reported Heated purge of sample/water slurry or 10 µg/kg method developed for volatile organic compounds Fisk (1986) of methanol extract of sample; trap; (wet weight) at concentrations of < 1 mg/kg; no interference desorption; separation-detection: GC/MS reported Biological Materials Mixture with dextrose and heating; 20 µg/litre method developed for simultaneous determination US NIOSH (1984b) HS analysis; GC/FID of MEK, toluene and ethenol in blood; recovery rates 90-98% Incubation in sealed vial; HS 100 µg/litre method developed for blood; uses 200-µl sample; Ramsey & Flanagan analysis; separation-detection: applicable to urine and tissue; no interference (1982) GC/FID and ECD reported Concentration by reverse-phase 100-150 method developed for MEK and its metabolites Kezic & Monster extraction column; separation- µg/litre in urine; no interference reported (1988) detection: GC/FID Table 2 (contd.) Methods Detection Comments Reference limits Derivatized with o-nitrophenylhydrazine 100 µg/litre method developed for the determination of MEK Van Doorn et al. and reacted with cyclohexane; in human urine (1989) centrifuge separation; reversed- phase HPLC; UV (254 nm) Steam distillation of slurry; HS 20 µg/litre method developed for cheese, but probably Lin & Jeon (1985) analysis; separation-detection: widely applicable; no interference reported GC/FID Homogenization; HS analysis; 6 mg/litre method developed for the identification of Deveaux & Huvenne GC/FT-IR solvents of abuse in biological fluids (1987) a Abbreviations used in the table DNPH 2,4-dinitrophenylhydrazine ECD electron-capture detector FID flame ionization detector FT Fourier transformed GC gas chromatograph HPLC high performance liquid chromatograph HS headspace IR infrared MS mass spectrometer UV ultraviolet b 0.05 µg/litre for drinking-water; 1.0 µg/litre for all other types of water The general procedure for analysis of MEK is summarized below: a) collect the sample, and if necessary, chemically stabilize it; b) separate MEK (and other volatile organic compounds) from the substrate; c) trap and concentrate MEK (plus other organic compounds); d) recover the trapped material; e) separate MEK and other organic compounds; f) detect and identify MEK; g) determine the quantity recovered; h) calculate the concentration present in the sample. In actual practice the procedure may be simplified by combining or omitting certain steps, or it may contain an additional step, i.e. the preparation of 2,4-dinitrophenylhydrazine (DNPH) derivatives of MEK and other aldehydes and ketones. The formation of DNPH derivatives quantitatively captures both aldehydes and ketones, and facilitates their subsequent separation with either gas or liquid chromatography. The preparation of DNPH derivatives also forms the basis for a simplified, non-specific method for roughly measuring high levels of ketones and aldehydes without the use of sophisticated laboratory equipment (Smith & Wood, 1972). The use of other derivatives, such as imines via phenylmethylamine (Hoshika et al., 1976), azines via 3-methyl-2-benzothiazolone (Chiavari et al., 1987), and O-(2,3,4,5,6-pentafluorobenzyl) oximes via pentafluorophenylhydrazine and pentafluorobenzyloxyamine (Kobayashi et al., 1980) has also been proposed. 2.4.2 Air A general methodology for determining MEK in air consists of trapping and concentrating MEK and other volatile organic compounds in sampling devices containing an absorbent material, charcoal (carbopack) or an artificial resin (Tenax GC(R), Ambersorb XE(R), Amberlites XAD(R)), followed by desorption and analysis. MEK decomposes when absorbed on charcoal and sample loss may occur after a few days (Elskamp & Schultz, 1983; Levin & Carleborg, 1987). Ambersorb XE(R) showed good capacity, and decomposition was insignificant (Levin & Carleborg, 1987). Kenny & Stratton (1989) evaluated various mixtures to find a solvent that would provide optimum desorption efficiency. For samples of MEK collected on charcoal tubes, a mixture of carbon disulfide with 10% amyl alcohol was found to be an effective desorption solvent. The substitution of hexyl for amyl alcohol gave comparable recovery but slower GC/FID analysis. Both thermal desorption and solvent desorption have been used to release the MEK from the trapping column. Collectors may be passive and dependent on diffusion or a packed tube through which a known volume of air is drawn. Passive collectors, often in the form of badges, avoid the need for specialized sampling equipment and are convenient for monitoring individual exposure. However, the results of several studies suggest that passive (diffusive) collectors not only show significant individual and brand variability but also variability in their speed of uptake of different solvent vapours (Hickey & Bishop, 1981; Feigley & Chastain, 1982), and thus may require calibration against a more quantitative method. The trapped organic compounds are desorbed either thermally by application of heat or microwave radiation, or by solution in carbon disulfide, and are separated with gas chromatography. A wide diversity of columns and packings have been found satisfactory for this separation. Using gas chromatography with a flame ionization detector, an overall precision (Sr) of 0.069 with a limit of detection of 0.004 mg/sample was achieved (US NIOSH, 1984a). Methodology for analysing air samples recommended by the United States Environmental Protection Agency (US EPA) can detect MEK and most other mono-functional aldehydes and ketones at the 3-6 µg/m3 (1-2 ppb) level (Riggin, 1984). Air is drawn through a mixture of isooctane and an acidified solution of 2,4-dinitrophenylhydrazine (DNPH), which reacts chemically with MEK. DNPH derivatives of aldehydes and ketones are extracted from the aqueous layer, separated with high performance liquid chromatography (HPLC) and detected by ultraviolet absorption. MEK vapour can also be detected and measured directly and instantaneously by absorption of infrared light. The method (detection limit, 3 mg/m3) appears suitable for use in the workplace where only a limited number of solvent vapours are present (Persson et al., 1984) but may not reliably detect MEK in the presence of a diverse mixture of organic vapours, due to overlapping of infrared absorption peaks (Puskar et al., 1986). Ying & Levine (1989) used Fourier transform-infrared spectrometry (FT-IR) to determine the concentration of MEK in mixtures of vapours in ambient air and obtained a detection limit of 1 mg/m3. Surface acoustic wave devices have been tested experimentally for the detection of MEK and other vapours (Rose-Pehrsson et al., 1988) and show promise for the development of electronic devices that can continuously monitor and analyse vapour mixtures at concentrations likely to be present in the work environment. 2.4.3 Water Water samples containing high levels of MEK (e.g., industrial waste water) can be analysed by direct injection of the sample into a gas chromatograph; the detection limit is 40 µg/litre (Middleditch et al., 1987). Samples with low levels of MEK (e.g., drinking-water) require some form of concentration such as distillation (Pellizzari et al., 1985) or adsorption on a hydrophobic zeolite (Ogawa & Fritz, 1985). GC/MS analysis gives a detection limit of 0.05 µg/litre (Pellizzari et al., 1985) whereas HPLC with UV detection has a detection limit of 2 µg/litre (Ogawa & Fritz, 1985). 2.4.4 Solids Analysis of solid and semisolid materials such as industrial wastes for MEK presents special difficulties in terms of both sampling and analysis. The sample must be representative and of adequate size, since substrates such as waste tend to be very non-homogeneous and MEK must be completely removed from both solid and liquid components. One method accomplishes this by extracting the sample with an appropriate solvent (tetraglyme) and purging MEK and other volatile organics from the tetraglyme with an inert gas (Gurka et al., 1984). Another method (Fisk, 1986) either directly purges MEK and other organic compounds from a water/solid material slurry held at an elevated temperature or purges a methanol extract of the solid material at an elevated temperature. 2.4.5 Biological materials Biological materials offer the same analytical problems as solid waste: MEK must be completely removed from both solid and liquid components of the sample. This can be accomplished by headspace analysis (Ramsey & Flanagan, 1982; US NIOSH, 1984b), steam distillation of a sample slurry followed by headspace analysis (Bassette & Ward, 1975; Lin & Jeon, 1985), derivation with o-nitrophenylhydrazine (Van Doorn et al., 1989) and, in the case of an entirely liquid substrate, separation and concentration by reverse-phase extraction (Kezic & Monster, 1988). For MEK in blood the United States National Institute of Occupational Safety and Health method (US NIOSH, 1984b), using GC/FID, has a detection limit of 20 µg/litre and the Ramsey & Flanagan (1982) method has a detection limit of 100 µg/litre. The latter method can also be used for the analysis of MEK in urine with the same limit of detection. Other methods for analysis in urine are those of Kezic & Monster (1988), using GC/FID, and Van Doorn et al. (1989) using HPLC/UV; both methods have limits of detection of 100 µg/litre. 3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE 3.1 Natural occurrence MEK occurs naturally at low concentrations. It has been identified in cigarette smoke (Osborne et al., 1956; Hoshika et al., 1976; Higgins et al., 1983). It also has been reported in chicken breast muscle (Grey & Shrimpton, 1967), weed residues (Bradow & Connick, 1988), southern pea seeds (Fisher et al., 1979), insect pheromones (Cammaerts et al., 1978; Attygalle et al., 1983), juniper leaves (Khasanov et al., 1982); marine macroalgae (seaweeds) (Whelan et al., 1982) and as a product of microbial metabolism (Patel et al., 1982; Mohren & Juttner, 1983; Zechman et al., 1986), including cultures isolated from fresh water and soil (Hou et al., 1983; Patel et al., 1983). Berseem clover, hairy vetch and crimson clover emitted volatile compounds including MEK (Bradow & Connick, 1990). Six amaranth species emit MEK which has been shown to cause significant inhibition of tomato and onion seed germination (Connick et al., 1989). Some bacteria (e.g., thermophilic obligate methane-oxidizing bacteria) can oxidise 2-butanol to produce MEK (Imai et al., 1986). Studies have shown that MEK is a normal component of flavour and odour in a wide range of foods, especially cheese and other fermented products (Zakhari et al., 1977), often as a result of bacterial activity (Lin & Jeon, 1985). Seven types of fish contain MEK, although reported levels were low relative to other compounds (Sakakibara et al., 1990). MEK has been detected in coyote urine (Schultz et al., 1988), in the urine of non-occupationally exposed humans (Tsao & Pfeiffer, 1957; Mabuchi, 1969), in human blood (Mabuchi, 1969) and in exhaled air (Conkle et al., 1975). The MEK in exhaled air may have been derived from food, but the observations of Poli et al. (1985) and other researchers (see section 6) strongly suggest that MEK and similar carbonyl compounds are minor products of normal mammalian metabolism. 3.2 Production levels, processes and uses 3.2.1 World production Although MEK is an important industrial chemical, world production figures are not available. Annual production in the USA, reported by the US International Trade Commission, ranged from 212 to 305 thousand tonnes over the period 1980-1987 and averaged 258 thousand tonnes (USITC, 1981-1988). Current (1987) annual capacity and production values for western Europe are 308 and 215 thousand tonnes, respectively (Chemical Business Newsbase, 1988). Japanese annual capacity and production figures in 1986 were 180 and 139 thousand tonnes, respectively (Chemical Business Newsbase, 1987). Argentinean annual capacity was 15 thousand tonnes in 1985 (Chemical Business Newsbase, 1986). A production plant opened in Brazil in 1991 but information on capacity and production is not available (personal communication from E. de S. Nascimento). 3.2.2 Production processes MEK is produced mainly by dehydrogenation of sec-butyl alcohol (Liepins et al., 1977; SRI International, 1985, 1988). In the USA, one process uses sec-butyl alcohol vapour at 400 to 550 °C oxidized with a zinc oxide catalyst. Reaction gases are condensed and the condensate fractionated in a distillation column. The yield of MEK is 85 to 90% (Lowenheim & Moran, 1975). Any uncondensed reaction gases are scrubbed with water or a non-aqueous solvent and the waste stream from the scrubber, which contains MEK and reaction by-products, is either recycled or discarded (Liepins et al., 1977). In Europe, sec-butyl alcohol is dehydrogenated over Rainey nickel or copper chromite catalyst at 150 °C (Papa & Sherman, 1978) MEK is also produced by the oxidation of n-butane, either as the main product or as a by-product in the manufacture of acetic acid (Liepins et al., 1977; Papa & Sherman, 1978). Liquid butane reacts with compressed air in the presence of a transition metal acetate catalyst, normally cobalt acetate, and the reaction product phase is separated. The hydrocarbon-rich phase is recycled to the reactor and the aqueous phase with MEK is withdrawn and purified. MEK and other organic compounds with low boiling points are separated from acetic acid by distillation. Reaction conditions determine whether MEK or acetic acid is the principal product (Lowenheim & Moran, 1975). Butane oxidation accounted for about 13% of the 1987 MEK production capacity in the USA (SRI International, 1988) but for none of the 1984 production capacity in western Europe (SRI International, 1985). Other methods exist for the commercial manufacture of MEK (Papa & Sherman, 1978), but there is no evidence that any of these alternatives are of current importance. 3.2.3 Other sources In addition to manufacture by the chemical industry, MEK and other carbonyls are incidentally produced as components of exhaust from jet (Miyamoto, 1986) and internal combustion engines (Seizinger & Dimitriades, 1972; Creech et al., 1982; Hampton et al., 1982) and from industrial activities such as retort distillation of oil shale (Hawthorne et al., 1985) and gasification of coal (Pellizzari et al., 1979). MEK comprises about 0.05% of the hydrocarbon exhaust gases of motor vehicles, and in 1987 the vehicle emission of MEK in the USA was estimated to be 1909 tonnes (Somers, 1989). Thus its anthropogenic production by vehicles plus an additional amount by stationary engines was no more than 0.1% of the industrial production in the USA. Grosjean et al. (1983) concluded, however, that during smog episodes in the Los Angeles basin much of the ambient level of MEK was produced photochemically. 3.2.4 Uses The major uses of MEK reflect its excellent characteristics as a solvent (Table 3). Its high solvency for gums, resins and many synthetic polymers permits formulations with high solid content and low viscosity. It is also inert to metal, evaporates rapidly, and is relatively low in toxicity compared with solvents like benzene which MEK replaced (Zakhari et al., 1977; Basu et al., 1981). Table 3. Major uses of MEK in the USAa End use % Solvent - protective coatings 65 Solvent - adhesives 15 Solvent - magnetic tape production 8 Lubricating oil dewaxing 5 Chemical intermediate 4 Miscellaneous 3 a From: Manville Chemical Products Corp. (1988) The largest single use of MEK is as a solvent for vinyl plastic used in coatings and moulded articles. Other important uses are as a solvent for lacquers and for cellulose nitrate, cellulose acetate, acrylics, and adhesive coatings. Its properties as a selective solvent make it ideal for dewaxing lubricating oils. MEK is also used for degreasing metals, in the manufacture of magnetic tapes, inks and smokeless powder, and as a chemical intermediate in the production of methyl ethyl ketoxime, MEK peroxide, methyl isopropyl ketone and many other compounds. In addition to industrial uses, MEK is an ingredient in a variety of consumer products such as lacquers, varnishes, spray paints, paint removers, sealers and glues (Zakhari et al., 1977). In both consumer products and industrial applications, MEK is frequently only one of several components in a mixture of organic solvents. MEK is also used as an extraction solvent in the processing of foodstuffs and food ingredients, e.g., in fractionation of fats and oils, decaffeination of tea and coffee, and extraction of flavourings. 3.3 Release into the environment Releases of MEK are mainly into the atmosphere (Reilly, 1988). These can result from: spillage; venting of gases and fugitive emissions during manufacture, transfer and use; solvent evaporation from coated surfaces; loss from landfills and waste dumps; and engine exhaust (Basu et al., 1981; LaRegina & Bozzelli, 1986). Relatively little MEK is lost during manufacture when the process is enclosed. The average annual release from four manufacturing plants in the USA was estimated to be 82 tonnes per site, equal to a total of 328 tonnes or about 0.1% of their annual production (Reilly, 1988). The bulk of MEK eventually evaporates to the atmosphere, since the major use of MEK is as a solvent for coatings and adhesives. In industry, some of the MEK evaporated from surface coatings or lost during cooking and thinning of resin is removed from the ventilation exhaust by absorption on charcoal filters or by incineration of the exhaust stream. The latter method can reduce emission by up to 97% (Gadomski et al., 1974), and removal is accomplished in a single step without generating a residue for subsequent disposal (DiGiacomo, 1973). The waste stream from MEK production contains acetic acid and a variety of alcohols, aldehydes, ketones and other organic compounds. It is likely that butane and other organic compounds are discharged into the atmosphere from the reaction section, but no specific information is available (Liepins et al., 1977). MEK is released from other industrial operations involving its use, and from activities such as retort distillation of oil shale and gasification of coal (Pellizzari et al., 1979; Hawthorne et al., 1985). It has been detected in drinking-water (Ogawa & Fritz, 1985), in well water (Jacot, 1983), in ground water (Botta et al., 1984) and in leachate from a hazardous waste site (Jacot, 1983). MEK occurs in water often as a result of natural processes (section 3.1). Atmospheric input and direct anthropogenic pollution contribute significantly to elevated levels (Grosjean & Wright, 1983). 4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION 4.1 Transport in the environment MEK appears to be highly mobile in the natural environment (Lande et al., 1976). It is water soluble (Windholz, 1983) and evaporates rapidly in air. The generally low values for MEK in outdoor air probably stem mainly from its rapid removal by photodecompo-sition. Scavenging by aqueous droplets and dry deposition, which also represent potential routes of loss from the atmosphere, are balanced to an unknown extent by evaporation of MEK from water and soil. There is no specific information on partitioning of MEK in the environment. Although Basu et al. (1981) estimated from its physical properties that MEK will "exhibit low sediment-water and soil-water partitioning and be susceptible to substantial leaching from soils to which it is not extensively chemically bound", there is no information on chemical binding of MEK to sediment particles. As mentioned above, MEK has, however, been detected in ground water and the leachate from hazardous waste sites (section 3.3). 4.2 Bioaccumulation and biodegradation On the basis of its octanol/water partition and water solubility, bioconcentration factors (BCF) of approximately 1 and 0.5, respectively, have been calculated for MEK (US EPA, 1985b). In view of its high water solubility, ecosystem modelling (Metcalf et al., 1973; Chiou et al., 1977) indicates that it is unlikely that MEK will accumulate in food webs. It is absorbed and metabolized by organisms present in the environment, e.g., in waste water (Dore et al., 1975; Bridie et al., 1979a) and in soil (Perry, 1968). It is rapidly metabolized by mammals (Di Vincenzo et al., 1976, 1978; Dietz et al., 1981; Miyasaka et al., 1982) and by many microbes (Gerhold & Malaney, 1966; Dojlido, 1977; Urano & Kato, 1986). MEK is nearly completely degradable at concentrations up to 800 mg/litre on the basis of biochemical oxygen demand (BOD), and the rate of degradation decreases with increasing concentration of MEK. Using activated sludge there was complete degradation of MEK in 8 days at a concentration of 200 mg/litre (200 ppm) and in 9 days at a concentration of 400 mg/litre (400 ppm) (Dojlido, 1979). At a concentration of 20 mg/litre in river water containing preadapted microbes, MEK was completely degraded in 2.5 days (Dojlido, 1977). Delfino & Miles (1985) reported a slower rate of decomposition in aerobic ground water; 1 mg/litre was fully degraded in 14 days. However, a bacterial species (Alcaligenes faecalis) found in sewage sludge metabolized MEK slowly if at all (Marion & Malaney, 1963). The data on mammals and microbes suggest that MEK is rapidly absorbed and metabolized by most living organisms (Basu et al., 1981). MEK in air is rapidly decomposed by photochemical processes, mainly through oxidation by hydroxyl free radicals as well as some decomposition by direct photolysis (Levy, 1973; Laity et al., 1973; Dilling et al., 1976; Grosjean, 1982; Seinfeld, 1989). Basu et al. (1981) estimated a half-life of 5.4 h for photochemical decomposition in urban atmospheres. They further concluded that the lower concentration of photochemically produced oxidants in rural air will lead to a substantially lower rate of photochemical decomposition in these areas. The concentration of MEK and other carbonyls is higher in urban air (Grosjean & Wright, 1983; Snider & Dawson, 1985). Greater anthropogenic emissions and photochemical synthesis of carbonyls from free radicals (Grosjean et al., 1983) may overwhelm the more rapid photochemical decomposition in urban atmospheres. Scavenging by aqueous droplets and dry deposition may also be important processes in the removal of atmospheric MEK (Grosjean & Wright, 1983). MEK (and other saturated aliphatic carbonyls) is not chemically reactive under conditions found in most natural waters and in general will not degrade rapidly from physical causes once deposited in water (US EPA, 1985b). The exception is water containing free halogens (such as chlorine) or hypohalides. MEK reacts with these to form a haloform and propionic acid (Basu et al., 1981). This can be a cause for concern in chlorinated waste water and water supplies, since the chloroform thus produced is more toxic than the original MEK (US EPA, 1985a). 5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE 5.1 Environmental levels 5.1.1 Air Although MEK is widely present in the natural environment, concentrations are always low even under conditions of pollution (Table 4). In minimally polluted outdoor air, the level is less than 3 µg/m3 (1 ppb), but 131 µg/m3 (44.5 ppb) has been measured under conditions of heavy air pollution in the Los Angeles basin. Volatilization of MEK from building materials and consumer products can pollute indoor air to levels above adjacent outside air. In a study of 15 Italian urban homes, De Bortoli et al. (1985, 1986) reported 8 µg/m3 as a mean indoor air value and 38 µg/m3 as a maximum value. Maximum and average values for MEK in outdoor air adjacent to these homes were 12 and 3.8 µg/m3 respectively. Shah & Singh (1988) reported four observations of MEK in indoor air in the USA; the median and mean values were 21 and 27 µg/m3 (7.1 and 9.2 ppb), respectively. In a confined and tightly sealed space, however, MEK concentrations can be much higher. Liebich et al. (1975) measured 1.9 to 4.4 mg/m3 (665 to 1505 ppb) in Space Lab IV. Human activities, other than the deliberate manufacture and use of MEK, may in some circumstances contribute significantly to environmental levels. MEK is a minor component, < 2.95 mg/m3 (1.0 ppm), of gasoline engine exhaust and also has been detected in the exhaust from diesel engines and jet aircraft. The US Environmental Protection Agency estimated that 1909 tonnes of MEK was emitted in motor vehicle exhaust in the USA in 1987 ("Mobile source estimates for methyl ethyl ketone"; personal communication by J.H. Somers, 1989). In addition, Grosjean et al. (1983) concluded that synthesis of MEK and other carbonyls from hydrocarbons in vehicle exhaust by photochemical reactions in the atmosphere may greatly exceed their direct production by motor vehicles. Thus, away from industrial areas where MEK is manufactured or used, it is likely that motor vehicles are an important and possibly major source of atmospheric pollution by MEK. Smoking cigarettes and other tobacco products contributes slightly to individual exposure. Although the concentration of MEK in cigarette smoke (Table 4) may exceed recommended levels of permissible occupational exposure (Table 5) by several a Assuming a respiratory volume of 20 m3 per day times, the total amount of MEK generated by smoking a single cigarette is about 1/74th of the acceptable human daily chronic intake in the USA (15.43 mg/day) (US EPA, 1986). MEK also has been detected in the gases from structural (building) fires (Lowry et al., 1981). 5.1.2 Water MEK concentrations in exposed natural waters are less than 0.1 mg/litre (0.1 ppm) and are usually below the level of detection. Ewing et al. (l977) analysed 204 samples from rivers with industrialized basins; only one sample contained MEK (0.023 mg/litre). Jungclaus et al. (1978) measured significant levels of MEK in waste water from a chemical plant but could not detect it in either water or sediment of the brackish Delaware River receiving this waste. Despite its rapid disappearance from water, trace amounts of MEK have been detected widely in drinking-water (US EPA, 1985b). A potential source is solvent leached from the cemented joints of plastic pipe (Wang & Bricker, 1979; Boettner et al., 1981). A single unexpectedly high value of 0.47 mg/litre (0.47 ppm) in mist from the landward edge of the Los Angeles basin probably resulted from scavenging of heavily polluted air (Grosjean & Wright, 1983). Data on MEK in sediment (US EPA, 1985b) were based on four samples and are difficult to interpret. Sawhney & Kozloski (1984) studied organic pollution of leachates from municipal landfill sites in Connecticut, USA. MEK concentrations ranging between 4.8 and 8.2 mg/litre were measured over a two-year period at one site. This high value may have resulted not only from a substantial input but also from reduced microbial activity and no evaporative loss to the air. Environmental concentrations in a number of media are shown in Table 4. Table 4. Concentrations of MEK in the environment Source Concentration Reference Air (rural) South-western USA 1.77 µg/m3 Snider & Dawson (1985) (0.6 ppb) Air (urban) South-western USA, Tucson 7.1 µg/m3 Snider & Dawson (1985) (2.4 ppb) USA, Los Angeles basin 0-131.3 µg/m3 Grosjean et al. (1983) (0-44.5 ppb) Sweden, traffic areas 7.7-94 µg/m3 Jonsson et al. (1985) (2.6-32 ppb) Italy < 2.1-12.1 µg/m3 De Bortoli et al. (1985) (< 0.7-4.1 ppb) Japan (air pollution) 12.7 µg/m3 Anonymous (1978) (4.3 ppb) Air (indoor) Italy (homes) < 2.1-38.1 µg/m3 De Bortoli et al. (1985) (< 0.7-12.9 ppb) USA (homes) detected in 3 out of Jarke et al. (1981) 87 samples Space Lab IV 1.96-4.44 mg/m3 Liebich et al. (1975) (0.665-1.505 ppm) Water Sea water (Gulf Stream) < 0.022 mg/litre Corwin (1969) Sea water (Mediterranean) < 0.008 mg/litre Corwin (1969) Mist (California, USA) < 0.47 mg/litre Grosjean & Wright (1983) Drinking-water (USA) < 0.0016 mg/litre Ogawa & Fritz (1985) Ground water (hazardous 4.8-8.2 mg/litre Sawhney & Kozloski (1984) waste landfill sites, USA) Table 4 (contd.) Source Concentration Reference Rivers (industrialized 0.023 mg/litrea Ewing et al. (1977) areas, USA) Waste water (oil well, USA) 1.5 mg/litre Sauer (1981) Waste water (Chemical plant, 8-20 mg/litre Jungclaus et al. (1978) USA) Waste water (plant, Poland) > 100 mg/litre Dojlido (1977) Sediment USA 0.050-23 mg/kg US EPA (1985b) Anthropogenic sources Automobile exhaust < 0.3-2.95 mg/m3 Seizinger & Dimitriades (0.1-1.0 ppm) (1972) Cigarette smoke 80-207 µg/cigarette Higgins et al. (1983) a MEK was detected in only one of 204 samples. Table 5. Levels of estimated daily MEK intake from different sources/routes of exposure Type/route of exposure Daily intake Foodstuffs 1590 µg Drinking-water (2 litres) 3.2 µg Ambient aira outdoor, rural 36 µg outdoor, urban < 2620 µg indoor < 760 µg Tobacco smoking (20 cigarettes) < 1620 µg a Assuming a respiratory volume of 20 m3 per day 5.1.3 Foodstuffs MEK is produced in small amounts by animals, higher plants, algae and microbes, and is a widespread, although generally minor, component of taste and odour in foods (Zakhari et al., 1977). It has been identified in some foodstuffs and beverages. Using the DNPH method with column or paper chromatography, it has been identified (but not quantified) in white bread (Ng et al., 1960), tomatoes (Schormüller & Grosch, 1964), cooked turkey meat (but not in raw meat and with more MEK in roasted than in boiled meat, the level increasing with roasting time) (Hrdlicka & Kuca, 1965), and in egg white (Sato et al., 1968). Using gas chromatography, traces of MEK were found in fresh chicken meat (pectoral muscle) with a marked increase in samples kept for 4 days at room temperature. It was not found in caecal gas from living chickens but was detected in the gas 18 and 24 h after death (Grey & Shrimpton, 1967). MEK has also been detected in cottonseed oil (Dornseifer et al., 1965), honey (Cremer & Riedmann, 1964), coffee (Gianturco et. al., 1966), roast barley (Shimizu et al., 1969), and in the mushroom Agaricus bisporus (Staüble & Rast, 1971). By means of GC/MS, Wong et al. (1967) detected MEK in codfish and Kahn et al. (1968) reported its presence in a low-boiling distillation fraction of Canadian whisky. In a study of compounds related to milk flavour, Wong & Patton (1962) determined MEK concentrations using the DNPH method with column separation and paper chromatography. The concentrations in two samples of untreated milk were 0.77 and 0.79 mg/litre and in two samples of cream were 0.154 and 0.177 mg/litre. Gordon & Morgan (1972) examined the influence of volatile compounds in milk on "feed" flavour and reported MEK concentrations of 0.25-0.35 mg/litre in moderately "feed" flavoured milk and 0.50-1.0 mg/litre in strongly flavoured milk, with a highest concentration detection of 1.4 mg/litre. They concluded that MEK is one of the compounds responsible for producing the unpleasant "feed" flavour in milk. Using the DNPH method and paper chromatography, Harvey & Walker (1960) detected MEK in New Zealand cheddar cheese one day after manufacture. The concentration increased during ripening, reaching 0.9 mg/kg at 40 weeks, and was related to the development of typical Cheddar cheese flavour. In another study of the chemical nature of USA Cheddar cheese flavour, Day et al. (1960) analysed the volatile flavour fraction of cheeses over 1 year old using DNPH and column partition chromatography, and reported approximate MEK concentrations of 12.5 mg/kg. Keen et al. (1974) postulated that the formation of MEK in New Zealand Cheddar cheese, for which levels as high as 19 mg/kg had been reported, occurred in steps carried out by different microbial species including Streptococcus cremaris, Pediococcus cerevisiae, Lactobacillus plantarum and Lactobacillus brevis. They considered that MEK was an important flavour constituent in the cheese. In another investigation of monocarbonyl compounds as flavour components, Mookherjee et al. (1965) measured MEK in fresh and stale (8 weeks old) potato chips with the DNPH method and liquid-liquid chromatography. In fresh potato chips the concentration of MEK was 1.8 µmoles/kg, and this increased to 2.2 µmoles/kg in stale chips. Amylomaize starches are heat treated in the production of films and fibres to concentrate the amylose. Bryce & Greenwood (1963) used gas chromatography to measure pyrolysis products (including MEK) of potato starch, potato amylose and amylopectin, maltose, isomaltose and glucose. MEK was not detected in untreated starch nor in starch pyrolysed in vacuo for 20 min at 200 and 220 °C. With increasing temperatures the concentration of MEK increased; at 230, 250, 300, 350 and 400 °C the MEK concentrations were 10, 15, 50, 65 and 70 moles x 107/g starch, respectively. Small quantities (up to 2 ng/1.5 g bean) were found in soybeans (Clycine max) and winged beans (Psophocarpus tetragonolobus) by means of dynamic headspace GC/MS (Del Rosario et al., 1984). However, the reported concentrations of MEK in foods are low and food consumption is not considered a significant source of population exposure. 5.2 General population exposure Stofberg & Grundschober (1984) calculated the consumption ratio between the quantity of a flavouring material consumed as an ingredient of basic and traditional foods and the quantity of that same flavouring material consumed as a component of added flavourings by a certain population. If the consumption ratio is more than 1, this substance is consumed predominantly as an ingredient of traditional foods. For MEK, this ratio may be up to 411. The annual consumption in the USA of MEK via apple juice is 85 kg, white bread 70 132 kg, butter 34 kg, carrot 154 kg, Cheddar cheese 30 139 kg, Swiss cheese 198 kg, fish 81 kg, potato chips 31 kg, tomato 31 878 kg, and yoghurt 1104 kg. Assuming a population of 230 millions, the estimated average daily intake in the USA amounts to 1.59 mg/kg foodstuff. Some information on the MEK contents of various foodstuffs is given in Table 6. The European Economic Community regulates the level of MEK in certain foodstuffs; these are given in Table 7. In addition to the MEK that is naturally present, foods may also contain MEK absorbed from plastic packaging materials. This can be derived from solvent left in the plastic during manufacture (Kontominas & Voudouris, 1982) or represent one of the many organic compounds produced during extrusion (Fernandes et al., 1986). It can also be produced by irradiation of polyethylene film during the sterilization of packaged foods with an electron beam (Azuma et al., 1983). Although the presence of MEK and associated organic compounds from packaging materials may affect the flavour of foods, it probably does not represent a significant source of population exposure. Table 6. MEK concentrations of certain foodstuffs Source Concentration Reference Bean seeds (raw) 0.5 mg/kg Del Rosario et al. (1984) Bean seeds (heated at 0.7-2.0 mg/kg Del Rosario et al. (1984) 190 °C) Pea seeds 0.074-0.39 mg/kg Fisher et al. (1979) Milk ("feed"-flavoured) 0.25-14 mg/litre Gordon & Morgan (1972) Bread 3.06 mg/kg Sosulski & Mahmoud (1979) Cheddar cheese 12.5 mg/kg Day et al. (1960) 19 mg/kg Bills et al. (1966) Table 7. Food uses of MEK permitted in the European Economic Community Conditions of use Maximum residue limits in the extracted foodstuff, food or food-contact material In manufactured or regenerated 0.6 mg/dm2 on the side in contact with cellulose film that comes into foodstuffs contact with fooda Fractionation of fats/oilsb 5 mg/kg in the fat/oil Decaffeination of, or removal of 20 mg/kg in the coffee or tea irritants and bitterings from, (as granules, powder, leaves, coffee and teab etc.) Preparation of flavourings 1 mg/kg in the foodstuff from natural flavouring materialsb a Council Directive 83/229/EEC b Council Directive 88/344/EEC Higgins et al. (1983) analysed the gas phase organic compounds in cigarette smoke. In cigarettes with high tar content (7-45 mg per cigarette), the MEK level was 63-131 µg/cigarette, whereas in ultralow tar delivery cigarettes (advertised value < 0.01-0.2 mg per cigarette), it was 0.93-4 µg MEK/cigarette. Levels of estimated daily MEK intake from different sources/ routes of exposure are given in Table 5. 5.3 Occupational exposure Information on measured levels of occupational exposure is summarized in Table 8. Some national occupational exposure limits for MEK in workplace air are shown in Table 9. In a study of an electronic parts plant in the USA, Lee & Parkinson (1982) reported that workers were exposed to mixtures of solvent vapours containing MEK. Inoue et al. (1983), in a nationwide survey of Japanese factories, found that MEK was widely used as a component of solvent mixtures. A study by Falla (1987) of 19 British plants manufacturing or applying surface coatings reported that none had MEK concentrations in excess of 295 mg/m3 (100 ppm). The highest TWA value, 723 mg/m3 (245 ppm), reported by Lee & Murphy (1982) represented a worker who entered the highly polluted vinyl dip room (502-1785 mg/m3; 170-605 ppm) only occasionally, but without always donning his respirator hood. A co-worker stationed in the vinyl dip room who wore his respirator hood constantly had TWA exposures of 307-617 mg/m3 (104-209 ppm). De Rosa et al. (1985) examined 504 work stations in 81 Italian plants (shoe factories, painting operations and printing plants) and found that the TLV for MEK (590 mg/m3, 200 ppm) was rarely exceeded. 5.4 Peri-occupational exposure Many small industries in the Netherlands are located in inner city areas. The influence of such industries on the quality of indoor air in adjacent houses was studied by Verhoeff et al. (1987), who monitored the indoor air of a car-body repair shop, an offset printing office and surrounding houses for organic solvents, including MEK. Monitoring was carried out for one week, and the individual exposure of workers and residents was investigated by biological monitoring of the exhaled breath with additional personal air sampling of the workers. Concentrations in both the factories were lower than 29.5 mg/m3, i.e. 5% of the Dutch MAC values (590 mg/m3). In the personal air samples of the employees, MEK was at or below the detection level. In the house located directly over the car-body repair shop, the average concentrations of MEK were about 50% those in the shop, while two floors above, MEK was detected only once. Table 8. Occupational exposure to MEK via air Activity Country Sampling Concentration Reference details mg/m3 (ppm) Printing and printing Japan personnel (62) 0-265 (0-90) Miyasaka et al. (1982) machine manufacturing Shoe factory Italy not given 0-300 (0-102) Brugnone et al. (1983) Shoe factories, painting Italy personnel (1 h) 0-1110 (0-376)a De Rosa et al. (1985) operations, printing plants (504) Miscellaneous factories Italy personnel (4 h) (65) 10-953 (3.4-323) Ghittori et al. (1987) Sheet metal shop Sweden area (continuous) approx. 3-44 Persson et al. (1984) (approx. 1-15) Organic chemical waste USA area (9) < 0.06 (< 0.02) Decker et al. (1983) incinerator (vicinity) personnel (7) < 0.06-189 (< 0.02-64) (exposed workers) Radio components USA personnel (range 0-38 (0-13) Lee & Parkinson (1982) manufacturing of jobs) Solvent recycling plant USA < LDQ-166 (< LDQ-38)b Kupferschmid & Perkins (1986) Plastic items factory USA no details 0.07-0.16 Ahrenholz & Egilman (1983) (0.024-0.054) Surface coatings United area (59) < 295 (100) Falla (1987) factories Kingdom Table 8 (contd) Activity Country Sampling Concentration Reference details mg/m3 (ppm) Lubricating oil USA personnel (38) 0.09-80 (> 0.029-27) Emmel et al. (1983) refineries Miscellaneous factories USA personnel (179) 0-0.59 (0-0.2) Whitehead et al. (1984) with spray application (non-exposed of glue and paint workers) 1.18-6.2 (0.4-2.1) (exposed workers) Aircraft maintenance USA personnel (9) 0-65 (0-22)c Thoburn & Gunter (1982) Athletic equipment USA personnel (12) 307-723 Lee & Murphy (1982) factory (104-245 TWA)d (vinyl dip room) 0.89-534 (0.3-181) TWA (elsewhere) a MEK was found in 85/504 samples b LDQ = lowest detectable quantity c MEK was detected in only one of nine samples d 8-h time-weighted average Table 9. Some national occupational exposure limits for MEK in air Country Exposure limit Category of limitb Reference mg/m3 (ppm) Argentina 590 (200) TWA IRPTC (1987, 1991) 885 (300) STEL Brazil 460 (155) for 48 h/week IRPTC (1987, 1991) Germany 590 (200) TWA IRPTC (1987, 1991) 1180 (400) 30 min. STEL (average value, 4 x per shift) Hungary 200 (68) TWA (8 h) IRPTC (1987, 1991) 1000 (2950) STEL (30 min) Italy 590 (200) TWA Notified by country Japan 590 (200) TWA IRPTC (1987, 1991) Netherlands 590 (200) TWA a Sweden 150 (50) TWA IRPTC (1987, 1991) 300 (100) STEL (15 min) United Kingdom 590 (200) TWA (8 h), OES Notified by country 885 (300) STEL (10 min), OES USA (NIOSH/OSHA) 590 (200) TWA (10 h) US NIOSH (1990) 885 (300) STEL (15 min) 8850 (3000) IDLH USA (ACGIH) 590 (200) TLV (TWA) ACGIH (1991) 885 (300) STEL 2 mg/litre BEI urine; end of shift Yugoslavia 295 (100) Notified by county TWA and TLV are, with one exception, in the range of 295-590 mg/m3 (100-200 ppm). The latter is stated to be the highest concentration which can be tolerated by humans without discomfort (ACGIH, 1986). In addition a short-term exposure limit of 885-1180 mg/m3 (300-400 ppm) has been established by some nations. a Dutch Expert Committee for Occupational Standards (1991) b Abbreviations: BEI = biological exposure index (ACGIH); IDLH = concentration immediately dangerous to life or health (US NIOSH); OES = occupational exposure standard (UK); STEL = short-term exposure limit; TLV = threshold limit value (ACGIH); TWA = time weighted average 6. KINETICS AND METABOLISM 6.1 Absorption 6.1.1 Percutaneous absorption Percutaneous absorption of MEK appears to be rapid (Munies & Wurster (1965) and Wurster & Munies (1965) reported that MEK was present in the exhaled air of human subjects within 2.5-3.0 min after it was applied to normal skin of the forearm, and the concentration of MEK in exhaled air reached a plateau in 2-3 h. The rate of absorption was controlled mainly by the moisture content of the skin. With dry skin, absorption was slow, and it took 4-5 h for the concentration of MEK in expired air to attain a plateau. With moist skin, absorption was very rapid initially. MEK was detected in expired air in measurable concentrations within 30 seconds after an application of MEK to the forearm, and a maximum concentration in expired air, averaging four times the plateau level for normal and dry skin, was achieved in 10-15 min. The concentration of MEK in expired air, and thus its absorption, subsequently declined to a plateau level somewhat above that for normal and dry skin because the MEK partially desiccated the moist skin with which it was in contact. Munies & Wurster (1965) concluded that the rapid percutaneous absorption of MEK reflected its olive oil-water partition coefficient, which is close to unity. Their data have been used to calculate minimum rates of percutaneous penetration of 0.46 µgÊcm-2Êmin-1 for dry or normal skin and 0.59 µgÊcm-2Êmin-1 for moist skin (JRB Associates, Inc., 1980). These rates are minimal because they are based solely on exhalation from the lungs and ignore all other excretion processes for MEK. As the elimination of MEK via inhalation constitutes only 5 to 10% of the total loss (Cushny, 1910), these rates should be multiplied by factors of 10-20. This gives a percutaneous absorption of 5-10 µgÊcm-2Êmin-1, which is identical to the value measured for methyl isobutyl ketone by DiVincenzo et al. (1978). 6.1.2 Inhalation absorption The absorption of MEK via the lungs was examined by Perbellini et al. (1984) in a study of workers exposed in industrial workplaces. The MEK concentration in alveolar and expired air correlated significantly with the environmental concentration, and averaged 30% of the latter. In more recent studies (Liira et al., 1988a, 1988b), values for pulmonary absorption ranging from 41.1% to 55.8% were obtained. Liira et al. (1988b) suggested that differences in their values may have reflected variations in breathing technique during the collection of samples rather than actual changes in uptake. Deeper inhalation increased the alveolar volume relative to the dead space (the non-alveolar volume of the respiratory system), and thus increased the apparent absorption (personal communication by J. Liira, 1989). When the alveolar retention of MEK (about 70%) measured by Perbellini et al. (1984/1985) is transformed to overall pulmonary retention, their observations and those of Liira et al. (1988a,b) are in agreement, showing that about 50% of inhaled MEK is taken up. Results of studies by Liira et al. (1988a,b, 1990a,b) indicated a rapid transfer of MEK vapour into the blood stream. Perbellini et al. (1984, 1985), reported that the concentrations of MEK in the blood and urine were significantly correlated with the environmental concentration, indicating rapid transfer to the blood and thence to other tissues. In human volunteer subjects, exercise during exposure markedly increased the MEK level in blood in comparison with sedentary behaviour (Liira et al., 1988b), indicating that the blood MEK level also depended on the rate of uptake. Ghittori et al. (1987) and Miyasaka et al. (1982) also found significant correlations between environmental levels of MEK and amounts excreted in the urine of exposed workers. The concentration of MEK in urine rose from essentially zero to 70% of its maximum value during the first 2 h of an 8-h shift (Miyasaka et al., 1982). Ong et al. (1991) studied biological monitoring of occupational exposure to MEK in 67 healthy male workers employed in plastic bag or video-tape production. Their ages ranged between 18 and 52 years with an average working experience of 8.6 years. For the majority of the workers, atmospheric MEK concentrations were in the range 30-885 mg/m3 (10-300 ppm). MEK concentrations in urine, blood and exhaled air were measured once weekly at the end of a shift. About 10% of absorbed MEK was eliminated in the exhaled air. Following exposure to 590 mg/m3 (200 ppm), the urinary concentration was 5.1 µmol/litre or 4.11 mg/g creatinine. The correlation coefficients (tau) between atmospheric MEK concentration and end-of-shift urine, blood and exhaled air were 0.89, 0.85 and 0.79, respectively. The authors also reported good correlation between blood and urinary MEK (tau = 0.86) and between blood and exhaled air concentrations (tau = 0.8) but poor correlation between exhaled air and urinary MEK concentrations. 6.1.3 Ingestion absorption In male rats given a large MEK dose (1505 or 1690 mg/kg) in water, blood concentrations reached maxima of 0.95 and 0.94 mg/ml 4 h after ingestion and subsequently declined sharply, indicating protracted absorption of this dose from the gastro-intestinal tract (Traiger & Bruckner, 1976; Dietz & Traiger, 1979; Dietz et al., 1981). 6.1.4 Intraperitoneal absorption The results of DiVincenzo et al. (1976) and Zakhari et al. (1977), who used intraperitoneal injections of MEK in research on metabolism and toxicity, suggest that absorption from the peritoneal cavity is rapid. 6.2 Distribution The distribution of MEK in human tissues was examined by Perbellini et al. (1984) in two solvent-exposed workers who died suddenly of heart attacks at the workplace. The results of this study (Table 10) indicate that the solubility of MEK is similar for all tissues. Brugnone (1985) calculated the uptake and distribution of MEK from the lungs. With a blood/air partition coefficient of 202, MEK can reach equilibrium concentration in a compartment in about 3 min. Distribution volumes were 6.0 for vessel-rich tissues, 39.6 for muscle and 12.8 for fat. Biological half-lives for the same tissues were 0.8, 21.8 and 23.3 min, respectively. The results of in vitro measurements at 37 °C of human tissue-gas partition coefficients, obtained by exposing samples of blood and tissue to a known concentration of MEK (Fiserova-Bergerova & Diaz, 1986), differed from the observations of Perbellini et al. (1984). The partition coefficients ranged from 96 to 162, but did not exceed 111, with the exceptions of whole blood (125), blood plasma (133) and fat (162). Other blood-gas partition coefficients measurements for MEK are 202 (Sato & Nakajima, 1979) and 215 (Pezzagno et al., 1983). Traiger & Bruckner (1976) and DiVincenzo & Krasavage (1974) provided evidence that MEK can enter the liver of rats and guinea-pigs, and Dowty et al. (1976) reported that it can cross the placenta and enter the human fetus. 6.3 Metabolic transformation MEK has been reported to be a metabolic end product of natural gas (methane 88%, ethane 5%, propane 5%, isobutane 2% with traces of tert-butyl mercaptan and methyl acrylate) inhaled for 2 h by ICR mice (Tsukamoto et al., 1985a). In an in vivo study of the metabolism of propane, n-butane and iso-butane inhaled for 1 h by ICR mice, it was found that n-butane gave rise to sec-butanol and MEK (Tsukamoto et al., 1985b). In vitro studies showed that mouse liver microsomal preparations metabolized n-butane to sec-butanol, the precursor of MEK (Tsukamoto et al., 1985b). Inhalation by male Sprague-Dawley rats of sec-butanol at concentrations of 5900 mg/m3 (2000 ppm) for 3 days or 1475 mg/m3 (500 ppm) for 5 days caused marked enzyme induction of cytochrome P-450 in liver and kidney but other butanol isomers did not have this effect (Aarstad et al., 1985, 1986). The authors concluded that the mechanism of induction was via the sec-butanol metabolite MEK. Table 10. Solubility (partition coefficient) of MEK in human tissuesa Tissues Tissue/air Tissue/blood Blood 183 1.00 Kidney 197 1.08 Liver 180 0.98 Brain 168 0.92 Fat 161 0.88 Muscle 212 1.16 Heart 254 1.39 Lung 147 0.80 a From: Perbellini et al. (1984) 6.3.1 Animal studies Traiger & Bruckner (1976) showed that the toxic effects of MEK and 2-butanol were essentially identical in rats, and that 2-butanol was rapidly oxidized to MEK. DiVincenzo et al. (1976) identified the metabolites of MEK in guinea-pigs as 2-butanol, 3-hydroxy-2-butanone and 2,3-butanediol. They hypothesized that the metabolism followed both oxidative and reductive pathways, with the latter leading to the production of 2-butanol. The former, employing microsomal omega-1 oxidization, oxidized MEK to 3-hydroxy-2-butanone, which was subsequently reduced to 2,3 butanediol. Further research utilizing rats (Dietz & Traiger, 1979; Dietz et al., 1981) clarified the pathways of rat MEK metabolism and permitted a calculation of rate constants for the elimination of MEK and its metabolites from the blood as well as for the metabolic transformations (Fig. 1). The body was divided into two compartments: (a) the liver, where metabolic transformations took place; and (b) the blood, which was the site of sampling. Experimental data and equations derived from this data indicated that the major metabolic pathway is butanol -> MEK -> 3-hydroxy-2-butanone -> 2,3-butanediol, with small or non-existent reverse flows. Dietz et al. (1981) estimated that an oral dose of 2-butanol or MEK resulted, on a molar basis, in the same blood level-time curve for 2,3-butanediol as 28-30% of this amount given as an intravenous dose of 2,3-butanediol. Their data indicated that transformations of 2-butanol to MEK and of 3-hydroxy-2-butanone to 2,3-butanediol are rapid and that transformation of MEK to 3-hydroxy-2-butanone is much slower. It is likely that 2-butanol, like ethanol (Mezey, 1976), inhibits oxidative pathways of drug metabolism and thus inhibits the hydroxylation step leading to 3-hydroxy-2-butanone. This possibility is supported by the observation of Dietz et al. (1981) that 2-butanol, at concentrations similar to those achieved in the blood of rats in the above experiment, significantly inhibited N-dealkylation of aminopyrine by rat liver microsomes in vitro . In rats exposed by inhalation to 1760 mg MEK/m3 (600 ppm), there were only marginal effects on microsomal cytochrome P-450 activities (Liira et al., 1991). However, a daily dose of 1.4 ml MEK per kg for 3 days increased the amounts of ethanol- and phenobarbital-inducible cytochromes P-450 (P-450 IIEI and P-450 IIB) (Raunio et al., 1990). In a study on male Sprague-Dawley rats, pretreatment with MEK elevated total microsomal cytochrome P-450 and NADPH-dependent cytochrome-c-reductase, the rates of oxidation of N-nitrosodimethylamine, benzphetamine and pentoxyresorufin, and also the levels of immunoreactive protein for both P-450 isozymes (Brady et al., 1989). In a study of hepatotoxicity in rats, Brondeau et al. (1989) found that MEK increased liver cytochrome P-450 content (33-86%) and glutathione-S-transferase (GST) activity (42-64%) but had no effect on serum glutamate dehydrogenase (GLDH) activity. Robertson et al. (1989) studied the effects on hepatic cytochrome P-450 activities of repeated daily doses of 1.87 ml/kg given by gavage to male Fischer-344 rats. The activity of 7-ethoxy coumarin- O-deethylase was increased by up to 500% after 1 to 7 days of MEK treatment, but there was practically no change in benzphetamine- N-demethylase activity. In another study in male Sprague-Dawley rats, administration of MEK by gavage caused an increase in hepatic acetanilide hydroxylase and a marginal increase in aminopyrine- N-demethylase activities (Traiger et al., 1989). 6.3.2 Human studies MEK has been identified as a minor but normal constituent of urine (Tsao & Pfeiffer, 1957), serum and urine of diabetics (Mabuchi, 1969), and expired air (Conkle et al., 1975). Its production in the body has been attributed to isoleucine catabolism (Tsao & Pfeiffer, 1957; Przyrembel et al., 1979). Although Smith (1981) mentioned MEK as a product of autoxidation of cholesterol, no evidence was offered that this process occurred in vivo . The studies of Perbellini et al. (1984) and Liira et al. (1988a,b) indicate that the same metabolites are produced and excreted in humans as in experimental animals. Liira (personal communication by J. Liira, 1989) further indicated that in an inhalation exposure to 590 mg MEK/m3 (200 ppm) the calculated areas under the curves of blood solvent concentration versus time (AUC) for MEK and 2,3-butanediol were equal, which suggests that MEK is almost completely transformed to 2,3-butanediol. The bulk of MEK absorbed thus enters the general metabolism and is transformed to simple compounds like carbon dioxide and water. 6.4 Elimination and excretion MEK and its metabolites are excreted by the lungs and kidneys. Liira et al. (1988a) reported that only 3% of the calculated absorbed dose during a 4-h exposure to 590 mg MEK/m3 (200 ppm) was secreted unchanged in the exhaled air of volunteers after the exposure. The fractional elimination of unchanged substance however depends on the efficiency of metabolic clearance. Since metabolic saturation for MEK in humans begins at relatively low levels (about 100 ppm) of exposure (Liira et al., 1990a), proportionally greater amounts of MEK would be expected to be excreted via the lungs (and kidneys) at high exposure levels. Relatively little of the absorbed MEK is excreted unchanged via the kidneys; a study of occupationally exposed workers revealed that it is less than 0.1% of the alveolar uptake (Miyasaka et al., 1982). In a similar study of workers occupationally exposed to a mixture of solvents, the excretion of MEK and a major recognizable metabolite, 3-hydroxy-2-butanone, was 0.1% of alveolar uptake (Perbellini et al., 1984). The concentrations of both MEK and 3-hydroxy-2-butanone in urine were significantly correlated with the environmental level of MEK. Other metabolites of MEK, 2-butanol or 2,3-butanediol, which DiVincenzo et al. (1976) identified in the serum of guinea-pigs, were not detected in the urine of the exposed workers. Liira et al. (1988a), however, reported that human excretion of 2,3-butanediol was individually variable but averaged 2% of the absorbed MEK. The urinary excretion of 2-butanol, a minor metabolite of MEK, was examined by Kamil et al. (1953), who found that clearance of 2-butanol administered by gavage in rabbits was about 14% of the administered dose and in the form of a glucuronide. Since MEK and 2,3-butanediol disappear from blood and urine and there is no evidence for accumulation elsewhere in the body, the above data suggest that the bulk of MEK absorbed by mammals enters the general metabolism and is eliminated from the body as simple compounds like carbon dioxide and water whose source is not readily identifiable. The specific pathways by which MEK is metabolized have not been identified. There also is no information on loss of MEK in faeces. 6.5 Turnover Both animal and human data indicate a rapid turnover of MEK. In guinea-pigs receiving an intraperitoneal dose of 450 mg MEK per kg, the half-life of MEK in blood serum was 4´ h and the clearance time for MEK in serum was 12 h. For the metabolites 2-butanol, 3-hydroxy-2-butanone and 2,3-butanediol, the clearance time in serum was 11 h (DiVincenzo et al., 1976). In rats given a 2.2-ml/kg oral dose of 2-butanol, the butanol was largely cleared from the blood in 15 h and the 2-MEK derived from the butanol was cleared in 24 h (Traiger & Bruckner, 1976). In a study by Dietz & Traiger (1979) on rats given an oral dose of 2-butanone of 2.1 mg/kg, there was a half-life of 3.6 h for MEK in blood if the rate of loss was assumed to be constant between the two times of measurement (4 h and 18 h) after dosing. Data from a study of Dietz et al. (1981) on rats receiving oral doses of 2-butanol or MEK also indicate a half-life of about 4 h for MEK. These authors reported that the clearance rate for 3-hydroxy-2-butanone and 2,3-butanediol was independent of dose for the two doses used (0.4 and 0.8 g/kg) and that the half-lives for these metabolites of MEK were 47 min and 3.45 h, respectively. Liira et al. (1988a) reported a steady increase in blood concentrations during 4-h exposures of human volunteer subjects to 590 mg/m3 (200 ppm) and observed a rapid elimination of MEK, with half-lives of 30 min during the first post-exposure hour and 81 min thereafter. An inhalation study with two volunteer subjects exposed to MEK for 4 h at concentrations of 74, 590 and 1180 mg/m3 (25, 200 and 400 ppm) indicated that the kinetics of MEK were dose dependent at higher exposure concentrations, i.e. much higher levels of MEK in blood were reached relative to inhaled concentrations, and the post-exposure elimination of MEK in blood was slower (zero-order kinetics). Simulated exposure to MEK for 8 h suggests that saturation kinetics are reached at about 295 mg/m3 (100 ppm) at rest and 148 mg/m3 (50 ppm) during light exercise (Liira et al., 1990a). 6.6 Metabolic interactions Ingestion of ethanol (0.8 g/kg) combined with an inhalation exposure to MEK (590 mg/m3, 200 ppm) inhibited the oxidative metabolism of MEK and led to a marked increase in the blood concentration of MEK (Liira et al., 1990b). There was a concurrent, even more pronounced elevation of the blood 2-butanol concentration; the most likely explanation is competitive inhibition by ethanol of the oxidation of 2-butanol back to MEK. Ethanol also appeared to interact with the further biotransformation of 2,3-butanediol as the urinary excretion of the metabolite was increased. Co-exposure with xylene, however, had no effect on the human metabolism or rate of elimination of MEK (Liira et al., 1988b). 6.7 Mechanisms of action There is very limited information on the mechanisms of toxic action of MEK. Relatively high inhaled concentrations 1475-29 500 mg/m3 (500-10 000 ppm) caused pulmonary vasoconstriction and hypertension in cats and dogs (Zakhari et al., 1977). From the toxicological point of view, interactions leading to the potentiation of effects, particularly neurotoxicity, by other intrinsically toxic substances constitute the main hazard of MEK. The mechanisms underlying these interactions are incompletely known (see section 10). 7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS 7.1 Acute exposure 7.1.1 Lethal doses Data on the acute toxicity of MEK to experimental animals are summarized in Table 11. Oral toxicity is low with LD50 values ranging from 2 to 6 g/kg for adult mice and rats. Intraperitoneal LD50 values are lower (approximately 1.5 g/kg for 24 h and 0.6 g/kg for 14 days). A larger dermal LD50 value for rabbits, i.e. > 8 g/kg (contact time: 24 h; observation time: 14 days), may reflect slower and less complete absorption via the skin, although it may also reflect species differences in sensitivity to MEK. The lethal dose of MEK given to rats in the only study using dosing by pulmonary aspiration (Panson & Winek, 1980) was 0.8 g/kg. This is well below the intraperitoneal 24-h LD50 for adult rats but similar to the 14-day value (Lundberg et al., 1986). It cannot be excluded that the high and rapid lethality of this aspirated dose to adult rats (5/6 deaths in < 24 h, with 4/6 reported as dying "instantly") may reflect serious damage to the lungs. Studies examining the effects of acute inhalation of MEK are not entirely comparable since they used not only different species but also different concentrations of MEK, exposure times, and periods over which survival was measured. For mice the LC50 (45-min exposure) was about 200 000 mg/m3. The lowest concentration lethal to all rats exposed by inhalation for 8 h was 47 200 mg/m3 and the lowest concentration producing lethality in a 4-h exposure was 5900 mg/m3 (2000 ppm). Guinea-pigs survived exposure to 29 500 mg/m3 for 4 to 4.7 h and showed no abnormal signs at 9735 mg/m3. A concentration of 97 350 mg/m3 was lethal to all exposed guinea-pigs in 3.3 to 4.2 h, whereas a slightly lower concentration, 73 750 mg/m3, although ultimately lethal to all animals, permitted some guinea-pigs to survive a 5.4-h exposure (Patty et al., 1935; Specht et al., 1940). 7.1.2 Non-lethal doses Non-lethal acute doses of MEK produced a number of measurable changes in experimental animals (Table 11). An oral dose of 1.5 g/kg to rats resulted in a 63% increase in liver triglycerides after 16 to 23 h, but did not alter liver histology or increase either of two enzymes, serum glutamic-pyruvic transaminase (alanine transferase (ALT)) and hepatic glucose-6-phosphatase (Traiger & Bruckner, 1976). These results suggest that this dose caused metabolic disturbances to the liver of rats. Much smaller acute intraperitoneal doses (0.049 to 0.194 g/kg) to rats appeared not to damage the liver. A single oral dose (15 mmol/kg) of MEK did not affect the hepatobiliary function of rats over an observation period of 10 to 96 h (Hewitt et al., 1986). A graded series of single doses of MEK to guinea-pigs revealed high sensitivity to small changes in dosage. The low dose (0.75 g/kg) appeared to produce no liver damage, whereas 1.5 g/kg produced slight liver damage and 2.0 g/kg produced major liver damage. These results also suggest that guinea-pigs and rats may be equally sensitive to MEK in terms of liver damage even though guinea-pigs survive acute exposure to higher concentrations of MEK vapour than do rats (DiVincenzo & Krasavage, 1974). Consistent increases in the frequency of food-reinforced lever pressing by rats were detected at MEK exposures as low as 74 mg/m3 (25 ppm) for 2 h (Garcia et al., 1978). Vestibulo-ocular effects were detected during continuous intravenous administration of MEK for 1 h via the caudal vein at a dosage as low as 0.005 g/kg per min (Tham et al., 1984). This dosage is roughly equivalent to inhalation of 2700 mg/m3 (900 ppm). Glowa & Dews (1987) reported that exposure of mice to 885 mg/m3 (300 ppm) for 30 min did not significantly alter schedule-controlled responses, whereas concentration-related suppression of response occurred at consecutive increasing concentrations (for 30 min) ranging from 2950 to 29 500 mg/m3 (1000 to 10 000 ppm) (see section 7.3.1). It can be concluded from observations of their behaviour that respiratory irritation occurred in guinea-pigs exposed to 29 500 mg/m3 or more within 2 min (Patty et al., 1935; Specht et al., 1940). In survivors, post-exposure recovery from this effect was rapid. Rats exposed for 8 h/day to 29 500 mg/m3 showed severe irritation of the upper respiratory tract after a "few days" (Altenkirch et al., 1979). 7.1.3 Skin and eye irritation In skin irritation studies, a small dose (8 mg) applied to clipped skin and covered by an impervious plastic film for 24 h (which was followed by a 14-day observation period) produced only minor irritation in male New Zealand albino rabbits (Smyth et al., 1962). A dose of 400 mg applied to the clipped dorsal skin of restrained albino rabbits in a gauze patch produced mild to moderate irritation in some cases (Weil & Scala, 1971). Data from this latter study, however, were highly variable and may reflect the fact that its purpose was intercomparison of laboratories rather than the effects of MEK on test animals. Neat MEK (0.1 ml) applied to the clipped skin of the flanks of guinea-pigs and rabbits daily for 10 days and left uncovered caused erythema and oedema after 24-72 h. These effects were more marked in rabbits (Wahlberg, 1984a). Table 11. Acute toxicity of MEK for mammalsa Species Number and sex Exposure concentration Exposure Study Effects Reference (strain) and range duration duration Oral studies Rat (Sprague- both sexes, 6-12 not stated one dose LD50 (7 days) < 0.8 g/kg b.w. Kimura et al. Dawley) new animals/group 2.5 (2.0-3.1) g/kg b.w. (1971) born 14 day 6 male/group 2.9 (2.3-3.5) g/kg b.w. young adult 2.7 (2.1-3.5) g/kg b.w. older adult Rat (Carworth- 5 female/group not stated one dose 14 days LD50 (14 days) Smyth et al. Wistar) 5.52 (4.50-6.80) g/kgb (1962) Rat (Sprague- 5 male/group 1.505 g/kg b.w. one dose 24 h after 16-23 h, no effect on Traiger & Dawley) liver histology, serum glutamic Bruckner (1976) pyruvic transaminase, or hepatic glucose-6-phosphatase; 63% increase in liver triglycerides Rat (Fischer- 32 male/group 0.072-1.082 g/kg b.w. one dose 24 h no evidence of liver Brown & Hewitt 344) dysfunction; slight damage at (1984) highest dose to kidney proximal tubules Mouse (CF1) 10 male/group in 2.0-5.1 g/kg b.w. one dose 24 h LD50 (24 h) Zakhari et al. each of 6 groups 3.14 ± 0.67 g/kg b.w. (1977) Inhalation studies Mouse (Swiss 10 male/group four levels over the 4 h decreased duration of De Ceaurriz et OF1) range 4726-7192 mg/m3 immobility in escape - al. (1983) (1602-2438 ppm) directed swimming test Table 11 (contd) Species Number and sex Exposure concentration Exposure Study Effects Reference (strain) and range duration duration Mouse (CD1) 12 males per five concentrations over continuous 2´ h increasing dose-related Glowa & Dews concentration the range 885-29 500 (30 min per behavioural effects starting (1987) mg/m3 (300-10 000 ppm) concentration) at 2950 mg/m3 (1000 ppm) Mouse (CF1) 6 groups of 147 500-294 000 mg/m3 45 min LC50 (45 min) 205 000 ± 32 500 Zakhari et al. 10 males (50 000-100 000 ppm) mg/m3 (69 500 ± 11 000 ppm) (1977) Mouse (white) both sexes, 6 303 850 mg/m3 until death average survival 43 min La Belle & animals/group (103 000 ppm) Brieger (1955) Rat (Sprague- 6 animals, sex 74-2360 mg/m3 2-6 h 2-6 h behavioural change (increased Garcia et al. Dawley) unspecified (25-800 ppm) frequency of lever pressing) (1978) which persisted for < 2 to > 11 days Rat (albino) 8 males/group 23 158-59 590 mg/m3 4 h 14 days LC50 (4 h) 34 500 mg/m3 La Belle & (7850-20 200 ppm) (11 700 ppm) Brieger (1955) Rat (Wistar) 5 males/group 23 600 mg/m3 8 h 14 days lethal to 3/6 within 14 days Smyth et al. (8000 ppm) (1962) Rat (Wistar) data not 47 200 mg/m3 8 h 14 days lethal to 6/6 within 14 days Smyth et al. supplied (16 000 ppm) (1962) Guinea-pig 6 unspecified approx. 9735 mg/m3 90-810 min 4-8 days no abnormal signs Patty et al. sex/group; (3300 ppm) (1935) 3 groups Guinea-pig 6 unspecified approx. 29 500 mg/m3 90-810 min 4-8 days 90 min: no injury; 280 min: Patty et al. sex/group; (10 000 ppm) unconsciousness and slight (1935) 3 groups injury, no deaths; 810 min: unconsciousness and moderate injury, no deaths Table 11 (contd) Species Number and sex Exposure concentration Exposure Study Effects Reference (strain) and range duration duration Guinea-pig 10 females 73 750 mg/m3 325 min < 2 days all animals died during or Specht et al. (25 000 ppm) post exposure (1940) Guinea-pig 6 unspecified approx. 97 350 mg/m3 30-250 min 4-8 days 30 min: slight injury; 90 min: Patty et al. sex/group; (33 000 ppm) unconsciousness and moderate (1935) 3 groups injury, no deaths; lethal to all animals in 200-250 min Guinea-pig 6 unspecified approx. 303 850 mg/m3 10-55 min 4-8 days 10 min: slight injury; 30 min: Patty et al. sex/group; (103 000 ppm) unconsciousness and moderate (1935) 3 groups injury, no deaths, lethal to all animals in 45-55 min Intravenous study Rat (Sprague- 18 females several infusion rates 60 min 60 min threshold limit for vestibulo- Tham et al. Dawley) including 5 mg/kg b.w. ocular disturbance (depression (1984) per min of nystagmus following accelerated rotation) 5 mg/kg b.w. per min; MEK concentration in blood, 101 mg/litre Pulmonary aspiration study Rat (Sprague- 3 males and 800 mg/kg b.w. one dose survivors lethal 5/6 in < 24 h; 4/6 died Panson & Winek Dawley) 3 females sacrificed instantly; produced pulmonary (1980) at approx. haemorrhage 24 h Intraperitoneal studies Mouse (CF1) 10 male/groups; 0.5-2.0 g/kg b.w. one dose 24 h LD50 (24 h) Zakhari et al. 5 groups 1.66 ± 0.74 g/kg b.w. (1977) Table 11 (contd) Species Number and sex Exposure concentration Exposure Study Effects Reference (strain) and range duration duration Rat (Sprague- 6 female/group; not stated one dose 24 h LD50 (24 h) 1.554 Lundberg et Dawley) number of groups (1.229-1.966) g/kg b.w. al. (1986) unstated Rat (Sprague- 6 female/group; not stated one dose 14 days LD50 (14 day) 0.607 Lundberg et Dawley) number of groups (0.486-0.748) g/kg b.w. al. (1986) unstated Rat (Sprague- 6 female/group; 0.049-0.194 g/kg b.w. one dose 18 h no significant elevation of Lundberg et Dawley) 3 groups serum sorbital dehydrogenase al. (1986) Guinea-pig 4 male/group 0.75-2.0 g/kg one dose 24 h 0.75 g/kg b.w.: no elevation DiVincenzo & in serum OCTc level, no Krasavage obvious lipid deposition in (1974) hepatocytes, no deaths; 1.5 g/kg b.w.: elevated serum OCT level, lipid deposition in hepatocytes, no deaths; 2.0 g/kg b.w.: elevation in serum OCT level, lipid deposition in hepatocytes, lethal to 1/4 animals Dermal studies Rabbit (Albino, 4 males amounts not stated; 24 h 14 days LD50 (14 days) > 8 g/kg Smyth et al. New Zealand) kept in contact with (1962) clipped flank skin under plastic film Rabbit (Albino, 5 males 8 mg on clipped one dose 24 h minor irritation Smyth et al. New Zealand) uncovered skin (1962) Table 11 (contd) Species Number and sex Exposure concentration Exposure Study Effects Reference (strain) and range duration duration Rat (Albino) 8 males 0.4 mg on 2.5 cm2 24 h 72 h variable results: no irritation Weil & Scala lightly covered pad to moderate irritation 1 day (1971) on clipped back skin and 3 days after applicationd Rabbit (Albino) 6 animals intact 0.5 ml to clipped 24 h 72 h slight erythema subsiding by Hazleton skin, 6 animals abdominal skin with 72 h (1963a) abraded skin, semi-occlusive sex unspecified cover Ocular studies Rabbit (Albino) 6 of unspecified 0.1 ml to one eye of 30 sec 14 days severe irritation, including Hazleton sex each rabbit corneal damage and scleral (1963b) haemorrhage in 2/6 rabbits Rabbit (Albino) 5 male per not stated 3 min solution of 130 g/litre water Larson et al. concentration produced significant oedema in (1955) eyelid in 1 h Rabbit (Albino, not specified 4 mg/eyee 1 min 18-24 h severe chemical burn Smyth et al. New Zealand) (1962) Rabbit (Albino) 6 males 8 mg to one eye of 20 sec 7 days variable results; no irritation Weil & Scala each rabbit to moderate irritation 1 day, 3 (1971) days and 7 days after applicationd Rabbit (Albino, 6 of unspecified 8 mg to one eye of described 7-10 days slight swelling, redness and MB Research New Zealand) sex each rabbit as brief discharge from eye for 4 to Lab. Inc. 10 days (1979) Table 11 (contd) a Values from the literature were recalculated as necessary to yield dosages in terms of ppm, g or g/kg b 95% confidence limits c OCT = ornithine carbamyl transferase d Purpose of study was intercomparison of testing laboratories that evaluated MEK and other substances by purportedly identical methods e Not clear whether one eye or both eyes were dosed The results of studies on eye irritation in rabbits are inconsistent. Smyth et al. (1962) reported that 0.005 ml (4 mg) created a severe chemical burn in the rabbit eye, whereas a study by MB Research Lab. Inc. (1979) reported less severe irritant effects from the larger dose of 0.1 ml (80 mg). Data from a study by Weil & Scala (1971) were also inconsistent but indicated that a dose of 0.1 ml (80 mg) produced minimal to moderate eye irritation. Undiluted MEK was used in all three studies but Smyth et al. (1962) used the grading system for eye injury described in Carpenter & Smyth (1946), whereas MB Research Lab., Inc. (1979) and Weil & Scala (1971) used the Draize scoring system (Draize et al., 1944). In all studies irritation disappeared or was markedly reduced by 7 days after treatment. Opaque corneas were apparent following exposure of guinea-pigs to 295 000 mg MEK/m3 (100 000 ppm) for 30 min; recovery from this effect was complete in 4-8 days (Patty et al., 1935). 7.2 Repeated exposures Data on repeated exposure of mammals to MEK are summarized in Table 12. None of the concentrations tested, not even the highest (17 700 mg/m3 (6000 ppm) 8 h/day for up to 7 weeks) was clearly lethal or even significantly harmful. The death of experimental animals (rats) at this highest dose was not associated with neurological signs and appeared to result exclusively from bronchopneumonia (Altenkirch et al., 1978, 1979). These authors did not comment on possible connections between bronchopneumonia and exposure to 17 700 mg/m3. Female rats exposed to 14 750 mg/m3 (5000 ppm) 6 h/day, 5 days per week, for 90 days showed only slightly increased liver weight, slightly decreased brain and spleen weights, and slightly altered blood chemistry in comparison with controls. Male rats receiving this exposure showed only a slightly increased liver weight. At lower concentrations of MEK (3688 and 7375 mg/m3 (1250 and 2500 ppm)) there was only slightly increased liver weight for female rats and no significant differences for males in comparison with controls (Cavender et al., 1983). In another subchronic inhalation study (Toxigenics, 1981), male and female rats were exposed to MEK concentrations of 3700, 7430 and 14 870 mg/m3 (1254, 2518 and 5041 ppm) 6 h/day, 5 days/week, for 90 days. No significant effects on food consumption, eyes or nervous system were observed. In addition, no MEK-induced morphological changes in the central or peripheral nervous system were detected. Lower levels of exposure resulted in few measurable effects. Inhalation of 2242 and 2360 mg/m3 (760 and 800 ppm) 6 h/day, 5 days/week, for 4 weeks by rats caused some enlargement of the liver and slightly modified the in vitro metabolism of liver microsomes (Nilsen & Toftgard, 1980; Toftgard et al., 1981). Ten intraperitoneal injections of 0.034 g/kg over 2 weeks produced no effect on the kidney (Bernard et al., 1989). However, exposure of rats to 590 mg/m3 (200 ppm) 12 h/day for 24 weeks transiently decreased nerve conduction velocity after 4 weeks (Takeuchi et al., 1983). Geller et al. (1978) reported that exposure of baboons to 295 mg/m3 (100 ppm) for 7 days increased the response time in a delayed "match to sample" task. However, this effect was transient and disappeared during the course of repeated exposure. Following intermittent exposure of rats to 3319 g/m3 for up to 5 months there was no morphological evidence of peripheral neurotoxicity (Saida et al., 1976). It is possible that the transient nature of the neurological and behavioural changes induced by MEK exposure may be due to behavioural and/or physiological adaptations. The latter may reflect more rapid metabolism of MEK with prolonged exposure. Short-term dermal exposure to small amounts of MEK resulted, at most, in mild local irritation. Two topical applications of an unstated amount to the ears of various strains of mice (Swiss, Balb/c, CBA, C5681/6, DBA/2, B6D2F1) produced no significant swelling or other signs (Descotes, 1988). In rabbits and guinea-pigs, a dose of 0.08 g applied to the skin, without covering, once a day for 10 days to the same site resulted in a slight to moderate increase in skinfold thickness, whereas a much smaller dose (8 mg) applied to the same site in rabbits 3 times a day for 3 days resulted in barely detectable irritation (Wahlberg, 1984a). Open application of MEK to the shaved flanks of guinea-pigs for 3 days produced slight erythema, epidermal thickening and dermal cell infiltration (Anderson et al., 1986). In the cat, injection of 150 mg MEK (99.98% pure) per kg, twice a day, 5 days/week, for up to 8.5 months, into subcutaneous tissue produced abscesses, skin ulceration and generalized weakness but no evidence of damage to the nervous system (Spencer & Schaumburg, 1976). In the one long-term dermal study of MEK (Horton et al., 1965), 8 mg dissolved in water was applied by dropper or brush to clipped skin of mice twice a week for a year. Few details were given of the results because this was the control for a study on carcinogenesis of the skin in which a MEK/water solution was the solvent for compounds under test. Horton et al. (1965) stated that the control mice did not develop skin tumours, and they did not mention any adverse effects from this prolonged application of MEK. Table 12. Repeated exposure of mammals to MEKa Route Species (strain), Exposure and range Results Reference number and sex Inhalation rat (Wistar), 590 ± 118b mg/m3 (200 ± 40b transient differences in nerve conduction claimed Takeuchi et al. 8 males ppm) 12 h/day for 24 weeks after 4 weeks, but no significant differences (1983) Inhalation rat (Albino), 693 ± 77b mg/m3 (235 ± 26b no significant gross or microscopic La Belle & 25, sex ppm) 7 h/day, 5 days/week pathological changesc Brieger (1955) unspecified for 12 weeks Inhalation rat (Wistar), 885 mg/m3 (range 867-894) no significant change in leucocyte or serum Li et al. (1986) 7 females (300 ppm (range 294-303)) alkaline phosphatase activity 8 h/day for 7 days Inhalation rat (Sprague- 3322 and 7723 mg/m3 (1026 at 7723 mg/m3 minor effects on dams, decreased Schwetz et al. Dawley), and 2618 ppm) 7 h/day food consumption and weight gain, and increased (1974) pregnant, 25 per for 10 days (gestation water consumption; no effects on dams at 1180 mg/m3 concentration days 6-15) Inhalation rat (Sprague- 2242 and 2360 mg/m3 (760 and significant enlargement of liver; no effect on Nilsen & Dawley), 4 800 ppm), 6 h/day, 5 days total liver microsomal concentration of cytochrome Toftgard (1980); male/concentration per week for 4 weeks P-450; in vitro liver microsomal metabolism of Toftgard et al. biphenyl unaffected, but slight effects on the (1981) metabolism of benzo[a]pyrene, 4-androstene-3,17- dione, and 5alpha-androstane-3alpha,17ß-diol Inhalation rat (Fischer- 3688, 7375, 14 750 mg/m3 females at 14 750 mg/m3 increased in liver weight, Cavender et al. 344), 15 males, (1250, 2500, 5000 ppm) smaller braind and spleene weight, slightly altered (1983) 15 females/ 6 h/day, 5 days/week for blood chemistryf; females at 3688, 7375 mg/m3 concentration 90 days nonsignificant increase in liver weight; males at 14 750 mg/m3 increased in liver weight. No pathological lesions, including peripheral nerves, attributed to MEK exposure. No effect on reproductive tissues (testis, epididymis, seminal vesicle, vagina, cervix, uterus, oviduct, ovary) Table 12 (contd) Route Species (strain), Exposure and range Results Reference number and sex Inhalation rat (Fischer- 3699, 7430, 14 870 mg/m3 elevated group mean body weights at 3699 and 7430 Toxigenics 344), 15 males, (1254, 2518, 5041 ppm) mg/m3; highest concentration: decreased group mean (1981) 15 females/ 6 h/day, 5 days/week for body weight, increased liver weight, liver/body concentration 90 days weight ratio and liver/brain weight ratio, increased mean corpuscular haemoglobin; highest concentration (males): increased kidney/body weight ratio, decreased spleen and brain weights; highest concentration (females): decreased brain/body weight ratio, increased kidney/brain weight ratio; no significant effects at any concentration on food consumption, eyes, nervous system, and no morphological changes in central or peripheral nervous systems Inhalation rat (Sprague- 3319 mg/m3 (1125 ppm) 24 h no morphological effects on peripheral nerves Saida et al. Dawley), 36 of per day for 16 days to 5 (1976) unspecified sex months Inhalation rat (Wistar), 29 500 mg/m3 (10 000 ppm) for severe irritation of upper respiratory tract; slight Altenkirch et 5 males a "few days"; 17 700 mg/m3 loss of weight; death during seventh week from al. (1978, (6000 ppm) ± 15% for 8 h/day, bronchopneumonia; no neurological signs 1979) 7 days/week for 15 weeks Inhalation guinea-pig, 693 ± 77b mg/m3 (235 ± 26b no significant effects La Belle & 15 of unspecified ppm) 7 h/day, 5 days/week for Brieger (1955) sex 12 weeks Inhalation baboon, 295 mg/m3 (100 ppm) for 7 days no impairment of discrimination in a behavioural Geller et al. 4 males test, slight increase in response time early in (1978) experiment, but not at end of experiment Table 12 (contd) Route Species (strain), Exposure and range Results Reference number and sex Intra- rat (Sprague- 0.034 g/kg, 5 days/week for no effect on kidney function Bernard et al. peritoneal Dawley), 2 weeks (1989) female, number unspecified Subcutaneous cat, 6 of 0.15 g/kg twice/day, 5 days abscesses, skin ulceration and generalized weakness Spencer & unspecified sex per week for up to 8.5 months in some animals; no damage to nervous Schaumburg system structure or function (1976) Dermal mouse (Swiss), unstated amount applied twice no significant swelling of treated ear Descotes (1988) 12 of unspecified to one ear sex Dermal mouse 8 mg (50 mg of 17% solution) no papilloma evident after 1 year Horton et al. (C3H/He) applied to clipped skin twice (1965) 10-25 males per week for 1 year Dermal guinea-pig, 3 80 mg rubbed into skin at slight to moderate increase in skinfold thickness Wahlberg (1984a) and rabbit, 4, same site once daily for at end of 10 days of unspecified 10 days sex Dermal guinea-pig 80 mg applied to 1.0 cm2 on no reaction until day 2; at end of experiment, Anderson et al. 10 females shaved flank 3 times daily slight redness and increase in dermal inflammatory (1986) for 3 days cell count a Values from the literature recalculated as ppm, mg or g/kg b Standard deviation of the mean c Depressed growth (average weight 70% of controls at end of experiment) may be due to causes other than MEK exposure; animals showed signs of "vitamin deficiency" (no details given) and infection in latter part of experiment. d Significantly different at the < 0.01 level from 0, 3688 and 7375 mg/m3 (0, 1250, and 2500 ppm) female group values e Significantly different at the < 0.05 level from 0, 3688 and 7375 mg/m3 (0, 1250, and 2500 ppm) female group values f Females in the 14 750 mg/m3 (5000 ppm) group had a small but significant elevation in serum potassium, alkaline phosphatase and glucose levels, and a significant reduction in serum glutamic-pyruvic transaminase level. 7.3 Neurotoxicity 7.3.1 Behavioural testing Neurotoxicity studies have been carried out on MEK, usually as part of studies on the neurotoxicity of methyl isobutyl ketone (MIBK) and MIBK/MEK mixtures. An increase in response rate (lever pressing) was reported in a group of six adult Sprague-Dawley rats (sex unspecified) exposed to MEK at various concentrations between 74 and 2360 mg/m3 (25 and 800 ppm) for 2 h at approximately weekly intervals. An increase in response rate also occurred in a group of four rats exposed to 74 mg/m3 (25 ppm) for 6 h at 2-day intervals (Garcia et al., 1978). Geller et al. (1978) examined behavioural effects (match to sample (MTS) test) in baboons exposed to MEK by inhalation. Four young male baboons (2 years old) were exposed continuously to MEK at a concentration of 295 mg/m3 (100 ppm) for 7 days. There were no effects on performance of the test in terms of the ability to discriminate visual stimuli but reaction time increased. However, in two of the baboons, response times returned to pre-exposure control values by day 7. Tham et al. (1984) examined the vestibulo-oculomotor reflex (VOR) during intravenous infusion of MEK into the caudal veins of 18 female Sprague-Dawley rats. The threshold for depression of the VOR was an infusion rate of 70 µmol/kg (0.005 g/kg per min) for 1 h (total dose 30 mg) and the associated arterial blood level was 1.4 mmol/litre. General depression of the central nervous system followed depression of the VOR. Glowa & Dews (1987) exposed continuously by inhalation a group of 12 adult male white mice (Charles River CD1) to concentrations of MEK that were increased at 30 min intervals until the mice failed to respond to a visual stimulus. The concentrations, in ascending order for each 30 min, were 885, 2950, 8850, 16 520 and 29 500 mg/m3 (300, 1000, 3000, 5600 and 10 000 ppm) with a total exposure time of 2´ h. Mice responded to a visual stimulus and the response rate was used as an indicator. There was no effect at a concentration of 885 mg/m3, a slight decrease in response rate at 2950 mg/m3 and a 75% decrease at 8850 mg/m3. Most mice ceased to respond at 16 520 mg/m3 and all failed to respond at 29 500 mg/m3. The response rate returned to the control value 30 min after exposure ended. The EC50 (concentration decreasing response rate by 50%) was calculated to be 8528 (SD = 2033) mg/m3 (2891 (SD = 689) ppm). An EC10 was calculated and dose-response estimates were derived. The concentrations of MEK producing a 10% decrease in response rate in 0.1%, 1% and 19% of a population were 50, 195 and 885 mg/m3 (17, 66 and 300 ppm), respectively. Couri et al. (1977) exposed continuously by inhalation young male Wistar rats to 2210 mg MEK/m3 (750 ppm) for either 7 or 28 days. In those exposed for 7 days there was a significant reduction in hexobarbital sleep times. In the group exposed for 28 days the reduction in sleep times was less marked. 7.3.2 Histopathology In chickens, cats, rats and mice exposed by inhalation to 3975 mg/m3 (1500 ppm) for periods of up to 12 weeks, there was no evidence of neuropathy and no histopathological changes were reported (Couri et al., 1974). Saida et al. (1976) exposed groups of 36 Sprague-Dawley rats (sex unspecified) continuously to MEK at a concentration of 3319 mg/m3 (1125 ppm) for periods of 16, 25, 35 and 55 days. Additional studies were carried out with up to 5 months of exposure. There were no abnormal clinical observations in any group. At the end of the exposure period, rats were sacrificed and the sciatic nerve and foot muscle excised. Spinal cord and dorsal root ganglion specimens were taken from the same rats. Quantitative histology (neurofilaments/µm2; frequency of inpouching of myelin sheath, denuded axons/mm2) showed no abnormality in rats exposed for up to 5 months. Cavender et al. (1983) reported no neurological abnormalities in Fischer-344 rats in a 90-day inhalation study on MEK alone. Groups of 15 male and 15 female rats were exposed 6 h/day, 5 days/week, for 90 days to MEK concentrations of 3688, 7375 and 14 750 mg/m3 (1150, 2500 and 5000 ppm). All rats were observed twice daily for clinical signs. At the end of the exposure period, the eyes of each animal were examined by ophthalmoscopy, and neurological function (posture, gait, tone and symmetry of facial muscles, and pupillary, palpebral, extensor-thrust and cross-extensor thrust reflexes) was evaluated. No abnormalities were found. Necropsy, including histopathology of the sciatic and tibial nerves, was carried out on all rats, with special neuropathological studies on the medulla, sciatic and tibial nerves in 5 male and 5 female rats from each group. There were no changes attributable to MEK. 7.4 Developmental toxicity Schwetz et al. (1974) exposed 44 pregnant rats from days 6-15 of gestation (sperm = day 0) to two concentrations of MEK vapour, i.e. nominally 2950 mg/m3 in 23 rats and 8850 mg/m3 in 21 rats (1000 and 3000 ppm), for 7 h/day; 43 rats were air-exposed as controls. The average values for measured concentrations in this study were 3322 and 7723 mg/m3 (1126 and 2618 ppm), respectively. There was no evidence of maternal toxicity. Fetal weight and crown-rump length were significantly decreased at 3322 mg/m3, but not at 7723 mg/m3. At 3322 mg/m3 there was also a significant increase, compared to the controls, in the number of litters with fetuses showing skeletal anomalies, and at 7723 mg/m3 a significantly increased incidence of litters with sternebral and soft tissue anomalies was reported. Four grossly malformed fetuses were found (two with brachygnathia and two acaudate with imperforate anus), all in different litters in the 7723 mg/m3 group. These malformations had not been observed previously in more than 400 control litters of this strain. Fetal body dimensions, however, were not significantly different from controls. When the data were analysed on a per litter basis, there was evidence of a teratogenic effect. However, the incidence of major malformations in these studies was sufficiently low that evidence for teratogenic effects was considered questionable, and the studies were repeated (John et al., 1980; Deacon et al., 1981). The methodology was identical except for the inclusion of an additional level of exposure, nominally 1180 mg/m3 (400 ppm). Average measured MEK concentrations during the experiment were 1215, 2956 and 8865 mg/m3 (412, 1002 and 3005 ppm). The only evidence for maternal effects was decreased weight gain and increased water consumption (no figures given) by dams exposed to 8865 mg/m3. None of the dosages had significant effects on the incidence of pregnancy, incidence of resorption, average number of implantations and live fetuses per dam, fetal weight and length, or incidence of external or soft tissue abnormalities. There were statistically significant differences in the incidences of some skeletal anomalies occurring in the 8875 mg/m3 group compared to the controls. There were increased incidences of lumbar ribs and delayed ossification of the cervical centra, but a decreased incidence of delayed ossification of the skull. Since these skeletal abnormalities occurred at low incidences among the population from which the experimental animals were drawn, the results of this study were interpreted as indicating a low level of fetotoxicity and no evidence for embryotoxic or teratogenic effects for MEK at exposure levels up to 8865 mg/m3. In a further study (Mast et al., 1989; Schwetz et al., 1991), groups of 10 virgin female Swiss CD1 mice and 33 plug-positive (day 0) females were exposed by inhalation on gestation days 6-15 to mean concentrations of 1174 ± 27, 2980 ± 83 and 8909 ± 233 mg/m3 (398 ± 9, 1010 ± 28 and 3020 ± 79 ppm). There was no evidence of maternal toxicity, although there was a slight, treatment-related increase in liver/body weight ratios that was significant at the highest dose level. Mild fetal toxicity was evident at this maternal dose level as a reduction in mean fetal body weight, statistically significant for males. There was no increase in the incidence of intrauterine death, but there was an increased dose-related incidence of misaligned sternebrae, statistically significant at the highest dose level. There were no significant increases in the incidence of malformations, although there were several malformations in one litter (cleft palate, fused ribs, missing vertebrae, syndactyly) in treated groups but not in the control group nor in contemporary control data. On the basis of this study it was concluded that the no-observed-adverse-effect level (NOAEL) was 2978 mg/m3 (1010 ppm) and the lowest-observed-adverse-effect level (LOAEL) was 8096 mg/m3 (3020 ppm). 7.5 Mutagenicity and related end-points Short-term genotoxicity tests in vitro and in vivo are summarized in Table 13. Although MEK has given negative results in most conventional assays, Zimmermann et al. (1985) found that MEK and certain other polar aprotic solvents were strong inducers of aneuploidy in the yeast. The induction of aneuploidy by MEK was markedly potentiated by coexposure to ethyl acetate (Mayer & Goin, 1988) or with nocodazol (methyl [5- (2-thienyl-carbonyl)-1 H-benzimidazol-2-yl]-carbamate) (Mayer & Goin, 1987). O'Donoghue et al. (1988) conducted mutagenicity studies on MEK. The test systems comprised the Salmonella/microsome (Ames) assay, the L5178/TK+/- mouse lymphoma (M/L) assay, the BALB/3T3 cell transformation (CT) assay, unscheduled DNA synthesis (UDS) and the micronucleus test. MEK was not found to be genotoxic in these assays. Other studies of MEK utilizing cultures of mammalian cells as test systems also yielded little or no evidence of mutagenicity and related effects. Perocco et al. (1983) tested MEK and other important industrial chemicals at concentrations of 10-2 to 10-4 mol/litre (0.72 to 0.0072 g MEK/litre) with an in vitro system utilizing cultures of human lymphocytes to determine toxicity and ability to inhibit DNA synthesis. Cultures were grown both with and without S9 mix derived from phenobarbital-induced rat liver. MEK at the concentrations tested showed no evidence of cytotoxic or genotoxic action. Chen et al. (1984) examined the effects of MEK on metabolic cooperation between 6-thioguanine-resistant and 6-thioguanine-sensitive Chinese hamster lung fibroblast V79 cells and obtained equivocal results. Holmberg & Malmfors (1974) also found some evidence of MEK cytotoxicity to ascites tumour cells cultured with the solvent for up to 5 h. Although there was no significant increase in irreversibly injured cells at MEK concentrations of 0.05 and 0.1 g/litre, there was a moderate increase in damaged cells at 0.1 g/litre. An ultrastructural study (Veronesi, 1984) utilizing a medium containing relatively high concentrations of MEK (0.3 g/litre) produced axoplasmic granularities in a few cultures. The relationship of this effect to possible MEK-induced neurotoxicity in vivo is not clear. 7.6 Carcinogenicity No long-term carcinogenicity studies have been reported. Table 13. Short-term genotoxicity tests on MEK Method Concentration Experimental conditions; comments Results References In vitro studies Bacterial assays 3 µmol/plate Salmonella typhimurium TA98, TA100, TA1535, - Florin et al. (1980) TA1537 with and without S-9 10 mg/plate S. typhimurium TA98, TA100, TA1535, - Nestmann et al. (1980) TA1537 with and without S-9 approx. S. typhimurium TA104: maximum non-toxic - Marnett et al. (1985) 1 mg/plate dose > 3 µmol 0.05-32 µl/plate S. typhimurium TA98, TA100, TA1535, - O'Donoghue et al. (1988) TA1537, TA1538 with and without S-9 4 mg/plate Escherichia coli WP2 and WP2 uvrA - Brooks et al. (1988) Mitotic gene 5 mg/ml Saccharomyces cerevisiae (JD1) - Brooks et al. (1988) conversion assay Induction of mitotic 3.54% S. cerevisiae (D61.M) + Zimmermann et al. (1985) aneuploidy 0.50-1.96% S. cerevisiae (D61.M) + Mayer & Goin (1987) Chromosome assay 1 mg/ml rat liver RL4 cells - Brooks et al. (1988) Cell transformation 9-18 µl/ml BALB/3T3 - O'Donoghue et al. (1988) UDS test 0.1-5.0 µl/ml primary rat (Sprague-Dawley) hepatocyte - O'Donoghue et al. (1988) Table 13 (contd) Method Concentration Experimental conditions; comments Results References In vivo studies Micronucleus test 1.9 ml/kg CD1 mice (male and female), - O'Donoghue et al. (1988) intraperitoneal time: 12, 24, 48 h 411 mg/kg Chinese hamster, time: 12, 24, 48, 72 h - Basler (1986) intraperitoneal 8. EFFECTS ON HUMANS 8.1 General population exposure The only record of non-occupational acute toxicity from MEK was a case of accidental self-poisoning (Kopelman & Kalfayan, 1983). A 47-year old woman inadvertently ingested an unknown amount of MEK and was found unconscious, hyperventilating, and suffering from severe metabolic acidosis. Her plasma concentration of MEK was 950 mg/litre. She responded promptly to an infusion of sodium hydrogen carbonate and was discharged from the hospital after a week. The metabolic effects of MEK ingestion by humans are not well characterized and it is uncertain that the acidosis was produced by MEK. 8.2 Effects of short-term exposure There are limited data on behavioural and other effects on humans of short-term exposure to MEK. Nakaaki (1974) found that exposure to 266-797 mg/m3 (90-270 ppm) for up to 4 h per day caused his subjects to underestimate times of 5 to 30 seconds. Dick et al. (1984, 1988, 1989), on the other hand, found that a 4-h exposure of human subjects to 590 mg/m3 (200 ppm) had no significant effect in a variety of behavioural tests. These included psychomotor, visual vigilance, dual task, sensorimotor and psychological tests. Solvent mixtures of 295 mg MEK/m3 (100 ppm) and 186 mg toluene/m3 (50 ppm), and of 295 mg MEK/m3 (100 ppm) and 298 mg acetone/m3 (125 ppm) similarly had no significant effect on the results of these behavioural tests. 8.3 Skin irritation and sensitization MEK (0.1 ml) rubbed into volar forearm skin daily for 18 days and left uncovered did not produce persistent erythema or swelling (Wahlberg, 1984a). A 5-min contact with 1.5 ml of analytical grade MEK confined to a 20-mm circle on the forearm produced a temporary whitening of the skin, but no visible erythema, alteration in cutaneous blood flow or other indication of irritation to the skin (Wahlberg, 1984b). A male painter developed dermatitis 18 months after commencing spray painting using an epoxy-polyamide paint (Varigos & Nurse, 1986). A patch test with "a small amount" of MEK applied to areas of skin 3 cm in diameter on each forearm caused these areas of skin to turn bright red within 10 min. The spots were itchy, but there was no induration or oedema. The reaction reached its maximum after 15 min and then gradually faded. The test was repeated after 2 days, and gave the same results. Application of the same grade of MEK to normal forearm skin of five male volunteers produced no reaction. 8.4 Occupational exposure 8.4.1 MEK alone There is no record that MEK toxicity has ever caused death or a large scale industrial accident, and only one acute occupational poisoning has been ascribed to MEK. An 18-year-old seaman with good vision and no previous eye problems was exposed to MEK vapour of unknown concentration while stripping paint, and promptly noted headache, mild vertigo and blurred vision (Berg, 1971). The diagnosis was retrobulbar neuritis. He was given vitamin B complex and steroid therapy, and his vision returned to normal in 36 h. However, blood analysis exhibited a positive methanol content (no value reported) according to the criteria given by Kaye (1961). Thus a potentiation of the effects due to combined exposure to MEK and methanol cannot be excluded. Data for occupational poisoning ascribed to chronic exposure to MEK in the absence of other solvents are equally limited. Long-term exposure of 51 Italian workers to MEK produced indications of neurotoxicity with slightly, but not significantly, reduced nerve conduction velocities and various other symptoms such as headache, loss of appetite and weight, gastrointestinal upset, dizziness, dermatitis and muscular hypotrophy, but no clinically recognizable neuropathy (Freddi et al., 1982). There has been a brief report of chronic exposure of American workers, in a factory producing coated fabric, to 885-1770 mg MEK/m3 (300-600 ppm) in the apparent absence of other solvents (Smith & Mayers, 1944). Workers complained of dermatoses and numbness of fingers and arms. 8.4.2 MEK in solvent mixtures It was reported that MEK was commonly present as part of solvent mixtures containing hexane, and that the TLV for hexane (148 mg/m3, 50 ppm) was exceeded in 89 of the work stations (mainly in shoe factories) (De Rosa et al., 1985). MEK potentiates the toxicity of hexane and these authors concluded that in these shoe factories the risk of neurotoxicity was extremely high. Observations by Tangredi et al. (1981), Brugnone et al. (1981) and Cresci et al. (1985) supported the widespread nature of this health problem in Italian industry, especially shoe factories. Arques Espi & Quintanilla Almagro (1981) also found that mixtures of solvent vapours including MEK and hexane posed an excessive risk in 95 of 114 work stations examined in a study of Spanish shoe factories. In a nationwide survey of Japanese factories, Inoue et al. (1983) found that MEK is widely used as a component of solvent mixtures. In a study of electronic parts plants in the USA (Lee & Parkinson, 1982), workers were found to be exposed to mixtures of solvent vapours containing MEK and as many as nine other components, although not hexane or other solvents whose toxic action MEK is known to potentiate. Observations on Finnish car painters (Husman, 1980; Husman & Karli, 1980) suggested that long-term exposure to complex solvent mixtures whose components individually and jointly are far below the legal concentration limits may produce significant adverse effects. Noma et al. (1988) similarly suggested that complex mixtures of volatile organic compounds, rather than a high concentration of any single compound, may be responsible for unhealthy air in buildings. Descriptions of the effects of occupational exposures to mixtures of solvent vapours not necessarily potentiated by MEK are summarized in Table 14. There are only two cases of acute occupationally related toxicity from such mixtures, and only a limited number of adequately documented cases of adverse effects from chronic occupational exposure which did not involve potentiation of hexacarbon toxicity. The only acute cases involved two young women waterproofing seams of raincoats with resins dissolved in acetone or MEK (Smith & Mayers, 1944). Post-exposure measurements revealed workplace concentrations of 785-1178 mg/m3 (330-495 ppm) for acetone and 1174-1655 mg/m3 (398-561 ppm) for MEK. The total solvent concentration was estimated to be 1000 ppm. Both women fainted at work and subsequently displayed a number of temporary neurological and other symptoms. However, the majority of these studies are difficult to interpret because they lack either a quantitative description of the solvent vapours in the work environment, or the description is based on post-exposure analyses that may not be typical of working conditions during most of the exposure. In these studies it is also impossible to make any certain assessment of the role(s) played by individual components. Mixed solvent exposures have been associated with alteration in nerve conduction velocity (Viader et al., 1975; Dyro, 1978; Triebig et al., 1983), memory and motor alterations (Binaschi et al., 1976), and dermatoses and vomiting (Lee & Murphy, 1982). Fagius & Grönqvist (1978) reported neurological effects in 3 out of 42 workers exposed in a steelworks to organic solvents. However, the actual role of MEK in these effects is not clear. Table 14. Effects of occupational exposure to mixtures of solvent fumes containing MEK Concentrations of MEK and other No. of workers Exposure Effects Defects in study Reference components exposed duration Acute MEK 1174-1655 mg/m3 (389-561 ppm), 2 few hours eyes watering, gastric measurements of work Smith & acetone 785-1178 mg/m3 (330-495 ppm) distress, fainting, environment limited Mayers convulsions, twitching, to post exposure (1944) headache, spinal pressure Chronic MEK 561-885 mg/m3 (190-300 ppm), 13 1 year or fatigue, frequent headache, measurements of work Lee & toluene 139-177 mg/m3 (37-60 ppm), more dizziness, incoordination, environment limited Murphy MIBK 37-41 mg/m3 (9-14 ppm) skin rashes, vomiting to post exposure (1982) MEK 30 mg/m3 (10 ppm), toluene 2 2 and 6.5 paraesthesis in extremities, measurements of work Dyro 94-488 mg/m3 (25-130 ppm) years reduced motor nerve conduction environment limited (1978) velocity, weakness in to post exposure individuals exposed for long periods, improvement following cessation of exposure MEK 62-51 mg/m3 (21-180 ppm), acetone 1 1 year altered consciousness and measurements of work Dyro 86-595 mg/m3 (36-250 ppm) EEG, reduced motor nerve environment limited (1978) conduction velocity to post exposure MEK 142 mg/m3 (48 ppm), isobutanol 8 not stated headache, dizziness, fatigue, measurements of work Binaschi 67 mg/m3 (23 ppm), MIBK 16 mg/m3 (4 ppm), depression, significant environment limited et al. toluene 180 mg/m3 (48 ppm), butyl acetate reduction in short-term to post exposure; (1976) 43 mg/m3 (9 ppm), xylene 347 mg/m3 memory and hand steadiness period of exposure (80 ppm) not indicated Table 14 (contd) Concentrations of MEK and other No. of workers Exposure Effects Defects in study Reference components exposed duration MEK 115 (avg), 2655 (max) mg/m3, n-butanol 9 8-35 significant changes in measurements of work Denkhaus 67 (avg), 1200 (max) mg/m3, isobutanol years several sub-populations of environment limited et al. 172 (avg), 3000 (max) mg/m3; 2-butoxy- peripheral blood lymphocytes: to post exposure (1986) ethanol 25 (avg), 350 (max) mg/m3, decrease in certain type of 2-ethoxyethanol 5 (avg), 53 (max) mg/m3, T-cells and helper cells, 2-methoxyethanol 6 (avg), 150 (max) mg/m3, increase in natural killer toluene 86 (avg), 750 (max) mg/m3, cells and B-lymphocytes m-xylene 19 (avg), 220 (max) mg/m3, MBK 2 (avg), 27 (max) mg/m3 MEK 9-124 mg/m3 (3-42 ppm), 5 > 10 years no significant differences measurements of work Lundberg xylene 0-6111 mg/m3 (0-1408 ppm), in serum activities of liver environment limited & toluene 0-1260 mg/m3 (0-336 ppm), enzymes between exposed to post exposure Hakansson isobutanol 0-1045 mg/m3 (0-345 ppm), workers and controls (1985) n-butanol 0-1548 mg/m3 (0-511 ppm), ethanol 0-1094 mg/m3 (0-582 ppm), ethyl acetate 0-2095 mg/m3 (0-582 ppm), n-butyl acetate 0-1691 mg/m3 (0-356 ppm), methyl acetate 3-181 mg/m3 (1-60 ppm), white spirit 2-30 mg/m3 (1-17 ppm), methylene chloride 10-2460 mg/m3 (3-707 ppm), isopropanol 5-260 mg/m3 (2-106 ppm), exposure to 2-8 solvents plus MEK MEK < 9-401 mg/m3 (< 3-136 ppm), 66 average significant reduction in measurements of work Triebig xylene < 4-82 mg/m3 (< 1-19 ppm), 5 years sensory conduction in environment limited et al. toluene 11-551 mg/m3 (3-147 ppm), comparison with controls, no to post exposure (1983) ethyl acetate < 11-302 mg/m3 (< 3-84 ppm) suggestion of neuropathy, trichloroethane 11-601 mg/m3 (2-110 ppm) change in conduction velocity correlated with exposure Table 14 (contd) Concentrations of MEK and other No. of workers Exposure Effects Defects in study Reference components exposed duration MEK 15-620 mg/m3 (5-210 ppm), acetone 17 not stated no health problems associated measurements of work Cohen & < 119-495 mg/m3 (< 50-208 ppm), xylene with exposure to MEK environment limited Maier < 22 mg/m3 (< 5 ppm), toluene < 19-34 to post exposure (1974) mg/m3 (< 5-9 ppm), petroleum naphtha (concentration unknown) MEK mainly < 443, max. 5115 mg/m3 42 0.5-8 1 likely and 2 suspected measurements of work Fagius & (mainly < 150, max. 1734 ppm), trichloroethylene years cases of polyneuropathy, environment limited Gronqvist mainly < 161 mg/m3 (< 30 ppm); slight loss of sensitivity to post exposure (1978) at low levels butanol, butyl acetate, butyl to vibration correlated diglycol, cyclohexanol, diacetal, ethyl with level of solvent ex- glycol acetate, ethanol, isoforone, posure during preceding methylene chloride, MIBK, toluene, 6 months xylene, "solvesso 100 and 150"a MEK 0-74 mg/m3 (avg, 3 mg/m3) (0-25 1006b average overall death rate below none Wen et al. (avg. 1) ppm), toluene 4 mg/m3 (1 ppm); 21.6 years expected level, excess of (1985) at very low levels benzene, "hexane", deaths from cancer associ- MIBK, xylene ated with lubricating oil, not MEK a hydrocarbon solvent mixtures b workers from lube oil and dewaxing plant In a study on a group of 9 parquet-flooring workers (age, 25-58 years; exposure time, 8-35 years), Denkhaus et al. (1986) noted significant changes in several subpopulations of peripheral blood lymphocytes, which could constitute an early indication of a haematological or immunological effect. Benzene was not detected in the ambient air and no investigation was made to determine which components of the solvent mixtures produced the observed changes in lymphocyte populations. Anshelm Olson et al. (1981) studied the simple reaction time (SRT) performance in a group of 42 workers (age, 18 to 52 years; employment, 0.5 to 8.1 years) from a plastic coating line of a steel factory (the same group had previously been studied by Fagius & Grönqvist, 1978). The study was longitudinal and covered a period of 27 months during which SRT was measured three times. Originally, the workers had been exposed to significant concentrations of MEK (up to 4000-5000 mg/m3 in certain regular tasks) and to much lower levels of other solvents. Five months after the completion of major improvements in the work environment which reduced the levels of MEK to about 20 mg/m3 (maximum of about 400 mg/m3), a second SRT measurement was made and a third measurement was performed 15 months later. The workers' performance on the SRT test improved over the three measurements. Moreover, on the first occasion SRT was correlated to the degree of exposure. The authors concluded that the workers' central nervous functioning had been adversely affected by solvent exposure. Mutti et al. (1982a) carried out a study of exposure to organic solvents in an Italian shoe factory. The exposed group consisted of 95 workers (24 males, 71 females) with an age range of 16-62 years (mean, 30.9 ± 11.7 years), and the exposure duration ranged from 1 to 25 years (mean, 9.1 ± 8 years). The approximate mean air concentrations in the breathing zone, over a 2-year period, for a number of solvents were: MEK, 115 mg/m3 (39 ppm); n-hexane, 317 mg/m3 (90 ppm); cyclohexane, 315 mg/m3 (92 ppm); and ethyl acetate, 205 mg/m3 (57 ppm). The exposed workers complained of sleepiness, dizziness, weakness, paraesthesia and hypo-aesthesia. Other neurological symptoms, such as headache, muscular cramps, neurasthenic syndrome and sleep disturbances, were found more often in exposed workers, but the differences in incidence between the exposed and reference group were not statistically significant. Among exposed workers the mean motor nerve conduction velocity was significantly reduced in the median and peroneal nerves but not in the ulnar nerve. The amplitude of the motor action potential (MAP) was significantly reduced in all nerves and its duration was increased in the ulnar nerve. There were no significant effects on the distal latency. The number of abnormal action potentials observed in the median and peroneal nerves of exposed workers was significantly increased. There was a correlation between the reduction in motor conduction velocity and exposure. In a follow-up study, electrophysiological measurements including somatosensory evoked potentials (SEPS) were recorded from a group of 15 female shoe factory workers aged 19-53 years (mean age, 26.6 ± 11.4 years) with a solvent exposure duration of 2-8 years (mean, 4.5 ± 2.3 years) (Mutti et al., 1982b). The mean air concentrations for various solvents in the breathing zone of the workers were: MEK, 177 mg/m3 (60 ppm); n-hexane, 690 mg/m3 (196 ppm); cyclohexane, 585 mg/m3 (170 ppm); and ethyl acetate, 360 mg/m3 (100 ppm). Electrophysiological measurements in peripheral nerves showed significant reductions in maximal motor and distal sensory nerve conduction velocities in the median and ulnar nerves and reduced maximal motor nerve conduction velocity in the peroneal nerve. The latency of the sensory peak action potential was significantly increased in the median and ulnar nerves. The amplitude of all peripheral nerve action potentials was slightly reduced but this was not statistically significant. There were also changes in the SEPs with significant increase in the latency of some early component peaks. The amplitude of some of the early peaks was significantly reduced. The neurotoxicity was attributed primarily to n-hexane. 8.5 Carcinogenicity In a historical prospective mortality study of 446 male workers in two MEK dewaxing plants, with an average follow up of 13.9 years, the observed deaths (46) were below the expected (55.51). There was a slight deficiency of deaths from neoplasms (13 observed; 14.26 expected) but there was a significant increase of deaths from tumours of the buccal cavity and pharynx (2 observed; 0.13 expected). However, there were significantly fewer deaths from lung cancer (1 observed; 6.02 expected). In view of the small numbers, it was concluded that there was no clear evidence of cancer hazard in these workers (Alderson & Rattan, 1980). 9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD 9.1 Microorganisms The effects of MEK on microorganisms have been studied in several species important to freshwater aquatic systems. As shown in Table 15, growth inhibition generally occurs at levels ranging from 120 mg/litre for the cyanobacterium (blue-green alga) Microcystis aeruginosa to 4300 mg/litre for the green alga Scenedesmus quadricauda. A number of bacterial species have also been tested, the effect levels ranging from 10 to 5050 mg/litre. Kulshrestha & Marth (1974a-f) conducted a series of studies to determine if MEK and other volatile compounds associated with the flavour of raw or mildly heated milk are able to inhibit the growth of certain pathogenic bacteria and other bacteria important in the manufacture of fermented dairy products. Nutrient broth laced with the organisms and MEK at levels of 1, 10, 100 and 1000 mg/litre was plated after 5, 8, 11 and 14 h of incubation in an air-tight vessel. Results are shown in Table 15. MEK was considered bacteriostatic to Escherichia coli, Salmonella typhimurium, Staphylococcus aureus, Leuconostoc citrovorum and Streptococcus thermophilus at levels as low as 10-100 mg/litre. However, Walton et al. (1989) reported that MEK at concentrations up to 1 g/kg dry weight of soil had little effect on the respiration of microorganisms in moist soil. Volskay & Grady (1988) found that MEK at 1 g/litre depressed respiration of sewage sludge organisms by only 11%. Ingram (1977) noticed changes in the fatty acid and phospholipid composition of the cell membranes of E. coli when cultured with a sublethal (0.218 mol/litre) concentration of MEK. Tests in fungi demonstrated that MEK has a very slight stimulating effect on the germination of uredospores in two species of rust (French, 1961; French et al., 1977). Growth of a mixture of other fungal species was inhibited by about 50% by 6.4 mg MEK/g seed when the fungi were cultured on moist wheat seed (Nandi & Fries, 1976). Growth of a mixture of other fungal species on moist wheat seed at 30 °C was inhibited by 50% following exposure for 5 days to approximately 6.4 mg MEK/g seed (Nandi & Fries, 1976). Germination of the wheat seeds was also reduced but there was no statistical evaluation of these data. Table 15. Effects of methyl ethyl ketone on microorganisms Organism Species, strain Concentration Effect Reference (mg/litre) Prokaryotes Cyanobacterium (Blue-green alga Microcystis aeruginosa 120 inhibition of cell multiplication Bringmann & Kuhn (1978) Bacteria Pseudomonas putida 1150 inhibition of cell multiplication Bringmann & Kuhn (1977a) Photobacterium phosphoreum 5050 50% reduction in light output Curtis et al. (1982) Escherichia coli, ML30 10 slight but not consistently Kulshrestha & Marth (1974a) significant inhibition of growth Escherichia coli, ML30 100 significant inhibition after 5 h Kulshrestha & Marth (1974a) incubation Escherichia coli, ML30 1000 10% reduction in growth Kulshrestha & Marth (1974a) Escherichia coli, B15 72.1 65% growth inhibition after 1 h; Egyud (1967) 30% after 2 h Salmonella typhimurium 1 & 10 slight but not consistently Kulshrestha & Marth (1974b) significant inhibition of growth Salmonella typhimurium 100 some inhibition of growth Kulshrestha & Marth (1974b) Salmonella typhimurium 1000 significant inhibition of growth Kulshrestha & Marth (1974b) Staphylococcus aureus, 100 1 no significant inhibition of growth Kulshrestha & Marth (1974c) Staphylococcus aureus, 100 10 some inhibition of growth Kulshrestha & Marth (1974c) Staphylococcus aureus, 100 100 & 1000 significant inhibition of growth Kulshrestha & Marth (1974c) Table 15 (contd) Organism Species, strain Concentration Effect Reference (mg/litre) Streptococcus lactis 1 no significant inhibition of growth Kulshrestha & Marth (1974d) Streptococcus lactis 10 significant inhibition at early Kulshrestha & Marth (1974d) stages of incubation Streptococcus lactis 100 & 1000 significant inhibition of growth Kulshrestha & Marth (1974d) Leuconostoc citrovorum 1 no significant inhibition of growth Kulshrestha & Marth (1974e) Leuconostoc citrovorum 10 some inhibition of growth Kulshrestha & Marth (1974e) Leuconostoc citrovorum 100 & 1000 significant inhibition of growth Kulshrestha & Marth (1974e) Streptococcus thermophilus, 1 no significant inhibition of growth Kulshrestha & Marth (1974f) ST4 Streptococcus thermophilus, 10 growth inhibition at later stages of Kulshrestha & Marth (1974f) ST4 incubation Streptococcus thermophilus, 100 & 1000 significant inhibition of growth Kulshrestha & Marth (1974f) ST4 Eukaryotes Fungi Puccinia graminis, var. tritica, not given slight stimulation to germination French (1961) spores (wheat stem rust) Uromyces phaseoli, (Reben), 1000 very slight stimulation to French et al. (1977) Wint., spores (bean rust) germination Puccinia helianthi, Schw. chr., 50 very slight stimulation to French (1984) spores (sunflower rust) germination Table 15 (contd) Organism Species, strain Concentration Effect Reference (mg/litre) Uromyces vignae, Barcl., 500 no effect on germination French (1984) spores (cowpea rust) various fungi found on wheat 6.4a slight inhibition of fungal growth Nandi & Fries (1976) seeds: Septoria nerdorum, Fusarium nivale, Aspergillus glaucus, Aspergillus candidus, Penicillium sp., Alternaria, sp. Green algae Chlorella sp. 806 no effect on chlorophyll content Dojlido (1979) after 7 days exposure Scenedesmus quadricauda 4300 slightly inhibited cell Bringmann & Kuhn (1978) multiplication Protozoa Entosiphon sulcatum 190 slightly inhibited cell Bringmann (1978) multiplication a mg/g seed. 9.2 Aquatic organisms MEK has been tested on numerous species of freshwater and marine vertebrates and invertebrates in short-term tests. The results indicate that MEK is generally of low toxicity to aquatic animals, with median lethal (LD50) levels ranging from 1382 to 8890 mg/litre (Table 16). Almost all of the acute tests were conducted under static conditions, in open vessels, with nominal measurements of MEK concentration, all of which may underestimate the true toxicity level of MEK. No chronic studies at low concentrations have been conducted. Curtis et al. (1982) related the effect of MEK on the bioluminescence of a bacterium to the 96-h LC50 in fathead minnows in an attempt to find an inexpensive yet accurate substitute for lethality tests. The relationship for ketones (r2 = 0.81, n = 7) is described by the equation: log LC50 = 0.74 (log EC50 + 0.79) where the EC50 (5 min) is the concentration causing a 50% reduction in light output. This relationship predicts a 96-h LC50 for fathead minnows of 3388 mg/litre, which agrees well with the measured value (Veith et al., 1983) of 3200 mg/litre. Similarly, LeBlanc (1984) found a highly significant correlation (P < 0.01) between the LC50 values reported for warm-water and cold-water fish, saltwater and freshwater fish, marine and freshwater invertebrates, and marine and freshwater algae. There was a remarkable similarity among acute toxicity values across three trophic levels. Data in Tables 15 and 16 lend further support to this conclusion. 9.3 Terrestrial organisms 9.3.1 Animals The effect of MEK on the alarm behaviour of social insects has been studied. It was considered inactive in producing alarm behaviour in the ant Iridomyrmex preinosus (Blum et al., 1966), the harvester ant Pogonomyrmex badius (Blum et al., 1971) and the honeybee Apis Mellifera (Boch & Shearer, 1971). Alarm behaviour was measured by the number of insects that were attracted to the chemical and was indexed relative to the activity of natural pheromones, including 2-heptanone. Table 16. Effects of methyl ethyl ketone on aquatic organisms Species Concentration Effects and comments pH Temperature Hardness Reference (mg/litre) (°C) Crustacea Artemia salina 1950 24-h LC50, bottles not not given not given not given Price et al. (1974) (brine shrimp) sealed and may have lost MEK during experiment Daphnia magna (water flea) < 180 no discernible effect 7.21 21.0-23.0 38 mg/litre Union Carbide Corp. CaCO3 (1980a) Daphnia magna (water flea) < 520 24-h LC50 8 ± 0.2 not given 173 ± 13 mg/l Le Blanc (1980) < 520 48-h LC50 < 70 no discernible effect Daphnia magna (water flea) 1382 48-h LC50 7.21 21.0-23.0 38 mg/litre Union Carbide Corp. (918-3349)a CaCO3 (1980a) Daphnia magna (water flea) 2500 24-h LC0 7.6-7.7 20-22 16 ° (German) Bringmann & Kuhn (1977b) Daphnia magna (water flea) 8890 24-h LC50 7.6-7.7 20-22 16 ° (German) Bringmann & Kuhn (1977b) Daphnia magna (water flea) 10 000 24-h LC100 7.6-7.7 20-22 16 ° (German) Bringmann & Kuhn (1977b) Fish Leucissus idus melanotus 4400b LC0, period not mentioned not given not given not given Juhnke & Luedemann (golden orfe) 4800b (1978) Leucissus idus melanotus 4600b LC50 not given not given not given Juhnke & Luedemann (golden orfe) 4880b (1978) Table 16 (contd) Species Concentration Effects and comments pH Temperature Hardness Reference (mg/litre) (°C) Leucissus idus melanotus 4800b LC100 not given not given not given Juhnke & Luedemann (golden orfe) 5040b (1978) Lebistes reticulatus 2000 disturbed behaviour not given 20 ± 1 27.5 mg/litre Dojlido (1979) (guppy) CaCl2 Lebistes reticulatus 5700 24-h LC50, open not given 20 ± 1 27.5 mg/litre Dojlido (1979) (guppy) containers may have lost CaCl2 MEK during test Pimephales promelas 3200 96-h LC50 7.5 25 ± 1 42.2 mg/litre Veith et al. (1983) (fathead minnow) CaCO3 Lepomis macrochirus < 1000 no discernible effect 7.93 21 240 mg/litre Union Carbide Corp. (bluegill sunfish) CaCO3 (1980b) Lepomis macrochirus 4467 96-h LC50 7.93 21 240 mg/litre Union Carbide Corp. (bluegill sunfish) CaCO3 (1980b) Gambusia affinis 5600 96-h LC50 7.8-8.3 room not given Wallen et al. (1957) (mosquito fish) temperaturec Cyprinodon variegatus 400 no discernible effect not given 25-31 not given Heitmuller et al. (sheepshead minnow) (1981) Carassius auratus > 5000 24-h LC50 7.0 20 ± 1 100 mg/litre Bridie et al. (1979b) (goldfish) a 95% confidence limits b Values are from different laboratories c No specific value given MEK was found to be moderately effective as a fumigant against the Caribbean fruit fly, Anastrepha suspensa (Davis et al., 1977). Treatments of 790 mg/m3 (286 ppm) for 2 h or 316 mg/m3 (107 ppm) for 7 h destroyed 100% of the larvae in naturally infected guavas, whereas exposure to 221 mg/m3 (75 ppm) for 3 h destroyed 92% of the larvae. Kwan & Gatehouse (1978) applied between 0.31 and 0.37 mg MEK topically to the dorsal thorax of tsetse flies (Glossina morsitans morsitans) weighing 17-19 mg. One day after treatment, the MEK had a significant effect on the activity of males but not females. The mortality in MEK-treated insects was marginally but consistently higher than in the untreated controls. No apparent inhibitory or other effect of MEK was noted with respect to feeding or mating. Vale et al. (1988) found MEK to be a very effective attractant for tsetse flies and used it in a successful control effort in which the flies were attracted to insecticide-coated netting. Uspenskii & Repkina (1974) reported that an unspecified dose of MEK caused an increase in the physiological age of the tick Ixodes perculcatus, an effect that increased the insect's sensitivity to DDT. 9.3.2 Plants MEK has an effect on the germination of seeds of several plant species. Nandi & Fries (1976) observed that the germination of wheat seeds was inhibited when the seeds were treated with 6.4 mg MEK/g seed. At this level, 10% of the experimental seeds germinated versus 60% of the control seeds. Germination of lettuce was inhibited 50% by MEK at 12.5 (± 4) mmol/litre (equivalent to 900 ± 288 mg/litre) dissolved in agar (Reynolds, 1977). Schulz et al. (1981) reported, however, that a mixture of acetone and MEK had no inhibitory effect on the growth of rye grass at concentrations up to 1 g/litre. 10. ENHANCEMENT OF THE TOXICITY OF OTHER SOLVENTS BY MEK The principal toxic effects noted with MEK exposure stem from its ability to potentiate the known toxicities of other solvents (Table 17). Two such interactions are described in detail below. 10.1 Hexacarbon neuropathy 10.1.1 Introduction MEK interacts with hexacarbon compounds and potentiates their neurotoxicity (WHO, 1991). Potentially MEK co-exposure could affect the metabolism of the hexacarbon compounds or the toxic process by which the hexacarbons induce the neuropathy. The critical metabolic pathway for hexacarbon induced neuropathy is outlined in Fig. 2, and the scheme by which the peripheral nerve axonal degradation is thought to occur is given in Fig. 3. The metabolic pathway involves hepatic microsomal oxidation to 2,5-hexanedione, the proximate neurotoxicant, which is thought to induce cross-linking of neurofilaments, blockage of transport at the node of Ranvier, and swelling from accumulated neurofilaments proximal to the nodes and axonal degeneration distal to the nodes. 10.1.2 Animal studies The phenomenon of potentiation of hexacarbon neurotoxicity by MEK has been firmly established by in vivo studies mainly on rats (Table 17) and also has been demonstrated in tissue culture (Veronesi et al., 1984). In every study in which the dose of n-hexane, methyl butyl ketone (MBK), or 2,5-hexanedione (2,5-HD) was large enough and sustained for a sufficient period, clinical signs of neural degeneration were produced. These signs were made more severe by co-exposure to MEK. In addition, the period prior to the onset of symptoms was frequently shortened by co-exposure to MEK. Minimum sustained continuous exposure concentrations that induced neuropathy in these experiments were 295/1408 and 590/1056 mg/m3 (100/400 and 200/300 ppm) MEK/ n-hexane mixtures. An intermittent exposure (8 h/day) of rats to 2950/31 680 mg/m3 (1000/9000 ppm) MEK/ n-hexane mixture produced severe neuropathy, whereas similar exposure to a lower concentration, i.e. 590/1760 mg/m3 (200/500 ppm) MEK/ n-hexane mixture yielded no evidence of hexacarbon neurotoxicity. Intermittent exposure (8 h/day, 5 days/week) to 5900/820 mg/m3 (2000/200 ppm) MEK/MBK mixture produced some neural degeneration and mild clinical signs. Oral dosing of rats once a day, 5 days/week, with a mixture of MEK/2,5-HD (0.159/0.253 g/kg) produced marked clinical signs (Ralston et al., 1985). Even with doses of MBK too low to produce significant neuropathy, studies generally indicated that co-exposure with MEK induced changes compatible with enhanced toxicity, such as reduced velocity of nerve conduction, elevated hepatic microsomal enzyme activity, or reduced clearance of 2,5-HD from the blood or the body. The only study reporting no evidence of potentiation (Spencer & Schaumburg, 1976) compared MBK at 0.150 g/kg with one tenth this concentration of MBK, 0.015 g/kg, in combination with MEK. Since there were no control data on the effects of 0.015 g/kg of MBK alone and the dose of MEK was very low, it is difficult to interpret the meaning of this study. Concentrations of MEK in inhalation studies did not exceed 3319 mg/m3 (1125 ppm), and continuous exposure to this concentration was demonstrated by Saida et al. (1976) not to produce neuropathological effects. Thus it is considered unlikely that MEK itself produces neuropathy. The property of potentiation of hexacarbon neurotoxicity is not unique to MEK, but is shared at least by methyl n-propyl ketone, methyl n-amyl ketone and methyl n-hexyl ketone, none of which appear intrinsically neurotoxic (Misumi & Nagano, 1985). The mechanism by which MEK potentiates hexacarbon neurotoxicity is not well understood, although potential metabolic interactions have been examined. In rats, simultaneous inhalation of n-hexane and MEK resulted in lower levels of 2,5-HD in vivo in urine (Iwata et al., 1984; Shibata et al., 1990a) and initially lower but later somewhat elevated levels of MBK and 2,5-HD in vivo in serum (Shibata et al., 1990b). Both Iwata et al. (1984) and Shibata et al. (1990b) concluded that potentiation of n-hexane neurotoxicity by MEK could not be explained solely by increased 2,5-HD formation. Pretreatment of rats with MEK (1.87 ml/kg, 4 daily doses) prior to inhalation of n-hexane resulted in higher levels of 2,5-HD in several tissues, including blood (Robertson et al. (1989). Abdel Rahman et al. (1976) reported that an 8-h simultaneous exposure of rats to MEK and MBK did not result in measurable levels of MBK or of 2,5-HD in blood, whereas a 6-day continuous co-exposure resulted in substantially raised levels of MBK and 2,5-HD. Continuous co-exposure for 23 days, however, resulted in further elevation of MBK in blood, but no measurable level of 2,5-HD. Simultaneous administration of 2,5-HD and MEK, either as a single dose or as six repeated daily doses in rats, resulted in a greater total area under the curve for 2,5-HD in blood compared with administration of 2,5-HD alone (Ralston et al., 1985). In guinea-pigs, increased levels of 2-hexanol and 2,5-HD were found following intraperitoneal administration of a MEK/MBK mixture compared with intraperitoneal administration of MBK alone (Couri et al., 1978). MEK administration has also been shown to induce a number of in vivo and in vitro parameters of hepatic oxidative metabolism (Couri et al., 1977; Wagner et al., 1983; Misume & Nagano, 1985; Raunio et al., 1990). Table 17. Interaction of MEK with other solvents and their metabolitesa Route of administration, Dose and/or Exposure Effects/results Reference species (strain), concentration number and sex n-Hexane Inhalation, rat, 820 mg/m3 (200 ppm) MBK; 8 h/day, muscular weakness lasted longer after exposure to Duckett et al. 9 sex unspecified 5900/820 mg/m3 5 days/week, MEK/MBK than after MBK alone (1974) MBK, 8 sex unspecified (2000/200 ppm) MEK/MBK 6 weeks MEK/MBK Inhalation, rat 1760 mg/m3 (500 ppm) 8 h/day, no significant enhancement of neuropathological Iwata et al. (Wistar), 6 males hexane; 1475/1760 mg/m3 7 days/week, damage evident in peripheral nerve (1984) per groupb (500/500 ppm) MEK/hexane 33 weeks electrophysiological function; 2,5-hexanedione and other hexane metabolites in urine reduced with co-exposure Inhalation, rat 923 mg/m3 (225 ppm) MBK; 24 h/day, MEK enhanced MBK-induced peripheral neurotoxicity Saida et al. (Sprague-Dawley), 3319/923 mg/m3 (1125/225 16-66 days (1976) 12/group, sex ppm) MEK/MBK unspecified Inhalation, rat 923 mg/m3 (225 ppm) MBK; 24 h/day, hexobarbital sleeping time significantly reduced in Couri et al. (Wistar), 5 males 2213/923 mg/m3 (750/225 7 or 28 days group exposed continuously for 7 days to MEK/MBK (1977) per group ppm) MEK/MBK (and in 2213 mg MEK/m3 (750 ppm) controls), but not in MBK group; interpreted as evidence for elevated microsomal enzyme activity in MEK/MBK and MEK groups Table 17 (contd) Route Route of administration, Dose and/or Exposure Effects/results Reference species (strain), concentration number and sex Inhalation, rat 1760, 2464 mg/m3 (500, 700 approx. experiments yielded similar results: potentiation of Altenkirch et (Wistar), 5 males ppm) hexane; 295/1408, 24 h/day, n-hexane neurotoxicity in MEK/hexane groups as al. (1982a) per group 590/1056, 590/1760 mg/m3 7 days/week, shown by shortened period before onset of clinical (100/400, 200/300, 200/500 9 weeks signs (weakness and paresis); hypersalination ppm) MEK/hexane in MEK/hexane groups only; neural degeneration in all solvent-treated groups (no mention of differences among these groups) Inhalation, rat 1760, 2464 mg/m3 (500, 700 8 h/day, every co-exposure to MEK/hexane resulted in earlier and Schnoy et al. (Wistar), 5 groups ppm) hexane; 295/1408, day for 1-89 more pronounced degeneration of pulmonary (1982); of 2-5 malesc 590/1056, 590/1760 mg/m3 days nerves than exposure to hexane alone; there were no Schmidt et al. (100/400, 200/300, 200/500 consistent differences in degeneration of alveolar (1984) ppm) MEK/hexane epithelium between the hexane and MEK/hexane groups Inhalation, rat 1640, 923 mg/m3 (400, 225 24 h/day, 2,5-HD detected in blood after 6 days but not after Abdel-Rahman (Wistar), unspecified ppm) MBK; 2213/923 mg/m3 6 or 23 days 23 days, and there were elevated MBK blood levels et al. (1976) number/group, male (750/225 ppm) MEK/MBK in group exposed to MEK/MBK; severe neuropathy in group exposed to MEK/MBK for 23 days Inhalation, rat 2464 mg/m3 (700 ppm) 8 h/day, no evidence of potentiation of hexane neurotoxicity: Altenkirch et (Wistar), 5 males hexane; 590/1760 mg/m3 7 days/week, no abnormal clinical signs or elevated al. (1982a) per group (200/500 ppm) MEK/hexane 40 weeks neuropathology in solvent-treated groups Inhalation, rat 3520 mg/m3 (1000 ppm) 8 hd 2,5-hexanedione in urine markedly less after co- Iwata et al. (Wistar), 5 males hexane; 2950/3520 mg/m3 exposure with MEK than after exposure to hexane (1983) per group (1000/1000 ppm) MEK/hexane alone Table 17 (contd) Route Route of administration, Dose and/or Exposure Effects/results Reference species (strain), concentration number and sex Inhalation, rat Experiment 1: 35 200 mg/m3 8 h/day, experiments yielded similar results; acceleration Altenkirch et (Wistar), 5 males (10 000 ppm) hexane; 3245/ 7 days/week, of n-hexane neurotoxicity in MEK/hexane group as al. (1978, per group 31 330 mg/m3 (1100/8900 15-19 weeks shown by more severe clinical signs (gait 1979) ppm) MEK/hexane disturbances, weakness and paresis), greater degeneration of peripheral nerves and shortened 5 males/group Experiment 2: 35 200 mg/m3 period before onset of signs and neural degeneration; (10 000 ppm) hexane; 2950/ hyper-salivation in MEK/hexane groups 31 680 mg/m3 (1000/9000 ppm) MEK/hexane 12 males/group Experiment 3: 35 200 mg/m3 (10 000 ppm) hexane; 2950/ 31 680 mg/m3 (1000/9000 ppm) MEK/hexane Inhalation, rat 35 200 mg/m3 (10 000 ppm) 4 h or 8 h, MEK did not enhance degeneration of intrapulmonary Schnoy et al. (Wistar), 7 groups hexane; 2950/31 680 mg/m3 8 h/day for nerves; enhanced degeneration of alveolar epithelium (1982); of 2-3 males (1000/9000 ppm) 2-14 days in the MEK/hexane group after 14 days exposure in Schmidt et al. MEK/hexanee comparison with the hexane group (1984) Inhalation, rat 352 mg/m3 (100 ppm) hexane; 12 h/day, marked impairment of nerve conduction velocities in Takeuchi et (Wistar), 8 males 590/352 mg/m3 (200/100 7 days/week, MEK/hexane group; no evidence of significant al. (1983) per group ppm) MEK/hexane 24 weeks potentiation of morphological effects of n-hexane Inhalation, mouse 615 mg/m3 (150 ppm) MBK; 24 h/day, hexobarbital sleeping time significantly lower in Couri et al. (Swiss), 5/group 2950/615 mg/m3 (1000/150 7 days MEK/MBK group than in MBK group (or in MEK 2950 (1978) sex unspecified ppm) MEK/MBK mg/m3 (1000 ppm) controls); interpreted as evidence for elevated microsomal enzyme activity in MEK/MBK group Table 17 (contd) Route Route of administration, Dose and/or Exposure Effects/results Reference species (strain), concentration number and sex Subcutaneous injection, 288 mg/kg b.w. MBK; 1/day, 5 days significantly enhanced neurotoxicity (slower motor Misumi & rat (Donryu), 288/288 mg/kg b.w. per week, 20 fibre conduction velocity and increased motor distal Nagano (1985) 8 males/group MEK/MBK weeks latency of tail nerve) and more pronounced weakness in rats receiving MEK/MBK than MBK alone Subcutaneous injection, 150 mg/kg b.w. MBK; 5 days/week peripheral neuropathological changes and paresis Spencer & cat, 9 (sex 135/15 mg/kg b.w. for up to 8.5 with MBK; possible nerve degeneration in animals Schaumburg unspecified) MBK, MEK/MBK months treated with MEK/MBK (1976) 4 (sex unspecified) MEK/MBK Other ketones Subcutaneous injection, 150 mg/kg b.w. MIBK; 2/day, 5 days no evidence of enhanced MIBK-induced Spencer & cat, 4, sex 135/15 mg/kg b.w. per week, up neuropathology Schaumburg unspecified, MIBK, MEK/MIBK to 8.5 months (1976) 6, sex unspecified, MEK/MIBK Inhalation, rat 3220 mg/m3 (700 ppm) EBK; 16-20 h/day, exposure to MEK/EBK at 2065 and 4130 mg MEK/m3 O'Donoghue et (Charles River), 15 207/3220, 2065/3200, 4 days produced a 2.5-fold increase in serum 2,5-Hpdn over al. (1984) males/group for 4130/3200 mg/m3 (70/700, that produced by EBK alone; no 2,5-HD detected in EBK and controls; 700/700, 1400/700 ppm) serum 5 males/group MEK/EBK for MEK/EBK Oral, rat (Charles 0.25 to 4 g/kg b.w. EBK; 1/day, 5 days 2 and 4 g EBK/kg b.w. fatal with or without MEK; O'Donoghue et River), 4 males in 1.5, 0.75/0.25-4 g/kg b.w. per week, 14 greater neurological damage and dysfunction, and al. (1984) control group MEK/EBK weeks around 1.5-fold increase in 2,5-Hpdn and 2,5-HD with 1.5/1.0 g/kg b.w. MEK/EBK than with 1.0 g EBK/kg b.w.; no neurotoxicity evident in doses of EBK below 1.0 g/kg b.w. Table 17 (contd) Route Route of administration, Dose and/or Exposure Effects/results Reference species (strain), concentration number and sex Oral, rat (Fischer- 253 mg/kg b.w. 2,5-HD; 1/day, 5 days marked neurological dysfunction consistently Ralston et al. 344), 5 males/group 159/253 mg/kg b.w. per week, ca. appeared weeks earlier in rats receiving (1985) MEK/2,5-HDf 13 weeks MEK/2,5-HD Oral, rat (Fischer- 253 mg/kg b.w. 2,5-HD; 1/day, 1 MEK did not alter gastric absorption of 2,5-HD; Ralston et al. 344), 5 males/group 159/253 mg/kg b.w. or 7 days clearance of 2,5-HD from blood slower from rats (1985) MEK/2,5-HD receiving MEK/2,5-HD Oral, rat (Fischer- 253 mg/kg b.w. 2,5-HD; 1/day, 5 days most of radiolabelled 2,5-HD bound to protein; Ralston et al. 344), 5 males/group 159/253 mg/kg b.w. per week, 1, greater binding to protein with MEK/2,5-HD (1985) MEK/2,5-HD 2 or 3 weeks after 1 week, but reverse true after 2 and 3 weeks Halogenated alkanes Oral, intraperitoneal, 1.505 g/kg b.w. MEK; 1 oral dose potentiation of CCl4 hepatotoxicity by MEK as Traiger & rat (Sprague- 0.16 g/kg b.w. CCl4 MEK followed shown by greater fatty vacuolation and necrosis Bruckner Dawley), 3-5 males/ 16 h later of liver, increased hepatic triglyceride and (1976) experimental group, with 1 ip GPT, and decreased hepatic glucose-6-phosphatase 5-20 males/control dose CCl4 group Oral, intraperitoneal, 1.691 g/kg b.w. MEK; 1 oral dose potentiation of CCl4 hepatotoxicity by MEK as Dietz & Traiger rat (Sprague- 0.16 g/kg b.w. CCl4 MEK followed shown by increased hepatic triglyceride and (1979) Dawley), males at 16 h later GPT least 5/group with 1 ip dose CCl4 Oral, intraperitoneal, 1.082 g/kg b.w. MEK; 1 oral dose potentiation of CHCl3 hepatotoxicity by both Hewitt et al. rat (Sprague- 0.797, 1.196 g/kg b.w. MEK followed doses of MEK as shown by increased GPT and OCT (1983) Dawley), 5 or 6 CHCl3 18 h later with males/groupg 1 ip dose CHCl3 Table 17 (contd) Route of administration, Dose and/or Exposure Effects/results Reference species (strain), concentration number and sex Oral, intraperitoneal, 0.072 to 1.082 g/kg b.w. 1 oral dose potentiation of CHCl3 renal and hepatic toxicity Brown & Hewitt neal, rat (Fischer- MEK; 0.797 g/kg b.w. MEK followed by MEK at all dosages as shown by histological and (1984) 344), 6 males per CHCl3 18 h later with several biochemical criteria; potentiation greatest experimental 1 ip dose at 0.361 to 0.721 g/kg b.w. MEK, and slightly group, 32 males CHCl3 reduced at 1.082 g/kg b.w.h in control group Oral, rat (Sprague- 1.082 g/kg b.w. MEK; 1 dose MEK MEK followed by CHCl3 did not induce cholestasis, Hewitt et al. Dawley), 6 males 0.797 to 1.594 g/kg b.w. followed 10 but MEK given 10 to 24 h prior to CHCl3 potentiated (1986) per group CHCl3 to 96 h later its ability to increase plasma bilirubin with 1 dose CHCl3 Oral, rat (Sprague- 1.082 g/kg b.w. MEK; 1 dose MEK MEK followed by CHCl3 10 to 48 h later potentiated Hewitt et al. Dawley), 6 males 0.797 g/kg b.w. CHCl3 followed 10 hepatotoxicity as indicated by elevated ALT and (1987) to 96 h later OCT levels by 1 dose CHCl3 a Values from the literature have been recalculated as ppm or g/kg body weight. b Iwata et al. (1984) states that 24 rats were divided into four groups, but does not specify that groups were of equal numbers. c Animals were apparently co-exposed with those described in Altenkirch et al. (1982a). d Description of exposure not entirely clear but a subsequent paper (Iwata et al., 1984) refers to this as a single exposure. e Animals co-exposed with those described in Altenkirch et al. (1978). f MEK and 2,5-HD doses were 0.317 and 0.506 g/kg, respectively, for first 8 days of experiment. g 5 males at the lower dose and 6 at the higher; 11 males in the control group h Measurements were made 24 h after the oral dose of CHC13 or CCl4. Abbreviations: GPT = plasma glutamic-pyruvic transaminase; OCT = plasma ornithine carbamyl transferase; ALT = plasma alanine aminotransferase; b.w. = body weight; ip = intraperitoneal; 2,5-HD = 2,5-hexanedione; 2,5-Hpdn = 2,5-heptanedione; CCl4 = carbon tetrachloride; CHCl3 = chloroform 10.1.3 Human studies 10.1.3.1 Solvent abuse Solvent abuse, the deliberate inhalation of solvent vapours for their euphoric effects, has been reported in many countries and has involved the use of lacquer thinners, glues and other readily available commercial items. Prockop et al. (1974) suggested that there were several hundred habitual "huffers", i.e. solvent abusers, in Tampa, Florida, USA, and Altenkirch et al. (1978) reported about 2000 in Berlin, Germany, in 1974. Outbreaks of polyneuropathy among Berlin huffers, which involved n-hexane toxicity potentiated by MEK, provided a major stimulus for research on the health effects of MEK and its interactions with hexacarbon solvents. Chronic huffers in Berlin inhaled fumes from about a half litre of liquid per day, either poured into a plastic bag or over rags (Altenkirch et al., 1977, 1982b). Inhalation sessions extended for as long as 10 or 12 h, and exposure periods of 5 to 7 years were not unusual. Prior to the end of 1975 no major damage to health resulting from chronic solvent abuse had been observed. At that time there appeared abruptly a number of cases of polyneuropathy among chronic huffers. All these were young males, mainly between 16 and 21 years of age. The initial symptom was paraesthesia of the toes accompanied in some cases by weakness of the legs. The paraesthesia ascended rapidly from distal to proximal and in 2 to 3 weeks affected the entire legs. This was followed rapidly by paraesthesia of the arms which also ascended from distal to proximal. Extensor muscles were always affected first and most severely. There also was severe muscle atrophy and a "glove and stocking" type sensory impairment of the hands and feet. In all cases there also was excessive sweating of the hands and feet, and in some cases discoloration and reduced skin temperature of these areas. In addition there was loss of weight and damage to the teeth. Head, neck and trunk muscles remained undisturbed, although in severe cases there was paresis of the phrenic nerve and reduced pulmonary function. The degree of impairment ranged from moderate crippling, which permitted walking with assistance, to complete tetraplegia. Symptoms continued to develop for 6 to 10 weeks after cessation of solvent abuse. Remission was slow, took up to a year, developed from proximal to distal, and was incomplete in those most severely affected. Neurophysiological and histological findings were similar to those reported in experimental animals (section 7). Motor and sensory nerve conduction velocities were reduced in proportion to the degree of paresis. There were lesions of the axons, paranodal axon swellings, clumping of the nerve filaments and demyelination. The outbreak of neuropathy came a few months after a change in the formulation of a solvent from one composed of n-hexane, benzene fraction, ethyl acetate and toluene to a similar mixture with less (16%) n-hexane and the addition of 11% MEK. This was interpreted as evidence that even long exposure to the substantial amount of n-hexane (31%) in the original formulation did not result in neuropathy, and that neuropathy developed only after the toxicity of n-hexane was potentiated by simultaneous exposure to MEK. The seven cases of polyneuropathy reported from Tampa, Florida, USA, apparently resulted from a small amount of n-hexane (0.5%) potentiated by other components in the solvent mixture (Prockop et al., 1974; Spencer et al., 1980). Two similar cases of polyneuropathy were reported from Japan (Goto et al., 1974), these were produced by chronic sniffing of a glue that contained 25% n-hexane and 20% MEK. What is puzzling in view of the absence of neuropathy in Berlin prior to the addition of MEK to a solvent mixture containing n-hexane is that Goto et al. (1974) also reported two cases of neuropathy resulting from chronic sniffing of glue solvent containing only n-hexane and toluene. However, Oh & Kim (1976) reported a case of polyneuropathy produced by chronic abuse of mixtures containing MEK, methyl isobutyl ketone (MIBK) and many other solvents, but apparently not n-hexane or MBK. There was, however, no analysis of the solvent mixtures. There was no experimental evidence to suggest that any of the solvents known to be present in the mixtures alone, or MEK and MIBK together, could produce this type of neuropathy. An alternative explanation is that the MIBK contained MBK as an impurity. 10.1.3.2 Occupational exposure Although there have been many cases of occupationally related poisoning by exposure to neurotoxic hexacarbon solvents (Spencer et al., 1980), poisonings in which neurotoxicity has been associated with concurrent MEK exposure are limited. In occupational health studies concentrating on n-hexane neurotoxicity, the study populations were exposed to several other solvents, including MEK (WHO, 1991). One of the most thoroughly investigated cases occurred in 1973 in a fabric factory in Ohio, USA (Allen et al., 1974). In 1973, an employee at the factory was found to have a severe sensorimotor neuropathy. Other co-workers at the plant were also found to have similar symptoms, which initiated a search for a causative agent in the workplace. Of the 1157 workers examined, 86 manifested signs and symptoms indicative of peripheral neuropathy, including parathesiae in arms and legs and weakness in the hands and legs. Subsequent investigation found that in the year prior to the confirmation of the first case, MBK had been substituted for MIBK as a co-solvent with MEK. Exposure occurred by skin contact and by inhalation. Measured concentrations of MEK in certain areas of greatest exposure were 251-2251 mg/m3 (85-763 ppm), while concentration for MBK ranged from 9 to 640 mg/m3 (2.3-156 ppm). The introduction of MBK into the factory was associated with the observed neurotoxicity. Spencer et al. (1980) pointed out that several animal studies showed that concurrent exposure to MEK accelerated MBK-induced neurotoxicity. 10.2 Haloalkane solvents 10.2.1 Studies in animals Carbon tetrachloride (CCl4), chloroform (CHCl3) and related haloalkane solvents are liver and kidney poisons as well as central nervous depressants (Gosselin et al., 1984). It has long been known that the hepatotoxic action of CCl4 is potentiated by ethanol, and more recently that the hepatic and/or renal toxicity of CCl4, CHCl3, trichloroethylene, 1,1,2-trichloroethane and related compounds is potentiated by n-hexane, ethanol, isopropanol, acetone, MEK, MBK, 2,5-HD, and other ketones or chemicals that are metabolized to ketones (Hewitt et al., 1980). Even an increase of naturally occurring ketones in the body via diabetes can precipitate potentiation. Decreasing the transformation of isopropanol to acetone by the administration of an inhibitor of alcohol dehydrogenase, pyrazole, has reduced potentiation of haloalkane toxicity. Although the phenomenon is referred to as haloalkane toxicity, experimental work in general appears largely or entirely limited to chlorinated compounds, and specific studies on MEK are limited to interactions with CCl4 and CHCl3. The effects of MEK and CCl4 or CHCl3 on rats are summarized in Table 17. At the doses used, MEK and the haloalkanes separately produced mild liver and kidney injury at most. When exposure to MEK was followed within 10 to 48 h by a haloalkane, there was severe injury to the liver, with marked and abrupt replacement of normal hepatic cells by necrotic and fatty, vacuolated tissue, an increase in hepatic triglyceride, and, presumably, a corresponding decrease in normal liver function. Hepatic enzymes were released into the blood by breakdown of liver tissue, resulting in elevated levels of plasma glutamic-pyruvic transaminase, plasma ornithine carbamyltransferase, and plasma alanine aminotransferase. There was also an increase in plasma bilirubin, although this was not accompanied by a decrease in bile secretion as was the case with some other ketones (Hewitt et al., 1986). A dose of MEK as small as 0.072 g/kg potentiated the effects of CHCl3 given 18 h later, and 1.505 g/kg MEK potentiated the effects of CCl4. Lower doses were not studied. The mechanism of potentiation is not fully understood but increased bioactivation of haloalkanes is believed to play a central part in the potentiation effect. CCl4 is metabolized in vivo with homolytic cleavage of the carbon-chlorine bond to produce highly reactive free radicals that exert their toxic effects a) by binding covalently to proteins and other elements, and b) via lipid peroxidation (Anders, 1988). It is likely that the toxic effects of CHCl3 also are produced in part by this mechanism (Hewitt et al., 1980, 1987). The toxicity of CCl4 is enhanced by pretreatment with various agents such as phenobarbital, ethanol, isopropanol, 2-butanol and the ketone metabolites of the last two compounds (acetone and MEK) (Cornish & Adefuin, 1967; Traiger & Plaa, 1972; Traiger & Bruckner, 1976; Gosselin et al., 1984). Recent studies have shown that the ethanol-inducible cytochrome P-450 isozyme (P-450IIE1) plays an important role in haloalkane metabolism (Johansson & Ingelman-Sundberg, 1985). It is a high affinity enzyme and operates at a low concentration range for various substrates (Nakajima et al., 1990). CCl4 hepatotoxicity in rats has been found to be potentiated by induction of the P-50IIE1 isozyme with ethanol and the injury was most marked in the perivenous liver cells where the expression of induction was the highest (Lindros et al., 1990). In addition to ethanol, P-450IIE1 is known to be induced by acetone and MEK (Ko et al., 1987; Raunio, et al., 1990; Albano et al., 1991). However, while a relatively large oral dose of MEK (1.4 ml for 3 days) to rats increased the amount of ethanol- and phenobarbital-inducible cytochromes P-450 (P-450IIE1 and P-450IIB, respectively) (Raunio et al., 1990), inhalation exposure of rats to 1770 mg MEK/m3 (600 ppm), 10 h/day for 7 days, caused only marginal effects on microsomal cytochrome P-450 activities in the liver (Liira et al., 1991). 10.2.2 Potentiation of haloalkane toxicity in humans There are no reports of MEK potentiation of haloalkane renal and hepatic toxicity in humans. 11. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT 11.1 Human health risks 11.1.1 Non-occupational exposure Low level non-occupational exposure to MEK is widespread from a variety of natural and anthropogenic sources, since MEK is a normal, though minor, mammalian metabolite. External sources include food, water and air. In the USA, the average daily per capita intake of MEK from food is estimated to be 1.6 mg. In addition to MEK present naturally, foods may contain MEK that has been added in food processing or absorbed from plastic packaging materials. MEK is rarely detected in exposed natural waters where it may originate from microbial activity and atmospheric and anthropogenic inputs, but is frequently detected at low concentrations in drinking-water where it presumably is leached from cemented joints of plastic pipes. Leaching from landfill hazardous waste dumps is another potential source of groundwater, and hence drinking-water, contamination. Measured concentrations in food and water are so low, however, that it is unlikely that either of these represent a significant source of exposure. In minimally polluted outdoor air, the MEK concentration is less than 3 µg/m3 (1.0 ppb) but has been measured at 134 µg/m3 (44.5 ppb) under conditions of dense smog. Away from industrial areas where MEK is manufactured or used, major sources may be vehicle exhaust and photochemical reactions in the atmosphere. In smog episodes, photochemical production of MEK may greatly exceed direct anthropogenic emission. Volatilization of MEK from building materials and consumer products can pollute indoor air to levels far above outdoor air, and a concentration as high as 48 µg/m3 (12.9 ppb) was measured in an Italian home. MEK also is present in tobacco smoke (e.g., 80-207 µg/cigarette). For the general population, daily MEK intake is estimated to range between 1.6 and 4.2 mg, depending on the location site (rural or urban), with an additional 1.6 mg in the case of smokers. There is no evidence of any adverse effects on the general population from exposure to MEK. Data from experimental animal studies show that toxic effects occur at dose levels that are 3 orders of magnitude higher than the estimated daily intake. Non-occupational poisoning from MEK alone is limited to a single case, which resulted in no lasting injury. MEK is, however, known to potentiate the toxicity of two classes of organic solvents, unbranched aliphatic hexacarbons and haloalkanes, and chronic exposure to consumer products containing MEK and n-hexane have produced outbreaks of polyneuropathy among individuals deliberately inhaling fumes from these mixtures for their euphoric effects. Co-exposure to MEK and either hexacarbons or haloalkanes via the abuse of consumer products remains a potential public health hazard. Injuries from such poisoning can be severe, permanently disabling and even fatal. 11.1.2 Occupational exposure MEK is an important industrial chemical which is used mainly as a component of solvent mixtures for application of a wide variety of coatings and adhesives. Moderate occupational exposure via air is widespread because losses to the environment result mainly from solvent evaporation from coated surfaces, and MEK is not viewed as an especially dangerous substance. Most national limits for occupational exposure are set at 590 mg/m3 (200 ppm), with a higher short-term exposure level of 885 mg/m3 (300 ppm), and these limits appear acceptable. On site measurements, however, indicate that workers may be chronically exposed to still higher MEK concentrations in small factories such as shoe factories, printing plants and painting operations, due to inadequate ventilation. Lesser amounts of MEK are lost to the air with concurrent worker exposure during manufacture, shipping, repackaging and preparation of coatings and adhesives. Industrial exposure from contact with liquid MEK does not appear an important problem. Chronic co-exposure to MEK and either unbranched aliphatic hexacarbon or haloalkane solvents represents a significant potential occupational hazard. Serious toxic effects could occur. Although there are no records of industrial accidents involving MEK potentiation of haloalkane toxicity, MEK potentiation of hexacarbon neurotoxicity may have caused at least one major industrial accident in which an outbreak of polyneuropathy followed introduction of MEK into a solvent mixture. Thus MEK, in the mixed solvent atmosphere of many industrial activities, can present a toxic hazard. 11.1.3 Relevant animal studies Acute MEK toxicity has been shown in animal studies to be low by the oral and inhalation route. The lowest oral dose modifying body structure (damage of kidney tubules) was 1 g/kg body weight in rats. Ten intraperitoneal injections of 34 mg/kg body weight over a 2-week period produced transient injection site irritations but no effect on the kidney. In a 90-day inhalation study, female rats exposed to 14.75 g/m3 for 6 h/day, 5 days per week, showed only slightly increased liver weight, slightly decreased brain and spleen weight, and slightly altered blood chemistry in comparison with controls; male rats showed only a slightly increased liver weight. A transient decrease in nerve conduction velocity was found following exposure to 590 mg/m3 (12 h/day for 24 weeks). The transient nature of neurological and behavioural changes induced by MEK may be due to adaptation or more rapid metabolism of MEK. Short-term dermal exposure to small amounts of MEK results in mild local irritation, at most. Results of studies of eye irritation are inconsistent, possibly due to different scoring techniques; 4 mg created severe chemical burns in the eye in one study whereas in other studies less severe signs were reported following a dose of 80 mg. An inhalation study provided evidence for low level fetotoxicity in the absence of maternal toxicity at 8825 mg/m3. Thus MEK may be a low grade teratogen in rats. There is a lack of data on other aspects of reproduction in animals, and no relevant data have been reported for humans. MEK has given negative results in most conventional mutagenicity assays. There is evidence of aneuploidy in yeast but this may not be relevant to humans or other mammals. 11.2 Effects on the environment MEK occurs naturally at low concentrations in air, water and soil. It is highly mobile in the natural environment and is not accumulated in any individual compartment. MEK is rapidly synthesized and destroyed by photochemical processes in the atmosphere. There is no specific information on either partitioning of MEK in any environmental compartment or on chemical binding to sediment particles. MEK is synthesized biologically and is rapidly metabolized by bacteria (even at high concentrations), mammals and probably many other organisms. Levels produced by fungi can cause inhibition of plant germination. Observations on microorganisms, higher plants, invertebrates, fish and mammals suggest a low level of toxicity. Environmental levels of MEK appear to be too low to cause any damage except in the immediate vicinity of highly polluted sites. Effects on the aquatic environment are likely to appear at levels between 1 and 10 mg/litre. The potentiation of solvent toxicity by MEK appears environmentally irrelevant, although substantial information is lacking. Overall, MEK does not represent a significant threat to the environment. 12. RECOMMENDATIONS FOR THE PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT 12.1 Human health protection MEK on its own appears a relatively safe organic solvent, but its use in combination with other solvents, in particular haloalkanes or unbranched aliphatic hexacarbons, should be avoided. Industries should be strongly encouraged to take all necessary precautions to ensure that workers are not exposed to both MEK and solvents whose toxicity is potentiated by MEK. 12.2 Environmental protection MEK is unlikely to present a hazard to the environment except in cases of major spills or discharges. 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Propriétés et méthodes d'analyse La méthyléthylcétone est un liquide limpide, incolore, volatil et très inflammable dont l'odeur rappelle celle de l'acétone. Elle est stable dans les conditions ordinaires, mais peut donner naissance à des peroxydes explosifs en cas de stockage prolongé. Elle peut aussi former des mélanges explosifs avec l'air. Elle est très soluble dans l'eau, miscible à de nombreux solvants organiques et forme des azéotropes avec l'eau et beaucoup de liquides organiques. Dans l'atmosphère, elle produit des radicaux libres qui peuvent favoriser la formation d'un smog photochimique. On dispose de plusieurs méthodes analytiques pour mesurer les concentrations de méthyléthylcétone dans l'air, l'eau, les échantillons biologiques, les effluents et d'autres milieux. Dans les méthodes les plus sensibles, la méthyléthylcétone est piégée et concentrée soit sur un sorbant solide, soit sous forme de dérivé de la 2,4-dinitrophénylhydrazine (DNPH). La méthyléthylcétone absorbée et les autres composés organiques volatils sont désorbés, séparés par chromatographie gazeuse et dosés à l'aide d'un spectromètre de masse ou d'un détecteur à ionisation de flammes. Le produit de dérivation de la méthyléthylcétone est séparé des substances apparentées par chromatographie liquide à haute performance et mesuré par spectrophotométrie ultraviolette. Dans des milieux tels que les déchets solides ou les substances biologiques, la méthyléthylcétone doit d'abord être séparée du substrat, par exemple par extraction à l'aide d'un solvant ou par distillation à la vapeur. Les concentrations élevées de méthyléthylcétone dans l'air peuvent être mesurées de façon continue par absorption infrarouge. Les limites de détection sont de 3 µg/m3 dans l'air, 0,05 µg/litre dans l'eau de boisson, 1,0 µg/litre dans les autres types d'eau, 20 µg/litre dans le sang et 100 µg/litre dans l'urine. 2. Sources d'exposition et usages 2.1 Production et autres sources Selon des statistiques récentes, la production annuelle (en milliers de tonnes) est la suivante: Etats-Unis d'Amérique, 212 à 305; Europe de l'Ouest, 215; Japon, 139. D'autres sources de contamination de l'environnement par la méthyléthylcétone sont les gaz d'échappement des réacteurs et des moteurs à combustion interne ainsi que certaines activités industrielles comme la gazéification du charbon. Elle est également présente en quantités notables dans la fumée de tabac. Aux Etats-Unis d'Amérique, la quantité de méthyléthylcétone émise par les moteurs représente plus de 1% de la quantité fabriquée volontairement. En cas de smog, la production photochimique de méthyléthylcétone et d'autres carbonyles à partir de radicaux libres peut être bien supérieure aux émissions résultant des activités humaines. La méthyléthylcétone peut aussi avoir une origine biologique et elle a été identifiée parmi les produits du métabolisme microbien. Elle a également été détectée dans divers produits naturels, notamment des végétaux supérieurs, des phéromones d'insectes, des tissus animaux, ainsi que chez l'homme dans le sang, l'urine et l'air expiré. Elle constitue probablement un produit mineur du métabolisme normal des mammifères. 2.2 Usages et pertes dans l'environnement La méthyléthylcétone est un excellent solvant, ce qui explique que sa principale application soit la fabrication de revêtements protecteurs et d'adhésifs. Elle est également utilisée comme intermédiaire chimique, comme solvant dans la fabrication des rubans magnétiques, pour l'élimination des cires dans les huiles de graissage et dans l'industrie alimentaire. Elle entre également dans la composition de nombreux produits d'usage courant comme les vernis et les colles. Dans la plupart de ces applications, la méthyléthylcétone est mélangée à d'autres solvants organiques. La libération dans l'environnement résulte principalement de l'évaporation des solvants à partir des surfaces enduites et concerne essentiellement l'atmosphère. La libération de méthyléthylcétone dans l'eau est la conséquence de sa présence dans les effluents provenant de sa fabrication et de diverses opérations industrielles. Elle a été détectée dans des eaux naturelles dans lesquelles sa présence pourrait s'expliquer par une activité microbienne, l'absorption à partir de l'atmosphère ou une pollution anthropogène. 3. Transport et distribution dans l'environnement La méthyléthylcétone est extrêmement mobile dans l'environnement naturel où elle se renouvelle rapidement. Elle est très soluble dans l'eau et s'évapore facilement. Dans l'atmosphère, elle subit une décomposition photochimique rapide, mais elle est également synthétisée par des processus photochimiques. Elle réagit avec les halogènes libres ou les hypochlorites et leurs homologues halogénés présents dans l'eau pour former un dérivé halogéné plus toxique que la molécule initiale. La méthyléthyl-cétone est transportée par l'air et par l'eau, mais elle ne s'accumule dans aucun compartiment et elle ne persiste pas longtemps là où il existe une activité microbienne. Elle est rapidement métabolisée par les microbes et les mammifères. Le phénomène de bioaccumulation n'a jamais été mis en évidence. La méthyléthylcétone existe naturellement dans certaines espèces de trèfle et elle est produite par des champignons à des concentrations qui peuvent affecter la germination de certaines plantes. 4. Concentration dans l'environnement et exposition humaine La population générale est fréquemment exposée à de faibles doses de méthyléthylcétone. Lorsque la pollution atmosphérique est très faible, la concentration est inférieure à 3 µg/m3 (< 1 ppb), mais on a mesuré jusqu'à 131 µg/m3 (44,5 ppb) en atmosphère très polluée. En dehors des zones industrielles de fabrication ou d'utilisation de la méthyléthylcétone, les gaz d'échappement des véhicules automobiles et les réactions photochimiques dans l'atmosphère peuvent être les principales sources de contamination. Les cigarettes et les autres formes de tabac à fumer contribuent à l'exposition individuelle (20 cigarettes en contiennent jusqu'à 1,6 mg). La volatilisation de la méthyléthylcétone présente dans les matériaux de construction et les produits de consommation peut entraîner une pollution de l'air des locaux bien supérieure à celle de l'air extérieur. Les concentrations dans les eaux naturelles dépassent rarement 100 µg/litre (100 ppb) et sont généralement inférieures au seuil de détection. Toutefois, on a souvent trouvé des traces de méthyléthylcétone dans l'eau de boisson (environ 2 µg/litre). Il est probable que les solvants entrant dans la composition des joints des tuyauteries de plastique sont à l'origine de cette pollution. Bien que la méthyléthylcétone soit un constituant normal de nombreux aliments, les concentrations sont faibles et l'alimentation ne peut être considérée comme une source importante d'exposition. Aux Etats-Unis d'Amérique, la quantité moyenne ingérée quotidiennement avec les aliments, principalement le pain blanc, les tomates et le fromage Cheddar, est estimée à 1,6 mg par personne. La méthyléthylcétone peut être présente naturellement dans les aliments, mais elle peut aussi se former lors de l'affinage du fromage, de l'entreposage de la viande de volaille, de la cuisson ou de la transformation des aliments, ou être absorbée à partir des emballages en matière plastique. L'exposition industrielle à des concentrations modérées de méthyléthylcétone est fréquente. Toutefois, dans certaines régions, le personnel travaillant dans de petites entreprises (fabriques de chaussures, imprimeries, ateliers de peinture) peut être exposé à des concentrations beaucoup plus élevées en raison d'une ventilation insuffisante. Ces travailleurs sont généralement exposés à un mélange de solvants, notamment au n-hexane. 5. Cinétique et métabolisme La méthyléthylcétone est rapidement absorbée par contact cutané, inhalation, ingestion et injection intrapéritonéale. Elle passe très vite dans le sang, et de là dans les autres tissus. Il semble que sa solubilité soit à peu près la même dans tous les tissus. L'élimination de la méthyléthylcétone et de ses métabolites est pratiquement totale chez les mammifères au bout de 24 heures. Elle est métabolisée dans le foie où la plus grande partie est oxydée en 3-hydroxy-2-butanone avant d'être réduite en 2,3-butanediol. Une petite partie peut être réduite en 2-butanol, mais celui-ci est rapidement oxydé pour redonner la molécule initiale. Chez les mammifères, la majeure partie de la méthyléthylcétone ingérée entre dans le cycle métabolique général et/ou est éliminée sous forme de molécules simples comme le dioxyde de carbone et l'eau. L'excrétion de la méthyléthylcétone et de ses métabolites caractéristiques se fait principalement par les poumons, bien que de petites quantités soient éliminées par les reins. La méthyléthylcétone augmente l'activité enzymatique du cytochrome P-450 microsomal. Il est possible que ce renforcement de l'activité enzymatique, et par conséquent du potentiel de transformation métabolique de l'organisme, explique pourquoi la méthyléthylcétone potentialise la toxicité des solvants du groupe des alcanes halogénés et des hydrocarbures aliphatiques à six atomes de carbone. 6. Effets sur les animaux d'expérience La méthyléthylcétone présente une toxicité faible à modérée pour les mammifères, qu'il s'agisse de toxicité aiguë, à court terme ou chronique. Chez la souris et le rat, la LD50 est de 2 à 6 g/kg de poids corporel, la mort survenant 1 à 14 jours après l'ingestion d'une dose unique. La dose moyenne entraînant la mort après une exposition unique aux vapeurs de méthyléthylcétone est d'environ 29 400 mg/m3 (10 000 ppm), bien que des cobayes aient survécu à une exposition de 4 heures à cette concentration. Dans des essais d'intoxication aiguë par voie orale, la dose la plus faible ayant entraîné une modification de structure des organes a été de 1 g/kg de poids corporel chez le rat. Cette dose a provoqué des lésions des tubules du rein. L'inhalation par des rats d'air contenant 74 mg/m3 (25 ppm) pendant 6 heures a provoqué des changements de comportement mesurables qui ont persisté pendant plusieurs jours. Une exposition répétée à 14 750 mg/m3 (5000 ppm) (6 h/jour, 5 jours/semaine) n'a provoqué la mort d'aucun animal. On n'a observé qu'un effet mineur sur la croissance et la structure et il n'y a eu aucune modification neuropathologique. Des poulets, des chats ou des souris exposés à 3975 mg/m3 (1500 ppm) pendant des périodes allant jusqu'à 12 semaines n'ont présenté aucun signe de changement neuropathologique. Des effets transitoires sur le comportement ou la neurophysiologie ont été détectés chez des rats et des babouins à la suite d'expositions répétées à des concentrations ne dépassant pas 295 à 590 mg/m3 (100 à 200 ppm). Une faible foetotoxicité a été observée à 8825 mg/m3 (3000 ppm), mais elle ne s'accompagnait d'aucune toxicité maternelle; aucun effet embryotoxique ou tératogène n'a été constaté à des concentrations inférieures à cette valeur. Après avoir exposé de façon répétée des rattes en gestation à une concentration de 8825 mg/m3, on a observé dans leur progéniture une augmentation légère mais significative de certains types d'anomalie du squelette rarement observés chez les animaux non exposés. Plusieurs épreuves classiques de mutagénicité ont été pratiquées, mais la seule qui ait donné un résultat positif a été une étude d'hétéroploïdie sur la levure Saccharomyces cerevisiae. La méthyléthylcétone ne présente pas de toxicité aiguë pour les poissons ou les invertébrés aquatiques, la CL50 se situant entre 1382 et 8890 mg/litre. Elle a un effet inhibiteur sur la germination de plusieurs plantes, même à des concentrations que l'on peut rencontrer dans la nature. La croissance des algues aquatiques est également inhibée. Des concentrations relativement élevées de méthyléthylcétone, comparées aux concentrations naturelles, ont été utilisées dans des expériences de fumigation. Cette substance s'est révélée un fumigant modérément efficace contre la mouche des fruits des Caraïbes. D'autre part, elle a un effet attractif très net sur la mouche tsé-tsé. Des concentrations allant jusqu'à 20 mg/litre retardent le processus de biodégradation mais ne l'arrêtent pas complètement. Jusqu'à 100 mg/litre, la méthyléthylcétone est bactériostatique pour différentes bactéries. A des concentrations plus élevées (1000 mg/litre et au-delà) elle inhibe la croissance des bactéries et des protozoaires. 7. Effets sur l'homme 7.1 Méthyléthylcétone seule Aucun effet notable n'a été observé lors de tests psychologiques et de comportement après exposition à 590 mg/m3 (200 ppm). Une exposition de courte durée à la méthyléthylcétone seule ne semble pas présenter de risques importants, que ce soit pour les professionnels ou pour le public en général. L'exposition, dans des conditions expérimentales, à une concentration de 794 mg/m3 (270 ppm), à raison de 4 heures par jour, a eu un effet à peu près nul sur le comportement et un contact de 5 minutes avec la substance liquide n'a provoqué qu'une décoloration temporaire de la peau. On n'a signalé qu'un seul cas d'intoxication aiguë par la méthyléthylcétone en dehors de tout contexte professionnel. Il s'agit d'un cas d'ingestion accidentelle qui ne semble pas avoir laissé de séquelles. On n'a jamais signalé de cas d'exposition professionnelle ayant entraîné la mort. Il existe deux rapports faisant état d'intoxication professionnelle chronique et un rapport d'intoxication professionnelle aiguë, mais ce dernier est sujet à caution. Dans un des cas d'intoxication chronique, l'exposition à 880-1770 mg/m3 (300-600 ppm) a provoqué des dermatoses, un engourdissement des doigts et des bras et divers symptômes, parmi lesquels des céphalées, des étourdissements, des troubles gastro-intestinaux et une perte d'appétit et de poids. Le faible nombre d'intoxications attribuées à la méthyléthylcétone seule tient à la fois à la faible toxicité de cette substance et au fait qu'elle est rarement utilisée seule, mais plutôt en mélange avec d'autres solvants. 7.2 Méthyléthylcétone dans les mélanges de solvants L'exposition à des mélanges de solvants contenant de la méthyléthylcétone a été associée à une réduction de la vitesse de conduction nerveuse, à des troubles de la mémoire, à des troubles moteurs, à des dermatoses et à des vomissements. Dans une étude longitudinale, des mesures consécutives du temps de réaction simple ont montré une amélioration des performances parallèlement à une diminution de la concentration de méthyléthylcétone jusqu'à un dixième de la valeur initiale (qui pouvait atteindre 4000 mg/m3 pour certaines tâches de routine). 8. Renforcement de la toxicité des autres solvants La méthyléthylcétone potentialise la neurotoxicité des solvants à six atomes de carbone ( n-hexane, méthyl- n-butylcétone et 2,5-hexanedione) ainsi que la toxicité hépatique et rénale des solvants de la famille des alcanes halogénés (tétrachlorure de carbone et trichlorométhane). La potentialisation des effets neurotoxiques des composés à six atomes de carbone a été démontrée chez l'animal pour les trois substances citées ci-dessus. Les neuropathies périphériques observées chez l'homme se sont produites à la suite de changements dans la formulation des solvants auxquels les sujets avaient été exposés, soit volontairement, soit en raison de leur activité professionnelle. Le mécanisme de cette potentialisation n'a pas été élucidé. La potentialisation de la toxicité hépatique et rénale des alcanes halogénés a été mise en évidence dans des études chez l'animal. La méthyléthylcétone active probablement la métabolisation des haloalcanes en substances toxiques pour les tissus en induisant la production des enzymes oxydantes responsables de cette transformation. RESUMEN 1. Propiedades y métodos analíticos La metiletilcetona (MEC) es un líquido transparente, incoloro, volátil, muy inflamable, de olor parecido a la acetona. Es estable en condiciones normales pero puede formar peróxidos si se almacena durante mucho tiempo; esos peróxidos pueden ser explosivos. La MEC también puede formar mezclas explosivas con el aire. Es muy soluble en agua, miscible con muchos disolventes orgánicos, y forma mezclas azeotrópicas con el agua y con numerosos líquidos orgánicos. En la atmósfera, la MEC produce radicales libres que pueden llevar a la formación de nieblas fotoquímicas. Existen varios métodos analíticos para medir los niveles ambientales de MEC en el aire, el agua, las muestras biológicas, los desechos y otros materiales. Con los métodos más sensibles, la MEC se separa y se concentra ya sea en un sorbente sólido o como derivado de la 2,4-dinitrofenilhidrazina (DNFH). La MEC y otros compuestos orgánicos volátiles absorbidos son desorbidos, separados mediante cromatografía de gases y medidos con un espectrómetro de masas o un detector de ionización de llama. La MEC derivada se separa de los compuestos afines mediante cromatografía de líquidos de alto rendimiento y se mide mediante absorción ultravioleta. En medios como desechos sólidos y material biológico, la MEC debe separarse en primer lugar del sustrato con métodos como la extracción por solventes o la destilación de vapores. Las concentraciones elevadas de MEC en el aire pueden controlarse de modo continuo mediante absorción infrarroja. Los límites de detección son 3 µg/m3 en el aire, 0,05 µg/litro en el agua potable, 1,0 µg/litro en otros tipos de agua, 20 µg/litro en sangre total y 100 µg/litro en orina. 2. Fuentes de exposición y usos 2.1 Producción y otras fuentes Las cifras más recientes de fabricación industrial anual (en miles de toneladas) son: EEUU, 212 a 305; Europa occidental, 215; Japón, 139. Además de su fabricación, las fuentes de MEC en el medio ambiente son los gases de escape de motores de reactores y de combustión interna, y las actividades industriales como la gasificación del carbón. Se encuentra en cantidades importantes en el humo de tabaco. En los Estados Unidos, la producción de MEC en motores no supera el 1% de su fabricación deliberada. En los episodios de nieblas, la producción fotoquímica de MEC y otros carbonilos a partir de radicales libres puede ser mucho mayor que la emisión antropogénica directa. La MEC se produce biológicamente y se ha identificado como producto del metabolismo microbiano. Se ha detectado asimismo en gran diversidad de productos naturales, entre ellos los vegetales superiores, las feromonas de insectos, en tejidos animales y en el hombre, en sangre, orina y aire exhalado. Probablemente es un producto secundario del metabolismo normal en el mamífero. 2.2 Usos y pérdidas al medio ambiente El uso principal de la MEC, la aplicación de revestimientos protectores y adhesivos, refleja sus excelentes características como disolvente. También se utiliza como intermediario químico, como disolvente en la producción de cintas magnéticas y para eliminar la cera del aceite lubricante, así como en la manipulación de alimentos. Además de las aplicaciones industriales, figura como ingrediente común en productos de consumo como barnices y pegamentos. En la mayoría de las aplicaciones, la MEC es componente de una mezcla de disolventes orgánicos. Las pérdidas al medio ambiente son principalmente al aire y se deben sobre todo a la evaporación de disolventes a partir de las superficies revestidas. Se libera al agua como componentes de los desechos de su fabricación y a partir de diversas operaciones industriales. Se ha detectado en aguas naturales, procedente probablemente de la actividad microbiana y del aporte atmosférico, así como de la contaminación antropogénica. 3. Transporte y distribución en el medio ambiente La MEC es sumamente móvil en el medio ambiente natural y está sometida a una transformación rápida. Es muy soluble en el agua y se evapora fácilmente a la atmósfera. En el aire, la MEC sufre una rápida descomposición fotoquímica y es también sintetizada por procesos fotoquímicos. En agua que contiene halógenos libres o hipohalitos, reacciona para formar un haloformo más tóxico que el compuesto original. La MEC se distribuye tanto por el aire como por el agua, pero no se acumula en ningún compartimento aislado, ni persiste mucho tiempo donde existe actividad microbiana. Se metaboliza rápidamente en los microbios y los mamíferos. No hay pruebas de bioacumulación. La MEC aparece naturalmente en algunas especies de trébol y es producida por hongos en concentraciones que afectan a la germinación de algunas plantas. 4. Niveles ambientales y exposición humana La exposición de la población general a bajos niveles de MEC es muy extensa. En aire poco contaminado, la concentración es inferior a 3 µg/m3 (< 1 ppmm), pero en condiciones de fuerte contaminación atmosférica se ha medido un nivel de 131 µg/m3 (44,5 ppmm). Lejos de las zonas industriales donde se fabrica o se usa la MEC, las principales fuentes pueden ser los escapes de vehículos y las reacciones fotoquímicas en la atmósfera. Los cigarrillos y otros productos del tabaco que se someten a combustión contribuyen a la exposición individual (20 cigarrillos contienen hasta 1,6 mg). La volatilización de la MEC de materiales de construcción y productos de consumo pueden contaminar el aire de interiores hasta niveles muy superiores a los del aire libre adyacente. Las concentraciones de MEC en aguas naturales expuestas rara vez se encuentran por encima de los 100 µg/litro (100 ppmm) y suelen encontrarse por debajo del nivel de detección. No obstante, se han detectado cantidades muy reducidas de MEC en el agua potable (aproximadamente 2 µg/litro) que probablemente proceden de disolventes lixiviados a partir del material de las juntas de las tuberías de plástico. Aunque la MEC es un componente normal de muchos alimentos, las concentraciones son bajas y el consumo de alimentos no puede considerarse una fuente significativa de exposición para la población. La ingesta media diaria por habitante de los Estados Unidos con los alimentos se calcula en 1,6 mg, en su mayor parte a partir del pan blanco, los tomates y el queso tipo Cheddar. Además de la MEC presente en el medio natural, puede producirse en la maduración de los quesos, el envejecimiento de la carne de ave, la cocción o la manipulación de alimentos, o por absorción a partir de los envases de plástico. La exposición industrial a niveles moderados de MEC está muy extendida. No obstante, en algunas regiones los trabajadores de fábricas pequeñas (por ejemplo, fábricas de calzado, imprentas y fábricas de pinturas) están expuestos a concentraciones mucho más elevadas por una ventilación insuficiente. En esas fábricas, la exposición se da por lo general a una mezcla de disolventes entre los que figura el n-hexano. 5. Cinética y metabolismo La absorción de MEC es rápida por contacto cutáneo, inhalación, ingestión e inyección intraperitoneal. Pasa rápidamente a la sangre y de ella a otros tejidos. La solubilidad de la MEC parece similar en todos los tejidos. La eliminación de la MEC y sus metabolitos en mamíferos se completa en su mayor parte en 24 horas. Se metaboliza en el hígado, donde se oxida a 3-hidroxi-2-butanona y a continuación se reduce a 2,3-butanodiol. Una pequeña porción puede reducirse a 2-butanol, pero éste se oxida rápidamente para dar de nuevo MEC. La mayor parte de la MEC que ingresa al organismo de mamíferos pasa al metabolismo general y/o se elimina en forma de compuestos simples, como dióxido de carbono y agua. La excreción de MEC y sus metabolitos reconocibles se hace principalmente por los pulmones, aunque pequeñas cantidades se eliminan por el riñón. La MEC aumenta la actividad enzimática del citocromo P-450 en los microsomas. Este aumento de la actividad enzimática y con ello del potencial del organismo para la transformación metabólica puede ser el mecanismo por el que la MEC potencia la toxicidad de los disolventes a base de haloalcanos y hexacarbonos alifáticos. 6. Efectos en los animales de experimentación La MEC tiene toxicidad aguda, a corto plazo y crónica de baja a moderada en los mamíferos. Los valores de la DL50 en ratones y ratas adultos son 2-6 g/kg de peso corporal; la muerte sobreviene en los días 1 a 14 después de una sola dosis por vía oral. Las concentraciones medias de vapor que producen letalidad en las ratas tras una sola exposición giran en torno a los 29 400 mg/m3 (10 000 ppm), aunque los cobayos sobrevivieron a una exposición de 4 horas a esta concentración. La dosis oral aguda más baja en modificar la estructura corporal fue de 1 g/kg de peso corporal, que produjo lesiones en los túbulos renales de la rata. La inhalación de 74 mg/m3 (25 ppm) durante 6 horas produjo en la rata cambios de comportamiento medibles que persistieron durante varios días. La exposición repetida de ratas a 14 750 mg/m3 (5000 ppm) (6 h/día, 5 días/semana) no produjo letalidad, tuvo sólo ligeros efectos en el crecimiento y la estructura, y no se observaron cambios neuropatológicos. No hubo pruebas de que la MEC produjera cambios neuropatológicos en pollos, gatos o ratones expuestos a 3975 mg/m3 (1500 ppm) durante periodos de hasta 12 semanas. Tras la exposición repetida de ratas y babuinos a concentraciones tan bajas como 295-590 mg/m3 (100 a 200 ppm) se observaron efectos transitorios en el comportamiento o la neurofisiología. Se ha observado un nivel bajo de fetotoxicidad sin toxicidad materna a 8825 mg/m3 (3000 ppm), pero no hay pruebas de efectos embriotóxicos o teratogénicos a niveles más bajos de exposición. La exposición repetida de ratas preñadas a 8825 mg/m3 indujo en sus crías un aumento pequeño pero significativo de ciertos tipos de anomalías esqueléticas cuya incidencia entre la población no expuesta es baja. Aunque se examinó en varios sistemas de ensayo de mutagenicidad convencionales, la única prueba de mutagenicidad se observó en un estudio sobre aneuploidia en la levadura Saccharomyces cerevisiae. La MEC no presenta toxicidad aguda para los peces ni los invertebrados acuáticos; los valores de la CL50 varían desde 1382 hasta 8890 mg/litro. La MEC inhibe la germinación de varias especies vegetales, incluso con niveles que se dan en la naturaleza. Inhibe el crecimiento de algas acuáticas. En comparación con los niveles de base naturales, se han utilizado concentraciones relativamente elevadas de MEC para fumigar en condiciones experimentales. Es moderadamente eficaz como fumigante contra la mosca caribeña de la fruta y atrae con gran eficacia a la mosca tse-tse. Con niveles de MEC de hasta 20 mg/litro se retrasa la biodegradación pero no se detiene el proceso por completo. Con niveles de hasta 100 mg/litro, la MEC es bacteriostatica para algunas bacterias. Con concentraciones más altas (1000 mg/litro o más) se inhibe el crecimiento de bacterias y protozoarias. 7. Efectos en el ser humano 7.1 MEC por sí sola La exposición a 590 mg/m3 (200 ppm) no tuvo efectos de importancia en varios ensayos comportamentales y psicológicos. La exposición a corto plazo a MEC por sí sola no parece constituir un riesgo de importancia, ni ocupacional ni para el público en general. La exposición experimental a una concentración de 794 mg/m3 (270 ppm) durante 4 h/día tuvo escaso o ningún efecto en el comportamiento, y un contacto de 5 minutos con MEC líquida no produjo más que un blanqueamiento temporal de la piel. Sólo hay un informe no ocupacional de toxicidad aguda a la MEC. Se debió a una ingestión accidental y no pareció producir lesiones duraderas. No hay pruebas de que la exposición ocupacional a la MEC haya originado ningún caso de muerte. Se han notificado dos casos de envenenamiento ocupacional crónico y uno dudoso de envenenamiento ocupacional agudo. En uno de los casos crónicos, la exposición a 880-1770 mg/m3 (300-600 ppm) dió lugar a dermatosis, endormecimiento de los dedos y los brazos, y diversos síntomas como dolor de cabeza, mareos, trastornos gastrointes-tinales y pérdida de apetito y de peso. Esta escasez de incidentes de envenamiento por MEC por sí sola refleja tanto su baja toxicidad como el hecho de que se usa más comunmente no por sí sola sino como componente de mezclas de disolventes. 7.2 La MEC en mezclas de disolventes La exposición a mezclas de disolventes con MEC se ha asociado a cierta reducción en la velocidad de conducción nerviosa, la memoria y alteraciones motoras, dermatosis y vómitos. En un estudio longitudinal, las medidas consecutivas de tiempo de reacción simple demostraron que mejoraba el rendimiento en paralelo al ir disminuyer de las con concentraciones de MEC hasta un décimo de los valores originales (que fueron de hasta 4000 mg/m3 para ciertas tareas rutinarias). 8. Potenciación de la toxicidad de otros disolventes La MEC potencia la neurotoxicidad de compuestos hexacarbonados ( n-hexano, metil- n-butilcetona y 2,5-hexanodiona) y la toxicidad hepatica y renal de los disolventes a base de haloalcanos (tetracloruro de carbono y triclorometano). La potenciación de los efectos neurotóxicos de los hexacarbonos se ha demostrado con los tres hexacarbonos en el animal. Las neuropatías periféricas observadas en humanos siguieron a cambios en la formulacion de disolventes a los que habían estado expuestos, ya sea voluntariamente o en el trabajo. El mecanismo por el que se produce esta potenciación no está claro. Las pruebas de la potenciación de la toxicidad hepática y renal de los haloalcanos proceden de estudios animales. La MEC activa probablemente el metabolismo de los haloalcanos de las especies que dañan los tejidos como resultado de la inducción de las enzimas oxidativas pertinentes.
See Also: Methyl ethyl ketone (CHEMINFO) Methyl ethyl ketone (ICSC)