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This report contains the collective views of international groups of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

Environmental Health Criteria 221

ZINC

First draft prepared by Drs B. Simon-Hettich and A. Wibbertmann, Fraunhofer Institute of Toxicology and Aerosol Research, Hanover, Germany, Mr D. Wagner, Department of Health and Family Services, Canberra, Australia, Dr L. Tomaska, Australia New Zealand Food Authority, Canberra, Australia, and Mr H. Malcolm, Institute of Terrestrial Ecology, Monks Wood, England.

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization

Geneva, 2001

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer-review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

Zinc.

(Environmental health criteria ; 221)

1.Zinc - analysis

2.Zinc - toxicity

3.Occupational exposure

4.Environmental exposure

5.Risk assessment I.Series

ISBN 92 4 157221 3

(NLM Classification: QD 181.Z6)

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.

©World Health Organization 2001

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 ZINC

PREAMBLE

ABBREVIATIONS

1. SUMMARY AND CONCLUSIONS

1.1 Identity, and physical and chemical properties

1.2 Analytical methods

1.3 Sources of human and environmental exposure

1.4 Environmental transport, distribution and transformation

1.5 Environmental concentrations

1.5.1 Human intakes

1.6 Kinetics and metabolism in laboratory animals and humans

1.7 Effects on laboratory animals

1.8 Effects on humans

1.9 Effects on other organisms in the laboratory and field

1.10 Conclusions

1.10.1 Human health

1.10.2 Environment

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, AND ANALYTICAL METHODS

2.1 Identity

2.2 Physical and chemical properties

2.2.1 Zinc metal

2.2.2 Zinc compounds

2.3 Analytical methods

2.3.1 Introduction

2.3.2 Sampling and sample preparation

2.3.3 Separation and concentration

2.3.4 Detection and measurement

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1 Natural occurrence

3.2 Anthropogenic sources

3.2.1 Production levels and processes

3.2.1.1 Production levels

3.2.1.2 Production processes

3.2.2 Uses

3.2.3 Emissions during production and use

3.2.3.1 Emissions to atmosphere

3.2.3.2 Emissions to aquatic environment

3.2.3.3 Emissions to soil

3.2.4 Emissions during combustion of coal and oil, and refuse incineration

3.2.5 Zinc releases from diffuse sources

3.2.5.1 Releases from atmospheric zinc corrosion

3.2.5.2 Releases from sacrificial zinc anodes

3.2.5.3 Household zinc emissions

4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION

4.1 Transport and distribution between media

4.1.1 Air

4.1.2 Water and sediment

4.1.2.1 Fresh water

4.1.2.2 Seawater

4.1.2.3 Wastewater

4.1.2.4 Groundwater

4.1.2.5 Sediment

4.1.3 Soil

4.2 Bioavailability

4.2.1 Factors affecting bioavailability

4.2.2 Techniques for estimation

4.3 Biotransformation

4.3.1 Biodegradation

4.3.2 Bioaccumulation

4.3.2.1 Aquatic organisms

4.3.2.2 Terrestrial organisms

5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

5.1 Environmental levels

5.1.1 Air

5.1.2 Precipitation

5.1.3 Water

5.1.3.1 Fresh water

5.1.3.2 Seawater

5.1.4 Soil

5.1.5 Sediments and sewage sludge

5.1.6 Aquatic and terrestrial organisms

5.1.6.1 Aquatic plants and animals

5.1.6.2 Terrestrial plants and animals

5.2 General population exposure

5.2.1 Air

5.2.2 Food

5.2.3 Drinking-water

5.2.4 Miscellaneous exposures

5.3 Occupational levels

5.4 Total human intake from all sources

5.4.1 General population

5.4.2 Bioavailability in mammalian systems

5.4.3 Occupational exposure

6. KINETICS AND METABOLISM IN MAMMALS

6.1 Absorption

6.1.1 Inhalation

6.1.1.1 Human studies

6.1.1.2 Animal studies

6.1.2 Oral

6.1.2.1 Human studies

6.1.2.2 Animal studies

6.1.3 Dermal

6.1.3.1 Human studies

6.1.3.2 Animal studies

6.2 Distribution

6.3 Excretion

6.4 Biological half-life

6.5 Zinc status and metabolic role in humans

6.5.1 Methods for assessment of zinc status in humans

6.5.1.1 Dietary methods to predict the proportion of the population at risk of inadequate intakes of dietary zinc

6.5.1.2 Static tests of zinc status

6.5.1.3 Functional tests of zinc status

6.5.1.4 New approaches

6.5.2 Metabolic role

6.5.2.1 Zinc metalloenzymes

6.5.2.2 Metallothionein

6.5.2.3 Other metabolic functions of zinc

6.5.3 Human studies

6.5.3.1 Copper

7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS

7.1 Single exposure

7.1.1 Lethality

7.1.2 Acute studies: summary of key findings

7.2 Short-term exposure

7.2.1 Oral exposure

7.2.2 Inhalation exposure

7.3 Long-term exposure

7.3.1 Oral exposure

7.4 Skin irritation

7.5 Reproductive toxicity, embryotoxicity and teratogenicity

7.6 Mutagenicity and related end-points

7.6.1 In vitro studies

7.6.2 In vivo studies

7.7 Carcinogenicity

7.8 Interactions with other metals

7.8.1 Zinc and copper

7.8.2 Zinc and other metals

7.9 Zinc deficiency in animals

8. EFFECTS ON HUMANS

8.1 Human dietary zinc requirements

8.1.1 Estimation of zinc requirements

8.2 Zinc deficiency

8.2.1 Clinical manifestations

8.2.2 Brain function

8.2.3 Immune function

8.2.4 Growth

8.2.5 Dermal effects

8.2.6 Reproduction

8.2.7 Carcinogenicity

8.3 Zinc toxicity: general population

8.3.1 Poisoning incidents

8.3.2 Dermal effects

8.3.3 Immune function

8.3.4 Reproduction

8.3.5 Zinc-induced copper deficiency

8.3.5.1 Controlled human studies

8.3.5.2 Case reports

8.3.6 Serum lipids and cardiovascular disorders

8.4 Occupational exposure

8.4.1 Acute toxicity

8.4.2 Short-term exposure

8.4.3 Long-term exposure

8.4.4 Epidemiological studies

8.5 Subpopulations at special risk

8.5.1 Dialysis patients

8.5.2 People with diabetes

8.5.3 Hospital patients

8.5.4 Other populations

8.6 Interactions

8.6.1 Copper

8.6.2 Iron

8.6.3 Calcium

9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD

9.1 Laboratory experiments

9.1.1 Microorganisms

9.1.1.1 Water

9.1.1.2 Soil

9.1.2 Aquatic organisms

9.1.2.1 Plants

9.1.2.2 Invertebrates and vertebrates

9.1.2.3 Effects on communities

9.1.3 Terrestrial organisms

9.1.3.1 Plants

9.1.3.2 Invertebrates

9.1.3.3 Vertebrates

9.2 Tolerance to zinc

9.3 Interactions with other metals

10. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT

10.1 Homeostatic model

10.2 Evaluation of risks to human health

10.2.1 Exposure of general population

10.2.2 Occupational exposure

10.2.3 Risks of zinc deficiency

10.2.4 Risks of zinc excess

10.3 Evaluation of effects on the environment

10.3.1 Environmental risk assessment

10.3.2 Components of risk assessment for essential elements

10.3.3 Environmental risk assessment for zinc

10.3.3.1 Environmental concentrations

10.3.3.2 Overview of toxicity data

11. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT

11.1 Human health

11.2 Environment

12. RECOMMENDATIONS FOR FURTHER RESEARCH

12.1 Zinc status

12.2 Functional indices of zinc status

12.3 Interactions with other trace elements

12.4 Supplementation

12.5 Occupational medicine

12.6 The molecular mechanism

12.7 Environment

REFERENCES

RÉSUMÉ ET CONCLUSIONS

RESUMEN Y CONCLUSIONES

 

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 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, Case postale 356, 1219 Châtelaine, Geneva, Switzerland (telephone no. + 41 22 - 9799111, fax no. + 41 22 - 7973460, E-mail irptc@unep.ch).

* * *

This publication was made possible by grant number 5 U01 ES02617-15 from the National Institute of Environmental Health Sciences, National Institutes of Health, USA, and by financial support from the European Commission.

Environmental Health Criteria

PREAMBLE

Objectives

In 1973 the WHO Environmental Health Criteria Programme was initiated with the following objectives:

(i)

to assess information on the relationship between exposure to environmental pollutants and human health, and to provide guidelines for setting exposure limits;

(ii)

to identify new or potential pollutants;

(iii)

to identify gaps in knowledge concerning the health effects of pollutants;

(iv)

to promote the harmonization of toxicological and epidemiological methods in order to have internationally comparable results.

The first Environmental Health Criteria (EHC) monograph, on mercury, was published in 1976 and since that time an ever-increasing number of assessments of chemicals and of physical effects have been produced. In addition, many EHC monographs have been devoted to evaluating toxicological methodology, e.g. for genetic, neurotoxic, teratogenic and nephrotoxic effects. Other publications have been concerned with epidemiological guidelines, evaluation of short-term tests for carcinogens, biomarkers, effects on the elderly and so forth.

Since its inauguration the EHC Programme has widened its scope, and the importance of environmental effects, in addition to health effects, has been increasingly emphasized in the total evaluation of chemicals.

The original impetus for the Programme came from World Health Assembly resolutions and the recommendations of the 1972 UN Conference on the Human Environment. Subsequently the work became an integral part of the International Programme on Chemical Safety (IPCS), a cooperative programme of UNEP, ILO and WHO. In this manner, with the strong support of the new partners, the importance of occupational health and environmental effects was fully recognized. The EHC monographs have become widely established, used and recognized throughout the world.

The recommendations of the 1992 UN Conference on Environment and Development and the subsequent establishment of the Intergovernmental Forum on Chemical Safety with the priorities for action in the six programme areas of Chapter 19, Agenda 21, all lend further weight to the need for EHC assessments of the risks of chemicals.

Scope

The criteria monographs are intended to provide critical reviews on the effect on human health and the environment of chemicals and of combinations of chemicals and physical and biological agents. As such, they include and review studies that are of direct relevance for the evaluation. However, they do not describe every study carried out. Worldwide data are used and are quoted from original studies, not from abstracts or reviews. Both published and unpublished reports are considered and it is incumbent on the authors to assess all the articles cited in the references. Preference is always given to published data. Unpublished data are used only when relevant published data are absent or when they are pivotal to the risk assessment. A detailed policy statement is available that describes the procedures used for unpublished proprietary data so that this information can be used in the evaluation without compromising its confidential nature (WHO (1999) Guidelines for the Preparation of Environmental Health Criteria. PCS/99.9, Geneva, World Health Organization).

In the evaluation of human health risks, sound human data, whenever available, are preferred to animal data. Animal and in vitro studies provide support and are used mainly to supply evidence missing from human studies. It is mandatory that research on human subjects is conducted in full accord with ethical principles, including the provisions of the Helsinki Declaration.

The EHC monographs are intended to assist national and international authorities in making risk assessments and subsequent risk management decisions. They represent a thorough evaluation of risks and are not, in any sense, recommendations for regulation or standard setting. These latter are the exclusive purview of national and regional governments.

Content

The layout of EHC monographs for chemicals is outlined below.

Summary a review of the salient facts and the risk evaluation of the chemical

Identity physical and chemical properties, analytical methods

Sources of exposure

Environmental transport, distribution and transformation

Environmental levels and human exposure

Kinetics and metabolism in laboratory animals and humans

Effects on laboratory mammals and in vitro test systems

Effects on humans

Effects on other organisms in the laboratory and field

Evaluation of human health risks and effects on the environment

Conclusions and recommendations for protection of human health and the environment

Further research

Previous evaluations by international bodies, e.g. IARC, JECFA, JMPR

Selection of chemicals

Since the inception of the EHC Programme, the IPCS has organized meetings of scientists to establish lists of priority chemicals for subsequent evaluation. Such meetings have been held in Ispra, Italy, 1980; Oxford, United Kingdom, 1984; Berlin, Germany, 1987; and North Carolina, USA, 1995. The selection of chemicals has been based on the following criteria: the existence of scientific evidence that the substance presents a hazard to human health and/or the environment; the possible use, persistence, accumulation or degradation of the substance shows that there may be significant human or environmental exposure; the size and nature of populations at risk (both human and other species) and risks for environment; international concern, i.e. the substance is of major interest to several countries; adequate data on the hazards are available.

If an EHC monograph is proposed for a chemical not on the priority list, the IPCS Secretariat consults with the Cooperating Organizations and all the Participating Institutions before embarking on the preparation of the monograph.

Procedures

The order of procedures that result in the publication of an EHC monograph is shown in the flow chart on p. xv. A designated staff member of IPCS, responsible for the scientific quality of the document, serves as Responsible Officer (RO). The IPCS Editor is responsible for layout and language. The first draft, prepared by consultants or, more usually, staff from an IPCS Participating Institution, is based initially on data provided from the International Register of Potentially Toxic Chemicals, and reference data bases such as Medline and Toxline.

The draft document, when received by the RO, may require an initial review by a small panel of experts to determine its scientific quality and objectivity. Once the RO finds the document acceptable as a first draft, it is distributed, in its unedited form, to well over 150 EHC contact points throughout the world who are asked to comment on its completeness and accuracy and, where necessary, provide additional material. The contact points, usually designated by governments, may be Participating Institutions, IPCS Focal Points, or individual scientists known for their particular expertise. Generally some four months are allowed before the comments are considered by the RO and author(s). A second draft incorporating comments received and approved by the Director, IPCS, is then distributed to Task Group members, who carry out the peer review, at least six weeks before their meeting.

The Task Group members serve as individual scientists, not as representatives of any organization, government or industry. Their function is to evaluate the accuracy, significance and relevance of the information in the document and to assess the health and environmental risks from exposure to the chemical. A summary and recommendations for further research and improved safety aspects are also required. The composition of the Task Group is dictated by the range of expertise required for the subject of the meeting and by the need for a balanced geographical distribution.

EHC Preparation Flow Chart

The three cooperating organizations of the IPCS recognize the important role played by nongovernmental organizations. Representatives from relevant national and international associations may be invited to join the Task Group as observers. Although observers may provide a valuable contribution to the process, they can only speak at the invitation of the Chairperson. Observers do not participate in the final evaluation of the chemical; this is the sole responsibility of the Task Group members. When the Task Group considers it to be appropriate, it may meet in camera.

All individuals who as authors, consultants or advisers participate in the preparation of the EHC monograph must, in addition to serving in their personal capacity as scientists, inform the RO if at any time a conflict of interest, whether actual or potential, could be perceived in their work. They are required to sign a conflict of interest statement. Such a procedure ensures the transparency and probity of the process.

When the Task Group has completed its review and the RO is satisfied as to the scientific correctness and completeness of the document, it then goes for language editing, reference checking and preparation of camera-ready copy. After approval by the Director, IPCS, the monograph is submitted to the WHO Office of Publications for printing. At this time a copy of the final draft is sent to the Chairperson and Rapporteur of the Task Group to check for any errors.

It is accepted that the following criteria should initiate the updating of an EHC monograph: new data are available that would substantially change the evaluation; there is public concern for health or environmental effects of the agent because of greater exposure; an appreciable time period has elapsed since the last evaluation.

All Participating Institutions are informed, through the EHC progress report, of the authors and institutions proposed for the drafting of the documents. A comprehensive file of all comments received on drafts of each EHC monograph is maintained and is available on request. The Chairpersons of Task Groups are briefed before each meeting on their role and responsibility in ensuring that these rules are followed.

WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR ZINC

Members

Dr H.E. Allen, Department of Civil and Environmental Engineering, University of Delaware, Newark, Delaware, USA

Dr G. Batley, CSIRO Centre for Advanced Analytical Chemistry, Division of Coal and Energy Technology, Lucas Heights Research Laboratories, Menai, Australia

Dr G. Cherian, Department of Pathology, University of Western Ontario, London, Ontario, Canada (Vice-Chairman)

Dr G. Dixon, Department of Biology, University of Waterloo, Waterloo, Ontario, Canada

Professor W.H.O. Ernst, Vrije University, Amsterdam, the Netherlands

Professor R. Gibson, Department of Human Nutrition, University of Otago, Dunedin, New Zealand

Dr C.R. Janssen, University of Ghent, Laboratory for Biological Research in Aquatic Pollution, Ghent, Belgium

Dr L.M. Klevay, US Department of Agriculture, Grand Forks Human Nutrition Research Center, Grand Forks, North Dakota, USA

Mr H. Malcom, Institute of Terrestrial Ecology, Monks Wood, Huntingdon, Cambridgeshire, United Kingdom (Co-Rapporteur)

Dr L. Maltby, Department of Animal and Plant Sciences, School of Biological Sciences, University of Sheffield, Sheffield, United Kingdom

Professor M.R. Moore, University of Queensland, National Research Centre for Environmental Toxicology, Coopers Plains, Brisbane, Australia

Dr G. Nordberg, Department of Occupational and Environmental Medicine, Environmental Medicine Unit, Umea University, Umea, Sweden

Dr H.H. Sandstead, University of Texas School of Medicine, Department of Preventive Medicine and Community Health, Galveston, Texas, USA

Dr B. Simon-Hettich, Fraunhofer Institute of Toxicology and Aerosol Research, Hanover, Germany

Dr J.H.M. Temmink, Wageningen Agricultural University, Department of Toxicology, Wageningen, Netherlands (Chairman)

Dr J. Vangronsveld, Limburgs University Centre, University Campus, Diepenbeek, Belgium

Dr D. Wagner, Chemicals Safety Unit, Human Services and Health, Canberra, Australia (Co-Rapporteur)

Observers/Representatives

Dr K. Bentley, Commonwealth Department of Health and Family Services, Canberra, Australia

Dr C. Boreiko, International Lead Zinc Research Organization, Inc., Research Triangle Park, North Carolina, USA

Dr P. Chapman, EVS Environment Consultants, Ltd., North Vancouver, Canada (Representing the International Lead Zinc Research Organization)

Dr T.M. Florence, Centre for Environmental Health Sciences, Oyster Bay, New South Wales, Australia

Dr T.V. ODonnell, University of Otago, Wellington South, New Zealand

Mr D. Sinclair, Pasminco Ltd., Melbourne, Victoria, Australia

Dr L. Tomaska, Australia New Zealand Food Authority, Canberra, Australian Capital Territory, Australia

Dr F. Van Assche, European Zinc Institute, Brussels, Belgium

Dr W.J.M. Van Tilborg, Rozendaal, Netherlands (Representing the European Chemical Industry Ecology and Toxicology Centre)

Mr H. Waeterschoot, Union Minière, Brussels, Belgium (Representing the International Zinc Association)

Secretariat

Dr G.C. Becking, International Programme on Chemical Safety, World Health Organization, Interregional Research Unit, Research Triangle Park, North Carolina, USA (Secretary)

Mr P. Callan, Environmental Health Policy, Department of Health and Family Services, Canberra, Australian Capital Territory, Australia

Dr A. Langley, Hazardous Substances Section, South Australia Health Commission, Adelaide, South Australia, Australia

Mr S. Mangas, Hazardous Substances Section, South Australian Health Commission, Adelaide, South Australia, Australia

WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR ZINC

A WHO Task Group on Environmental Health Criteria for Zinc met in McLaren Vale, Australia, from 16 to 20 September 1996. The meeting was sponsored by a consortium of Australian Commonwealth and State Governments through a national steering committee chaired by Dr K. Bentley, Commonwealth Department of Health and Family Services, Canberra. The meeting was co-hosted and organized by the South Australian Health Commission, Dr A. Langley and Mr S. Mangas being responsible for the arrangements. Participants were welcomed on behalf of the host organizations by Dr I. Calder, Director, Environmental Health Branch, South Australian Health Commission. Dr G.C. Becking, IPCS, opened the meeting and, on behalf of the Director, IPCS and the three cooperating organizations (UNEP/ILO/WHO), thanked the Australian Commonwealth and State Governments for their funding of the Task Group as well as their financial and in-kind support for the preparation of the first draft of the Environmental Health Criteria for Zinc. He thanked the staff of the Hazardous Substances Section, South Australian Health Commission for their excellent work in organizing the Task Group. The Task Group reviewed and revised the draft criteria monograph, and made an evaluation of the risks to human health and the environment from exposure to zinc.

The first draft of this monograph was prepared by Dr B. Simon-Hettich and Dr A. Wibbertmann, Fraunhofer Institute of Toxicology and Aerosol Research, Hanover, Germany; Mr D. Wagner, Commonwealth Department of Health and Family Services, Canberra, Australia; Dr L. Tomaska, Australia New Zealand Food Authority (ANZFA), Canberra, Australia, and Mr H. Malcolm, Institute of Terrestrial Ecology, Monks Wood, United Kingdom. The draft reviewed by the Task Group, incorporating the comments received from the IPCS Contact Points, was prepared through the cooperative efforts of the Commonwealth Department of Health and Family Services, ANZFA, Institute of Terrestrial Ecology, and the Secretariat.

Dr G.C. Becking (IPCS Central Unit, Interregional Research Unit) and Ms S.M. Poole (Birmingham, England) were responsible for the overall scientific content and technical editing, respectively, of this monograph.

The efforts of all who helped in the preparation and finalization of the monograph are gratefully acknowledged.

ABBREVIATIONS

AAS

atomic absorption spectroscopy

AES

atomic emission spectroscopy

ASV

anodic stripping voltametry

BAF

bioaccumulation factor

BCF

bioconcentration factor

CRIP

cysteine-rich intestinal protein

CSV

cathodic stripping voltametry

DNA

deoxyribonucleic acid

DP-ASV

differential pulse-anodic stripping voltametry

DTPA

diethylenetriamine pentaacetic acid

dw

dry weight

Eh

redox potential

EC50

effective concentration, affecting 50% of test organisms

EDTA

ethylenediaminetetraacetic acid

EPA

Environmental Protection Agency (USA)

ESOD

Cu, Zn erythrocyte superoxide dismutase

FAAS

flame atomic absorption spectroscopy

GF-AAS

graphite furnace atomic absorption spectroscopy

HDL

high-density lipoprotein

ICP-AES

inductively-coupled plasma-atomic emission spectroscopy

ICP-MS

inductively-coupled plasma-mass spectrometry

Ig

immunoglobulin

IGF

insulin-like growth factor

LC50

lethal concentration killing 50% of test organisms

LDL

low-density lipoprotein

LOEC

lowest-observed-effective concentration

LT(50)

lethal time(50) for specified concentration of chemical killing 50% of test organisms

MS

mass spectrometry

NAA

neutron activation analysis

NHANES

National Health and Nutrition Examination Survey (USA)

NOEC

no-observed-effect concentration

NOEL

no-observed-effect level

RNA

ribonucleic acid

SEM

standard error of the mean

TFIIIA

transcription factor IIIA

UV

ultraviolet

XRF

X-ray fluorescence

 

1. SUMMARY AND CONCLUSIONS

1.1 Identity, and physical and chemical properties

Zinc metal does not occur in the natural environment. It is present only in the divalent state Zn(II). Ionic zinc is subject to solvation, and its solubility is pH and anion dependent. Zinc is a transition element and is able to form complexes with a variety of organic ligands. Organometallic zinc compounds do not exist in the environment.

1.2 Analytical methods

Because zinc is ubiquitous in the environment, special care is required during sampling, sample preparation and analysis to avoid sample contamination. Sample preparation for solid samples typically involves microwave-assisted mineralization with concentrated acids. For water samples, solvent extraction in the presence of complexing agents and chelating resin separation have been used to preconcentrate zinc.

Inductively-coupled plasma atomic emission spectrometry (ICP-AES), graphite furnace atomic absorption spectrometry (GF-AAS), anodic stripping voltammetry (ASV) and ICP-mass spectrometry (ICP-MS) are commonly used instrumental techniques for zinc determination. For low-level analyses, GF-AAS, ASV and ICP-MS are preferred.

With special care, zinc concentrations as low as 0.006 g/litre and 0.01 mg/kg are detectable in water and solid samples, respectively.

Speciation analyses in water require the application of separation techniques with any of the above methods or use of the labile-bound discrimination offered by ASV.

1.3 Sources of human and environmental exposure

Most rocks and many minerals contain zinc in varying amounts. Commercially, sphalerite (ZnS) is the most important ore mineral and the principal source of the metal for the zinc industry. In 1994, world metal production of zinc was 7 089 000 tonnes and zinc metal consumption amounted to 6 895 000 tonnes.

Zinc is widely used as a protective coating of other metals, in dye casting and the construction industry, and for alloys. Inorganic zinc compounds have various applications, e.g., for automotive equipment, storage and dry cell batteries, and dental, medical and household applications. Organo-zinc compounds are used as fungicides, topical antibiotics and lubricants.

Zinc becomes malleable when heated to 100150 C and is then readily machined into shapes. It is capable of reducing most other metal states and is therefore used as an electrode in dry cells and in hydrometallurgy.

The largest natural emission of zinc to water results from erosion. Natural inputs to air are mainly due to igneous emissions and forest fires. Anthropogenic and natural sources are of a similar magnitude. The main anthropogenic sources of zinc are mining, zinc production facilities, iron and steel production, corrosion of galvanized structures, coal and fuel combustion, waste disposal and incineration, and the use of zinc-containing fertilizers and pesticides.

1.4 Environmental transport, distribution and transformation

Zinc in the atmosphere is primarily bound to aerosol particles. The size of particle is determined by the source of zinc emission. A major proportion of the zinc released from industrial processes is adsorbed on particles that are small enough to be in the respirable range.

The transport and distribution of atmospheric zinc vary according to the size of particles and the properties of the zinc compounds concerned. Zinc is removed from the atmosphere by dry and wet deposition. Zinc adsorbed on particles with low densities and diameters can be transported over long distances.

The distribution and transport of zinc in water, sediment and soil are dependent upon the species of zinc present and the characteristics of the environment. The solubility of zinc is primarily determined by pH. At acidic pH values, zinc may be present in the aqueous phase in its ionic form. Zinc may precipitate at pH values greater than 8.0. It may also form stable organic complexes, for example, with humic and fulvic acids. The formation of such complexes can increase the mobility and/or solubility of zinc. Zinc is unlikely to be leached from soil owing to its adsorption on clay and organic matter. Acidic soils and sandy soils with a low organic content have a reduced capacity for zinc absorption.

Zinc is an essential element and in vivo levels are therefore regulated by most organisms. Zinc is not biomagnified. The absorption of zinc by aquatic animals tends to be from water rather than food. Only dissolved zinc tends to be bioavailable, and bioavailability depends on the physical and chemical characteristics of the environment and biological processes. Consequently, environmental assessment must be conducted on a site-specific basis.

1.5 Environmental concentrations

Zinc occurs ubiquitously in environmental and biological samples. Concentrations in soil sediments and fresh water are strongly determined by local geological and anthropogenic influences and thus vary substantially. Natural background total zinc concentrations are usually < 0.150 g/litre in fresh water, 0.002- 0.1 g/litre in seawater, 10300 mg/kg dry weight (dw) in soils, up to 100 mg/kg dw in sediments, and up to 300 ng/m3 in air. Increased levels can be attributed to natural occurrence of zinc-enriched ores, to anthropogenic sources or to abiotic and biotic processes. In anthropogenically contaminated samples, zinc levels of up to 4 mg/litre in water, 35 g/kg in soil, 15 g/litre in estuarine water, and 8 g/m3 in air are found.

Zinc concentrations in representative organisms during exposure to water-borne zinc are in the range 2002000 mg/kg.

Concentrations in plants and animals are higher near anthropogenic point sources of zinc contamination. Interspecies variations in zinc content are considerable; intraspecies levels vary, for instance, with life stage, sex, season, diet and age. Normal levels of zinc in most crops and pastures are in the range 10100 mg/kg dw. Some plants are zinc accumulators, but the extent of the accumulation in plant tissues varies with soil and plant properties.

Only negligible quantities of zinc are inhaled from ambient air, but a broad range of exposures to dusts and fumes of zinc and zinc compounds is possible in occupational settings.

1.5.1 Human intakes

Estimated ranges of daily dietary intakes of total zinc are 5.6- 10 mg/day for infants and children aged 2 months11 years, 12.313.0 mg/day for children aged 1219 years, and 8.8- 14.4 mg/day for adults aged 2050 years. Mean daily zinc intake from drinking-water is estimated to be < 0.2 mg/day.

Dietary reference values for zinc vary according to the dietary pattern of the country, assumptions on the bioavailability of dietary zinc, and age, sex and physiological status. Dietary reference values range from 3.3 to 5.6 mg/day for infants aged 012 months, 3.8 to 10.0 mg/day for children aged 110 years, and 8.7 to 15 mg/day for adolescents aged 1118 years. Adult values range from 6.7 to 15 mg/day for those aged 1950 years, 7.3 to 15 mg/day during pregnancy, assuming diets of moderate zinc availability, and 11.7 to 19 mg/day during lactation, depending on the stage.

1.6 Kinetics and metabolism in laboratory animals and humans

For inhalation studies (nose only) in guinea-pigs, rats and rabbits, retention values of 520% in the lung were observed after exposure to zinc oxide aerosols at a concentration of 512 mg/m3 for 36 h. The intestinal absorption of zinc is controlled by a homeostatic mechanism which is not fully understood but is mainly controlled by pancreatic and intestinal secretion and faecal excretion. Homeostasis may involve metal-binding proteins such as metallothionein and cysteine-rich intestinal protein. Other unknown mechanisms may also exist. The uptake from intestinal mucosa may involve both active and passive transport processes. In animals, absorption can vary in the range 1040% depending on nutritional status and other ligands in the diet. Dermal absorption of zinc from zinc oxide and zinc chloride can occur and is increased in zinc deficiency. Absorbed zinc is mainly deposited in muscle, bone, liver, pancreas, kidney and other organs. The biological half-life of zinc is about 450 days in rats, depending on the administered dose, and about 280 days in humans.

1.7 Effects on laboratory animals

Acute oral toxicity in rodents exposed to zinc is low, with LD50 values in the range 30600 mg/kg body weight, depending on the zinc salt administered. Acute effects in rodents following inhalation or intratracheal instillation of zinc compounds include respiratory distress, pulmonary oedema and infiltration of the lung by leukocytes.

Toxic effects of zinc in rodents following short-term oral exposure include weakness, anorexia, anaemia, diminished growth, loss of hair and lowered food utilization, as well as changes in the levels of liver and serum enzymes, morphological and enzymatic changes in the brain, and histological and functional changes in the kidney. The level at which zinc produces no adverse symptoms in rats has been set at about 160 mg/kg body weight. Pancreatic changes were observed in calves exposed to high levels of dietary zinc. Short-term inhalation exposure of guinea-pigs and rats to zinc oxide at concentrations of > 5.9 mg/m3 resulted in inflammation and pulmonary damage.

Long-term oral exposure to zinc indicated the target organs of toxicity to be the haematopoietic system in rats, ferrets and rabbits; the kidney in rats and ferrets; and the pancreas in mice and ferrets. The no-observed-effect level (NOEL) with respect to growth and anaemia for zinc sulfate in the diet was reported to be < 100 mg/kg in rats. Increases in zinc concentrations in the bodies of experimental animals exposed to zinc are accompanied by reduced levels of copper, suggesting that some of the signs of toxicity ascribed to exposure to excess levels of zinc may be caused by zinc-induced copper deficiency. Moreover, studies have shown that exposure to zinc alters the levels of other essential metals, including iron, in the bodies of exposed animals. Some signs of toxicity observed in animals exposed to high levels of zinc can be alleviated by the addition of copper or iron to the diet.

Very high levels of zinc are toxic to pregnant mice and hamsters. Rats exposed to zinc at 0.5% and 1% in the diet for 5 months were unable to conceive until the zinc was withdrawn. High levels of zinc in the diet (2000 mg/kg) were also associated with an increase in resorptions and stillbirths in mice and rats; a finding also observed in sheep and hamsters. Resorptions were increased in one study in which rats were exposed, throughout the entire gestation period, to zinc at doses as low as 150 mg/kg. In another rat study, however, no deleterious effects on the developing fetus were observed at doses of 500 mg/kg. Exposure of rats to dietary zinc levels of 4000 mg/kg post coitus was shown to interfere with the implantation of ova. Elevation of zinc levels in rat pups exposed to zinc was accompanied by reductions in the levels of copper and iron.

Genotoxicity studies have been conducted in a variety of systems. Most of the findings have been negative, but a few positive results have been reported.

Zinc deficiency in animals is characterized by reduction in growth, cell replication, adverse reproductive effects, adverse developmental effects, which persist after weaning, and reduced immunoresponsiveness.

1.8 Effects on humans

Poisoning incidents with symptoms of gastrointestinal distress, nausea and diarrhoea have been reported after a single or short-term exposure to concentrations of zinc in water or beverages of 1000- 2500 mg/litre. Similar symptoms, occasionally leading to death, have been reported following the inadvertent intravenous administration of large doses of zinc. Kidney dialysis patients exposed to zinc through the use of water stored in galvanized units have developed symptoms of zinc toxicity that were reversible when the water was subjected to activated carbon filtration.

A disproportionate intake of zinc in relation to copper has been shown to induce copper deficiency in humans, resulting in increased copper requirements, increased copper excretion and impaired copper status. Pharmacological intakes of zinc have been associated with effects ranging from leukopenia and/or hypochromic microcytic anaemia to decreases in serum high-density lipoprotein concentrations. These conditions were reversible upon discontinuation of zinc therapy together with copper supplementation.

The human health effects associated with zinc deficiency are numerous, and include neurosensory changes, oligospermia, impaired neuropsychological functions, growth retardation, delayed wound healing, immune disorders and dermatitis. These conditions are generally reversible when corrected by zinc supplementation.

There is no single, specific and sensitive biochemical index of zinc status. The most reliable method for detecting deficiency is to show a positive response to zinc supplementation in controlled double-blind trials (in the absence of other limiting nutrient deficiencies). This approach is time-consuming and often impractical, however, and determination of a combination of dietary, biochemical and functional physiological indices is generally preferred. Several concordant abnormal values are more reliable than a single aberrant value in diagnosing a zinc deficiency state. The inclusion of functional physiological indices, such as growth, taste acuity and dark adaptation with a biochemical test (e.g., plasma or hair zinc concentration) allows the extent of the functional consequences of the zinc deficiency state to be assessed.

Inhalation exposure to zinc chloride following the military use of "smoke bombs" has resulted in effects that include interstitial oedema, interstitial fibrosis, pneumonitis, bronchial mucosal oedema, ulceration and even death under extreme exposure conditions in confined spaces. These effects are possibly attributable to the hygroscopic and astringent nature of the particles released by such devices.

Occupational exposure to finely dispersed particulate matter formed when certain metals, including zinc, are volatilized can lead to an acute illness termed "metal-fume fever", characterized by a variety of symptoms including fever, chills, dyspnoea, nausea and fatigue. The condition is generally severe but transient, and individuals tend to develop tolerance. Exposure of volunteers to zinc concentrations of 77150 mg/m3 for 1530 min gave rise to symptoms in some of the subjects, a marked dose-related inflammatory response with increased polynuclear lymphocytes in broncheoalveolar lavage fluid, and a marked increase in cytokines. Occupational asthma has been reported among those working with soft solder fluxes, but the evidence was not sufficient to indicate a causative relationship. A rare case suggesting such a relationship has been diagnosed recently in a worker from a hot-dip (zinc) galvanizing plant.

1.9 Effects on other organisms in the laboratory and field

Zinc is important in membrane stability, in over 300 enzymes, and in the metabolism of proteins and nucleic acids. The adverse effects of zinc must be balanced against its essentiality. Zinc deficiency has been reported in a wide variety of cultivated plants and animals, with severe effects on all stages of reproduction, growth and tissue proliferation. Zinc deficiencies in various crops have resulted in large crop losses worldwide. Zinc deficiency is rare in aquatic organisms in the environment, but can be induced under experimental conditions.

The toxicity of zinc can be influenced by both biotic and abiotic factors, such as organism age and size, prior exposure, water hardness, pH, dissolved organic carbon and temperature. The integration of environmental chemistry and toxicology has allowed a better prediction of the effects on organisms in the environment. This has led to the now accepted view that the total concentration of an essential element such as zinc in an environmental compartment is not, taken alone, a good predictor of its bioavailability.

Acute toxicity values of dissolved zinc to freshwater invertebrates range from 0.07 mg/litre for a water flea to 575 mg/litre for an isopod. Acute toxicity values for marine invertebrates range from 0.097 mg/litre for a mysid to 11.3 mg/litre for a grass shrimp. Acutely lethal concentrations for freshwater fish are in the range 0.0662.6 mg/litre; the range for marine fish is 0.1917.66 mg/litre.

Zinc has been shown to exert adverse reproductive, biochemical, physiological and behavioural effects on a variety of aquatic organisms. Zinc concentrations of > 20 g/litre have been shown to have adverse effects on aquatic organisms. However, the toxicity of zinc to such organisms is influenced by many factors, such as the temperature, hardness and pH of the water, and previous zinc exposure.

Zinc toxicity in plants generally causes disturbances in metabolism, which are different from those occurring in zinc deficiency. The critical leaf tissue concentration of zinc for an effect on growth in most species is in the range 200300 mg/kg dw.

Field studies have revealed adverse effects on aquatic invertebrates, fish and terrestrial plants close to sources of zinc contamination. Zinc tolerances in terrestrial plants, algae, microorganisms and invertebrates have developed in the vicinity of areas with elevated zinc concentrations.

1.10 Conclusions

1.10.1 Human health

There is a decreasing trend in anthropogenic zinc emissions.

Many pre-1980 environmental samples, in particular in water samples, may have been subject to contamination with zinc during sampling and analysis and, for this reason, zinc concentration data for such samples should be viewed with extreme caution.

In countries where staple diets are based on unrefined cereals and legumes, and intakes of flesh foods are low, dietary strategies should be developed to improve the content and bioavailability of zinc.

Preparations intended to increase the zinc intake above that provided by the diet should not contain zinc levels that exceed dietary reference values, and should contain sufficient copper to ensure a ratio of zinc to copper of approximately 7, as is found in human milk.

There is a need for better documentation of actual exposures to zinc oxide fume in occupational settings. Workplace concentrations should not result in exposure levels as high as those known to have given rise to inflammatory responses in the lungs of volunteers.

The essential nature of zinc, together with its relatively low toxicity in humans and the limited sources of human exposure, suggests that normal, healthy individuals not exposed to zinc in the workplace are at potentially greater risk from the adverse effects associated with zinc deficiency than from those associated with normal environmental exposure to zinc.

1.10.2 Environment

Zinc is an essential element in the environment. The possibility exists both for a deficiency and for an excess of this metal. For this reason it is important that regulatory criteria for zinc, while protecting against toxicity, are not set so low as to drive zinc levels into the deficiency area.

There are differences in the responses of organisms to deficiency and excess.

Zinc bioavailability is affected by biotic and abiotic factors, for instance: organism age and size, prior history of exposure, water hardness, pH, dissolved organic carbon and temperature.

The total concentration of an essential element such as zinc, alone, is not a good predictor of its bioavailability or toxicity.

There is a range of optimum concentrations for essential elements such as zinc.

The toxicity of zinc will depend on environmental conditions and habitat types, thus any risk assessment of the potential effects of zinc on organisms must take into account local environmental conditions.

 

2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES,
AND ANALYTICAL METHODS

Zinc is the twenty-fifth most abundant element. It is widely found in nature and makes up 0.02% by weight of the earths crust (Budavari, 1989). Zinc normally appears dull grey owing to coating with an oxide or basic carbonate. It is extremely rare to find zinc metal free in nature (Beliles, 1994).

Some zinc compounds, synonyms and formulae are given in Table 1.

Table 1. Chemical names, synonyms and formulae of elemental zinc and zinc compounds

Chemical name

CAS registry number

Formula

Synonyms

Zinc

7440-66-6

Zn

-

Zinc acetate

557-34-6

Zn(C2H3O2)2

-

Zinc arsenite

10326-24-6

Zn(AsO2)2

zinc meta-arsenite, ZMA

Zinc bromide

7699-45-8

ZnBr2

-

Zinc carbonate

3486-35-9

ZnCO3

-

Zinc chloride

7646-85-7

ZnCl2

butter of zinc

Zinc cyanide

557-21-1

Zn(CN)2

-

Zinc diethyldithiocarbamate

14324-55-1

Zn[SC(S)N(C2H5)2]2

-

Zinc fluoride

7783-49-5

ZnF2

-

Zinc hexafluorosilicate

16871-71-9

ZnSiF6.6H2O

zinc silicofluoride; zinc fluosilicate

Zinc iodide

10139-47-6

ZnI2

-

Zinc laurate

-

Zn(C12H33O2)2

-

Zinc nitrate

7779-88-6

Zn(NO3)2

-

Zinc oleate

557-07-3

Zn(C17H33COO)2

-

Zinc oxide

1314-13-2

ZnO

Chinese white; zinc white; flowers of zinc; philosopher's wool

Zinc permanganate

23414-72-4

Zn(MnO4)2.6H2O

 

Zinc peroxide

1314-22-3

ZnO2

zinc dioxide; zinc superoxide; ZPO

Zinc-1,4-phenolsulfonate

127-82-2

Zn (SO3C6H4OH)2.8H2O

p-hydroxybenzenesulfonic acid zinc salt; zinc sulfophenate; zinc sulfocarbolate

Zinc phosphate

7779-90-0

Zn3(PO4)2

zinc orthophosphate; zinc phosphate, tribasic

Zinc phosphide

1314-84-7

Zn3P2

-

Zinc silicate

13597-65-4

Zn2SiO4

zinc orthosilicate

Zinc sulfate

7733-02-0

ZnSO4.7H2O

white vitriol; white copperas; zinc vitriol

Zinc sulfide

1314-98-3

ZnS

wurtzite; sphalerite; zinc blende

Zinc telluride

1315-11-3

ZnTe

-

Zinc thiocyanate

557-42-6

Zn(SCN)2

zinc thodanide; zinc sulfocyanate

Zinc dimethyldithiocarbamate

137-30-4

Zn(SCSNCH3CH3)2

Ziram

Zinc ethylene-bis(dithiocarbamate)

12122-67-7

Zn(CS2NHCH2)2

Zineb

 

2.1 Identity

Pure zinc is bluish-white and lustrous when polished. It has the atomic number of 30 and the relative atomic mass of 65.38, and belongs to group 2b and the fourth period of the periodic table. The configuration of the outermost electrons is 3d104s2. Thus, its valence in chemical compounds is +2. In nature, zinc is a mixture of five stable isotopes: 64Zn (49%), 66Zn (28%), 68Zn (19%), 67Zn (4.1%), and 70Zn (0.62%) (Budavari, 1989). A further 19 radioactive isotopes (57Zn63Zn, 65Zn, 68Zn80Zn) are known; 65Zn is the most stable with a half-life of 243.8 days, but most have very short half-lives (Lide, 1991).

2.2 Physical and chemical properties

2.2.1 Zinc metal

Zinc possesses a low to intermediate hardness (Mohs hardness 2.5) and crystallizes in a distorted hexagonal close-packed structure. Because of its density of 7.13 g/cm3, it is called a heavy metal. It has an electrical conductivity of 28.3% of the international annealed copper standard (Kirk & Othmer, 1982). At ordinary temperatures the metal is too brittle to roll, but it becomes malleable and ductile when heated to 100150 C. At temperatures of > 210 C, zinc becomes brittle and pulverizable, and, at higher temperatures, again soft and malleable (Budavari, 1989; Beliles, 1994). Since zinc is very reactive, it reacts strongly with other elements, such as oxygen, chlorine and sulfur, at elevated temperatures (Melin & Michaelis, 1983). Zinc has reducing and also several transitional properties (see below).

The metal burns in air with a bluish-green flame. It is stable in dry air, but on exposure to moist air it becomes covered with an adherent film of zinc oxide or basic carbonate (2ZnCO33Zn(OH)2), so isolating the underlying metal and retarding further corrosion.

Zinc is amphoteric and dissolves in strong alkalis and mineral acids with evolution of hydrogen and soluble zinc salts. Oxidizing agents or metal ions, e.g., Cu2+, Ni2+ and Co2+, accelerate the dissolution of zinc. Zinc is capable of reducing most metals except aluminium and magnesium (E o(aq) Zn/Zn2+, 0.763 eV; Budavari, 1989).

In solution, four to six ligands can be coordinated with the zinc ion. Complexes are formed with polar ligands, e.g., ammonia, amines, cyanide and halogen ions. Zinc is a reactive amphoteric metal. The hydroxide is precipitated in alkaline solution, but with excess base, it redissolves to form "zincates", ZnO22-, which are hydroxo complexes such as Me+[Zn(OH)3]-, Me2+[Zn(OH)4]2-and Me2+[Zn(OH)4(H2O)2]2- (Budavari, 1989).

2.2.2 Zinc compounds

Zinc has a strong tendency to react with acidic, alkaline and inorganic compounds. Because of its amphoteric properties, zinc forms a variety of salts, which are all nonconducting, nonmagnetic and white or colourless, with the exception of those with a chromophore group, such as chromate. Some physical and chemical data for zinc and selected zinc compounds are given in Table 2.

Table 2. Physical and chemical properties of zinc and some of its compoundsa

Chemical name

Relative atomic/ molecular mass

Melting point
(
C)

Boiling point
(
C)

Relative density (g/cm3)
(
C)

Crystalline form

Solubility

Zinc

65.38

419.58

907

7.14 (25)

distorted hexagonal close packed

soluble acid, alkali; insoluble H2O,

Zinc acetate

183.47

237

200b

1.735

monoclinic

soluble H2O, alcohol

Zinc bromide

225.19

394

690

4.201 (25)

rhombic

soluble H2O, alcohol, ether

Zinc carbonate

125.39

300b

ND

4.398

rhombohedral

soluble acid, alkali; slightly soluble H2O

Zinc chloride

136.29

283

732

2.907 (25)

hexagonal, deliquescent

soluble H2O, acid, acetone, alcohol

Zinc fluoride

103.38

872

ca. 1500

4.95 (25)

monoclinic or triclinic

soluble HCl, HNO3, NH4OH; slightly soluble H2O, aqueous HF

Zinc hexafluorosilicate

207.46

NDb

ND

2.104

crystalline powder

soluble H2O

Zinc hydroxide

99.39

125b

ND

3.053

rhombic

soluble acid, alkali; very slightly soluble H2O

Zinc iodide

319.19

446

624b

4.736 (25)

hexagonal

soluble H2O, alcohol, ether

Zinc nitrate, hexahydrate

297.48

36.4

105131 (-H2O)

2.065 (14)

tetragonal

soluble H2O, alcohol

Zinc oxide

81.38

1975

ND

5.606

hexagonal

soluble dilute acetic acid, alkali; insoluble H2O, alcohol

Zinc phosphate

386.08

900

ND

3.998 (15)

rhombic

soluble acid, NH4OH; insoluble H2O, alcohol

Zinc phosphide

258.09

> 420

1100 (sublimes in H2)

4.55 (13)

tetragonal

soluble benzene, CS2; insoluble H2O, alcohol

Zinc sulfate

161.44

600b

ND

3.54 (25)

rhombic

soluble H2O, MeOH, glycerol

alpha-Zinc sulfide

97.44

1700 20

ND

3.98

hexagonal

very soluble alcohol; insoluble acetic acid

beta-Zinc sulfide

97.44

NDb

ND

4.102 (25)

cubic

very soluble acid

a From: Lide (1991); ND = not determined.

b Decomposition.

Zinc oxide is a coarse white or greyish powder, odourless and with a bitter taste. It absorbs carbon dioxide from the air and is soluble in acids and alkalis but insoluble in water and alcohol. The compound is used as a pigment in paints and as an ultraviolet (UV) absorber in several products. It has the greatest UV absorption of all commercial pigments (Lide, 1991). Its major use (see section 3.2.2) is as a vulcanizing agent in the production of rubber products (Melin & Michaelis, 1983).

Zinc chloride, chlorate, sulfate and nitrate are readily soluble in water, whereas the oxide, carbonate, phosphates, silicates, sulfides and organic complexes are practically insoluble in water, with the exception of zinc diethyldithiocarbamate (Budavari, 1989).

Zinc halogenides are hygroscopic. Zinc chloride forms hydrates with 1.334 mol H2O and exerts a water-extracting and condensing action on many organic compounds. Owing to the high polarizing effect, zinc protolyses part of the water envelope and forms hydroxo complexes. Thus, concentrated zinc chloride solutions react like strong acids because of the formation of the acids H[ZnCl2OH] and H2[ZnCl2(OH)2] (Giesler et al., 1983). Zinc chloride and fluoride have catalytic properties and are used in organic synthesis and also in wood preservation and for antiseptic purposes (Budavari, 1989).

Zinc carbonate occurs naturally as zinc spar. When heated to 150 C, the compound decomposes into zinc oxide and carbon dioxide. Basic zinc carbonate, zinc carbonate hydroxide, is known in variable composition and is usually characterized as 3Zn(OH)2 2ZnCO3. It occurs as the mineral hydrozincite, a weathering product of zinc spar.

Zinc sulfide is a white powder that appears in two different modifications: the hexagonal close packed alpha-modification (wurtzite), the form preferred by the pigment industry (n 2.37); and the cubic beta-modification (sphalerite), which is substantially converted to wurtzite when heated to 725 C in the absence of air. Because of its semiconducting and luminescent properties, zinc sulfide is used industrially as a pigment and as phosphors in X-ray and television screens (Neumueller, 1983; Budavari, 1989).

Some organo-zinc compounds (diethyl zinc, diphenyl zinc) are sensitive to air and water. The lower alkyl compounds are autoflammable when exposed to air.

Other organo-zinc compounds, such as zineb (zinc ethylene-bis(dithiocarbamate)) and ziram (zinc dimethyl-dithiocarbamate), are used as agricultural fungicides (Neumueller, 1983).

2.3 Analytical methods

2.3.1 Introduction

Because zinc is ubiquitous in the environment, special care is required during sampling, sample preparation and analysis to avoid sample contamination. Precautions must be taken to avoid contamination arising from such sources as sampling apparatus, filtration equipment, and atmospheric exposure during collection and analysis. Clean room conditions and sample handling using apparatus rigorously cleaned with acid by operators wearing polyethylene gloves and appropriate lint-free clothing are desirable (Batley, 1989a). The necessary detection limits for trace analysis are often affected by problems related to inadequate reagent purity or contamination introduced during the course of the sampling and analytical manipulations. With adequate care, however, zinc concentrations as low as 0.006 g/litre in water and 0.1 mg/kg in solid samples are detectable, using modern instrumental analysis techniques.

For many environmental samples, zinc concentrations are sufficiently high to obviate the need for the precautions described above. Nevertheless, appropriate quality assurance during both sampling and analysis is necessary to ensure confidence in the methods of analysis used and the subsequent data that they generate.

2.3.2 Sampling and sample preparation

The background concentrations of dissolved zinc in many natural water samples are frequently below 1 g/litre. However, contamination leading to levels as high as 20 g/litre is quite possible during sampling and filtration of waters. Containers must be carefully selected and precleaned before use. Teflon containers are preferable; polyethylene is acceptable and superior to Pyrex glass, but soda glass should be avoided (Batley, 1989a). Precleaning is best carried out by prolonged soaking in 2 mol/litre nitric or hydrochloric acids, although hot nitric acid has been used (Mart, 1979). The containers should be rinsed with distilled water and thoroughly rinsed with sample before collection. The need for rigorous care with water sampling has been elegantly demonstrated by Ahlers et al. (1990).

Water sample preservation is achieved by acidification to < pH 2, generally after filtration if dissolved metals are being sought. For zinc speciation analysis, acidification is unacceptable, and storage at 4 C minimizes species transformations or losses. Similar constraints apply to biological fluids.

For ultratrace analysis, the use of a clean laboratory or at least a laminar flow work station is highly recommended to avoid contamination from airborne particulates. Typical unfiltered urban room air may contain zinc at concentrations as high as 1 g/m3 (Henkin, 1979). In general laboratory operations, care should be taken to avoid galvanized laboratory fittings (especially retort stands and clamps), rubber materials and powdered gloves, all of which contain zinc.

Contamination of soil and sediment samples, in which zinc concentrations may vary in the range 102000 mg/kg, is less of a problem. Where sediment samplers are likely to contaminate the sample, the outer sample layers should be discarded and only those portions not in contact with contaminating surfaces should be subsampled. Coring is usually carried out with PVC or Perspex tubes; where metal corers are used, it is usual for them to have polyethylene or polycarbonate liners. Where sieving of samples is undertaken, stainless steel or nylon sieves are unlikely to cause sample contamination.

If the measurement of zinc present in soils or sediments in specific mineral phases is required, the sample should be frozen as soon as possible after collection and air excluded to avoid oxidation of metal sulfides and transformation of chemical forms. When selective extractions are to be undertaken, the sample is thawed and homogenized by mixing. An aliquot of the moist sample is then taken for analysis, with moisture content being determined in replicate aliquots (Batley, 1989a).

Sampling of plant material from the field requires procedures that take into account a number of abiotic and biotic factors (Quevauviller & Maier, 1994; Ernst, 1995). The former include climate, i.e., sampling before or after rain and, in the case of roots, soil type. Biotic factors include age of material and the presence of parasites (e.g., mildew) or mycorrhizal fungi.

For total zinc analysis, sample preparation involves drying at 110 C followed by acid digestion. Total mineralization requires a mixture of concentrated acids, e.g., nitric, hydrochloric and hydrofluoric acids, and the digestion is performed on a hot plate in a heated block assembly or microwave oven. Microwave digestion is being increasingly used to minimize sample contamination. The detection of acid-soluble metals, as stipulated by US EPA method 200.8, uses only nitric and hydrochloric acids (Long & Martin, 1991).

Biological samples comprise aquatic and terrestrial organisms and may include human tissue, hair, sweat, blood, urine and faeces. Again, care is required in the handling of samples to avoid contamination (Batley, 1989a), avoiding metal surfaces and using appropriately cleaned plastic containers. The method of sample preparation depends to a large extent on sample type. Animal and human tissue samples are usually analysed without drying, and wet weight concentrations are reported. In some instances freeze-drying has been employed. Plant tissue samples have been dried at 110 C, freeze-dried and, in some instances, ashed at 500 C to facilitate dissolution. In recent years, however, it has been realized that temperature can have a significant effect on the quality of plant material during drying and mineralization prior to analysis. Owing to burning of carbohydrates, drying at 110 C will diminish the real dry mass, leading to overestimation of the zinc concentration. Ashing at 500 C should be avoided as it causes loss of zinc as volatile compounds. Plant samples are therefore now usually oven-dried at 80 C for 48 h (Ernst, 1995; Rengel & Graham, 1995). Freeze-drying remains an option, especially in zinc compartmentation studies.

Dissolution is usually undertaken by wet ashing with nitric acid, either on a hot plate or by microwave-assisted digestion (White, 1988). The use of perchloric acid is generally avoided nowadays, and complete decomposition of organic compounds is not required for most spectroscopic analysis techniques. For marine organisms, hydrogen peroxide is usually added during the dissolution process. Tissue solubilizers such as tetramethylammonium hydroxide or potassium hydroxide have been used for effective dissolution of biological tissue samples (Martin et al., 1991).

Care should be taken in the acid dissolution of blood and urine samples, as frothing of natural surfactants in the sample during digestion can lead to losses. Allowing the sample to stand overnight after the addition of acid can often obviate this problem.

It should be noted that in all of the above analyses, care must be paid to the quality of acids and other reagents used. For analysis of zinc at low concentrations, reagents of an appropriately high purity are essential.

For air sampling with high-volume samplers, low-ash filters are required. Glass fibre filters are sources of zinc contamination and membrane filters made of cellulose acetate or Teflon are preferred (Batley, 1989a). Samples are analysed after dissolution of particulates in nitric acid, although ashing has also been used (NIOSH, 1984).

2.3.3 Separation and concentration

Given the low detection limits of modern analytical techniques, separation techniques, such as ion exchange or solvent extraction, that preconcentrate zinc from solution, are less frequently used nowadays, although they are required for ultratrace detection. Any additional sample manipulation, however, increases the opportunity for sample contamination. A range of preconcentration techniques has been applied, but only those currently in common use are discussed here.

Most appropriate is the use of the complexing agents ammonium pyrrolidine dithiocarbamate (APDC) or diethyldithiocabamate (DDC) to extract zinc, using trichloroethane or chloroform as the solvent. Apte & Gunn (1987) have described a micro solvent extraction procedure with analysis by graphite furnace atomic-absorption spectrometry (GF-AAS); detection of zinc concentrations as low as 20 ng/litre in seawater and other natural waters is possible.

Chelating resins have also been widely used for preconcentration. Chelex-100 or equivalent iminodiacetate resins in the sodium or calcium forms effectively remove zinc from seawater or fresh waters at pH values greater than 6. It should be noted that zinc associated with colloids will not be satisfactorily removed. The use of immobilized 8-hydroxyquinoline, dithiocarbamates or other zinc-binding ligands has also been reported. The former is incorporated in at least one in situ water sampler (Willie et al., 1983; Batley, 1989b).

In natural water systems, measurements typically involve either total zinc, dissolved zinc or some form of zinc speciation analysis. Water quality criteria are frequently based on total analyses. Acidification of the sample, with heating, is therefore used as a pretreatment option. Filtration through 0.45-m membrane filters provides the accepted means of separating particulate species, and a separate analysis can then be performed on each phase. For speciation, the principal concern is for bioavailable species, and a range of procedures has been applied, including ultrafiltration, dialysis, ligand exchange, chelating resin separations and measurement techniques, such as anodic and cathodic stripping voltammetry (ASV and CSV) that discriminate between labile and non-labile zinc. These have been comprehensively reviewed elsewhere (Florence & Batley, 1980; Batley, 1989b; Apte & Batley, 1995).

2.3.4 Detection and measurement

For environmental and biological samples, the required detection limits necessitate the use of modern instrumental methods of analysis. Traditional titrimetric and gravimetric methods are not sufficiently sensitive. Spectrophotometric methods offer greater sensitivity, but are tedious and subject to numerous interferences (Cherian & Gupta, 1992). A summary of analytical methods for zinc in various environmental media is given in Table 3.

Table 3. Analytical methods for zinc

Sample

Preparationa

Analytical methodb

Limit of detection

Reference

Atmospheric particulates

collection on membrane filter, ashing with HNO3

F-AAS

2.6 pg/litre

Ottley & Harrison (1993)

Atmospheric particulates

polystyrene filter collection, pressed into pellets

NAA

not given

Zoller et al. (1974)

Atmospheric particulates

cellulose filter collection

NAA

0.4 pg/litre

Amundson et al. (1992)

Water

filtration, acidification

FAAS

50 g/litre

Greenberg et al. (1992)

Water

APDC/MIBK extraction

FAAS

not given

Greenberg et al. (1992)

Water

filtration, acidification

GF-AAS

0.1 g/litre

Greenberg et al. (1992)

Water

filtration, acidification

ICP-AES

2 g/litre

Greenberg et al. (1992)

Water

filtration, acidification US EPA Method 200.8

ICP-MS

1.8 g/litre

Long & Martin (1991)

Water/seawater

APDC/trichloroethane extraction

GF-AAS

0.02 g/litre

Apte et al. (1998)

Water/seawater

acidification, ultraviolet irradiation

DP-ASV

0.05 g/litre

Batley & Farrar (1978)

Seawater

APDC chelation

CSV

0.006 g/litre

Van den Berg (1986)

Water/seawater

chelating resin preconcentration

ICP-MS

0.05 g/litre

Sturgeon et al. (1981)

Water, leachates

acidification

XRF

5 mg/litre

Cornjeo et al. (1994)

Soil, sediments

US EPA Method 200.8

ICP-MS

0.7 mg/kg

Long & Martin (1991)

Soil, sediments

HCl/HNO3/HF microwave digestion

ICP-MS

0.7 mg/kgc

Dale (unpublished data)

Biota (fish, oysters, mussels, etc.)

HNO3/H2O2 microwave digestion

ICPA-ES

0.2 mg/kgc

Martin et al. (1991)

Biota (fish, oysters, mussels, etc.)

tetramethylammonium hydroxide dissolution, US EPA Method 200.11

ICP-AES

0.2 mg/kgc

Martin et al. (1991)

Biota (fish, oysters, mussels, etc.)

homogenization, freeze-drying, HNO3/H2O2 dissolution

IDMS

1.5 ng absolute

Waidmann et al. (1994)

Biological samples

solid

XRF

0.1 mg/kg

Heckel (1995)

Plant material

homogenization, digestion in HNO3/HCl in Teflon bomb

AAS/F-AAS

not given

Harmens et al. (1993)

Food

dry ashing, HNO3 /H2O2 digestion

ICP-MS (isotope dilution)

not given

Veillon & Patterson (1995)

Food

homogenization, freeze-drying, acid microwave digestion

ICP-AES

2 mg/kgc

Copa-Rodriguez & Basadre-Pampin (1994)

Blood serum

dilution with HNO3/HCl

ICP-AES

1050 g/litre

Que Hee & Boyle (1988)

Biological tissues, whole blood, faeces

heating with HNO3 , Parr bomb digestion, addition of HClO4

ICP-AES

1050 g/litre

Que Hee & Boyle (1988)

Blood, plasma

dilution with water

GF-AAS

6 g/litre

Schmitt et al. (1993)

Human milk

ultrafiltration

GF-AAS

1.6 g/litre

Arnaud & Favier (1992)

Faeces

drying, digestion with H2SO4/HClO4

F-AAS

not given

Dastych (1990)

Saliva

-

GF-AAS

0.4 g/litre

Henkin et al. (1975)

a

APDC = ammonium pyrrolidine dithiocarbamate; MIBK = methyl isobutyl ketone; US EPA = United States Environmental Protection Agency.

b

CSV = cathodic stripping voltammetry; DP-ASV = differential pulse anodic stripping voltametry; F-AAS = flame atomic-absorption spectrometry; GF-AAS = graphite furnace atomic-absorption spectrometry; ICP-AES = inductively-coupled plasma atomic emission spectrometry; ICP-MS = inductively-coupled plasma mass spectrometry; IDMS = isotope dilution studies; NAA = neutron activation analysis; XRF = X-ray fluorescence.

c

Dependent upon the mass of sample taken and the dilution.

 

To achieve the necessary detection limits, spectrophotometric methods will usually require some form of sample preconcentration. The achievable detection limit is frequently limited in practice by the purity of the reagents used.

Instrumental techniques offer element-specific detection at low concentrations. The most common are atomic absorption or emission spectrometry (AAS and AES), X-ray fluorescence (XRF), electroanalytical techniques, such as polarography or stripping voltammetry, and neutron activation analysis.

XRF and other focused particle beam methods require solid samples. The detection limit for zinc by direct microprobe analysis is only around 240 mg/kg (Kersten & Forstner, 1989). For liquid samples, preconcentration by adsorption or complexation onto solid phases has been used. A relatively new XRF procedure based on polarized X-rays has a detection limit for zinc of 0.1 mg/kg in biological materials (Heckel, 1995).

Flame atomic absorption spectrometry (F-AAS) has for many years been the basis of the standard method for determining zinc in waters (Hunt & Wilson, 1986). The method is very sensitive: for direct F-AAS analysis, the instrumental detection limit is 5 g/litre, although the optimal concentration range is 502000 g/litre. This can be further enhanced with preconcentration by complexation/solvent extraction or using solid-phase adsorbents. GF-AAS offers improved detection limits for direct analysis, but is subject to matrix interferences, particularly in saline waters (Slavin, 1984).

Inductively-coupled plasma atomic emission spectrometry (ICP-AES) is considerably more sensitive than F-AAS, and detection of 2 g/litre is possible by direct analysis (Greenberg et al., 1992), although with the latest axial plasma instruments with ultrasonic nebulization, the limit is as low as 0.2 g/litre. Calibration by standard additions is essential. This technique offers adequate sensitivity for zinc in contaminated waters or for acid digests of soil, sediment and biological samples. The multi-element capability offered by ICP-AES is a considerable advantage over AAS methods.

ICP mass spectrometry (ICP-MS) offers excellent sensitivity. The instrumental detection limit for zinc in fresh waters is 20 ng/litre using conventional nebulization systems. With aerosol desolvation devices, the detection limit is about one order of magnitude better. However, these detection limits are not achievable unless stringent procedures to avoid zinc contamination are implemented, including the use of ultrapure reagents. A content of solids in excess of 0.1%, as in seawater samples, creates problems during nebulization. These are best overcome by complexation and extraction of zinc as described earlier. The technique is ideally suited to digests of soils, sediments and biological samples; the greater sensitivity means that any difficulties due to a high content of solids are overcome by dilution. In addition, because of its mass resolution, ICP-MS enables isotopic ratio analysis (67Zn/68Zn/70Zn) or isotope dilution studies using 65Zn (Ward, 1987). Isotope tracers have been used to study zinc absorption following administration of the isotope in food (Johnson, 1982; Watson et al., 1987).

Neutron activation analysis (NAA) is a useful technique for the non-destructive analysis of solid samples, and requires a minimum of sample preparation (Fredrickson, 1989; Heydorn, 1995). Its main advantage is its multi-element capability; the great disadvantage is its limited availability, and long analysis time. It has largely been superseded by ICP-MS, which offers a similar capability and is more widely available. For zinc, the sensitivity of NAA is poor.

Of the electroanalytical techniques, polarography is rarely employed except for samples containing high zinc concentrations (> 10 g/litre), such as digests of ores. For ambient water concentrations, stripping voltammetric techniques are essential. Differential-pulse ASV (DP-ASV) offers detection limits in natural waters in the ng/litre range (Florence, 1989). An advantage of ASV is the in situ preconcentration achieved during the accumulation step, which avoids the contamination problems associated with the greater sample manipulation of other preconcentration techniques. A disadvantage is the potential interference from high concentrations of natural organic compounds in some samples, which may adsorb to the mercury electrode and limit zinc deposition. Although this is not a problem for most natural water samples, complete digestion of biological samples or highly contaminated waters, to decompose interfering surface-active organic compounds, is essential.

CSV has also been successfully applied to the detection of baseline concentrations of zinc in seawater (Van den Berg, 1986). It requires the formation of a zinc complex with APDC, which can be accumulated at a mercury electrode and stripped using a cathodic scan. CSV is best used with pristine samples, where interference due to other metals or adsorbing ligands is less likely.

It should be noted that voltammetric techniques applied to water samples will only measure an operationally-defined labile fraction unless the sample is pretreated by UV irradiation to destroy non-labile zinc complexes, and acidification to dissociate zinc bound to natural colloids. This property can be an advantage in speciation studies, where the ASV-labile concentration has been related to the zinc fraction that is bioavailable (Florence & Batley, 1980; Florence, 1992).

New zinc-specific fluorophores have been developed to measure and visualize intracellular zinc. One of these, Zinquin, has been successfully used in lymphoid, myeloid and hepatic cells to detect labile intracellular zinc (Zalewski et al., 1993; Coyle et al., 1994), although the interaction between Zinquin and the zinc-binding protein, metallothionein (see section 6.5.1.4) needs further study (Coyle et al., 1994).

In all analyses, the use of appropriate quality assurance procedures is required. In particular, standard reference materials are essential. These are currently available for waters, sediments and soils, as well as for plant and other biological materials.

3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE

3.1 Natural occurrence

Zinc is a chalcophilic element like copper and lead, and a trace constituent in most rocks. Zinc rarely occurs naturally in its metallic state, but many minerals contain zinc as a major component from which the metal may be economically recovered (Table 4). The mean zinc levels in soils and rocks usually increase in the order: sand (10- 30 mg/kg), granitic rock (50 mg/kg), clay (95 mg/kg) and basalt (100 mg/kg) (Adriano, 1986; Malle, 1992). Sphalerite (ZnS) is the most important ore mineral and the principal source for zinc production. Smithsonite (ZnCO3) and hemimorphite (Zn4(Si2O7) (OH)2XH2O) were mined extensively before the development of the froth-flotation process (Melin & Michaelis, 1983; Jolly, 1989). The main impurities in zinc ores are iron (114%), cadmium (0.10.6%), and lead (0.12%), depending on the location of the deposit (ATSDR, 1994).

Table 4. CAS chemical names and registry numbers, synonyms, trade names and molecular formula of zinc oresa

Chemical name

CAS registry number

Synonyms and trade names

Composition

Formula

Zinc oxide

1314-13-2

zincite

80.34% Zn, 19.66% O

ZnO

Zinc phosphate

7779-90-0

hopeite

50.80% Zn, 33.16% O, 16.04% P

Zn3(PO4)2.4H2O

Zinc silicate

13597-65-4

willemite

58.68% Zn, 28.72% O, 12.60% Si

Zn2SiO4

Zinc sulfide

1314-98-3

sphalerite, wurtzite

67.09% Zn, 32.91% S, up to 25% Fe

ZnS

Zinc carbonate

3486-35-9

smithsonite, zincspar

52.14% Zn, 38.28% O, 9.58% C

ZnCO3

Hemimorphite

-

-

58.28% Zn

Zn4(Si2O7)(OH)2XH2O

Franklinite

-

-

1525% ZnO, 1016% MnO

(Zn, Fe, Mn).(FeMn)2O4

Hydrozincite

-

zinc bloom

-

Zn5(OH)6(CO3)2

Tetrahedrite

-

-

89% Zn

(Cu,Zn)12Sb4S14<