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    WORLD HEALTH ORGANIZATION             FOOD AND AGRICULTURE
                                          ORGANIZATION
    ORGANISATION MONDIALE DE LA SANTE     ORGANISATION POUR L'ALIMENTATION
                                          ET L'AGRICULTURE

                                                      VBC/DS/80.42

                                                      ORIGINAL: ENGLISH






    DATA SHEETS ON PESTICIDES No. 42

    March 1980

    DIMETHOATE






         It must be noted that the issue of a Data Sheet for a
    particular pesticide does not imply endorsement of the pesticide by
    WHO or FAO for any particular use, or exclude its use for other
    purposes not stated. While the information provided is believed to
    be accurate according to data available at the time when the sheet
    was compiled, neither WHO nor FAO are responsible for any errors or
    omissions, or any consequences therefrom.

    The issue of this document does    Ce document ne constitue pas une
    not constitute formal              publication. Il ne doit faire
    publication. It should not be      l'objet d'aucun compte rendu ou
    reviewed, abstracted or quoted     résumé ni d'aucune citation sans
    without the agreement of the       l'autorisation de l'Organisation
    Food and Agriculture               des Nations Unies pour
    Organization of the United         l'Alimentation et l'Agriculture
    Nations or of the World Health     ou de l'Organisation Mondiale de
    Organization.                      la Santé.

                             CLASSIFICATION:

                             Primary use: Insecticide

                             Secondary use: Acaricide

                             Chemical group: Organophosphorus compound

                             Date issued: March 1980

    1.  GENERAL INFORMATION

    1.1  COMMON NAME:

    Dimethoate (ISO)

    1.1.1  Identity:

    O,O-dimethyl S-[2-(methylamino)-2-oxoethyl] phosphorodithioate

    Structural Formula

    1.1.2  Synonyms:

    OMS-94              Rogor(R)
    OMS-111             Cygon(R)
    Perfekthion(R)      Ridmite(R)
    De-Fend(R)          Roxion(R)

    Local synonyms:

    1.2  SYNOPSIS:

    An insecticide and acaricide of moderate mammalian toxicity which is
    used in housefly control and against a broad range of agricultural
    insect and mite pests. It is active after metabolism, both as a
    contact and as a systemic insecticide.

    1.3  SELECTED PROPERTIES

    1.3.1  Physical characteristics

    The pure compound forms colourless crystals with a camphor-like
    odour, m.p. 51-52°C. Technical dimethoate, about 93% pure, varies
    from off-white crystals to a grey, semi-crystalline material.

    1.3.2  Solubility

    Water: 25 g/l at 21°C. Soluble in most organic solvents except
    saturated hydrocarbons such as hexane.

    1.3.3  Stability

    Stable in aqueous solution but is readily hydrolysed by aqueous
    alkali; heating converts it to the SCH3 isomer.

    1.3.4  Vapour pressure

    8.5 x 10-6 mmHg at 25°C.

    1.4  AGRICULTURE, HORTICULTURE AND FORESTRY

    1.4.1  Common formulations

    200, 400 and 500 g/l emulsion concentrates, 200 g/kg
    water-dispersible powder, 50 mg/kg granules: 200 g/technical
    compound/l for ULV.

    1.4.2  Susceptible pests

    Aphids, leafhoppers, lygus bugs, pear psylla. Various mites of crops
    and ornamental plants.

    1.4.3  Use pattern

    As a systemic pesticide on various fruits and vegetables, on alfalfa
    and fodder crops well before harvesting or grazing, on cotton
    against sucking insects: as a residual insecticide at 1 g/m2 for
    houseflies in barns, stables, piggeries, etc. Also used for
    houseflies in sugar baits.

    1.4.4  Unintended effects

    Cytotoxic for some fruit trees, including citrus.

    1.5  PUBLIC HEALTH PROGRAMMES

    No recommended use.

    1.6  HOUSEHOLD USE

    No recommended use.

    2.  TOXICOLOGY AND RISKS

    2.1  TOXICOLOGY - MAMMALS

    2.1.1  Absorption route

    Readily absorbed by the gastrointestinal tract and to a lesser
    extent through the intact skin and by inhalation.

    2.1.2  Mode of action

    Cholinesterase inhibition after metabolism. Oral dimethoate is
    biotransformed in the liver microsomes by conversion into its oxygen
    analogue, which is the active form, by hydrolysis of the methyl
    ester group, and by removal of the methyl-amido group.

    2.1.3  Excretion products

    Studies on rat and man with 32p-labelled dimethoate show rapid
    absorption and excretion, 79-90% of the radioactivity being found in
    the urine after 24 hours. Direct degradation of a C-N bond is
    considered largely responsible for its selective toxicity and
    relative safety for mammals. The chief metabolite in animals is the
    thiocarboxy derivative of dimethoate.

    2.1.4  Toxicity, single dose

    Oral: LD50 rat 500-600 mg/kg:   pure dimethoate
                 about 150 mg/kg:   technical product

    The difference in toxicity between pure dimethoate and the technical
    product is due to a trace impurity produced by partial hydrolysis
    which potentiates the toxicity of pure dimethoate. Therefore the
    LD50 of a given technical product may vary within these limits.
    The oral LD50 value of 150 mg/kg is recommended for classification
    purposes.

    Dermal: LD50 rat 353 mg/kg

    2.1.5  Toxicity repeated doses

    Oral: Given 30 (mg/kg)/day for three weeks, guinea-pigs showed
    some weight loss and weakness but no cholinergic effects. In similar
    tests on rats, the highest non-lethal dose was 20 (mg/kg)/day.

    Inhalation: Twelve male rats were kept for 28 days in a miniature
    greenhouse, with plants and sides of the chamber sprayed daily with
    0.570 aqueous dimethoate formulation. No blood cholinesterases
    inhibition or toxic effects occurred.

    Cumulation of compound: Dimethoate is not cumulative in body
    tissues.

    Cumulation of effect: Repeated daily exposure may cumulatively
    inhibit cholinesterase activity.

    2.1.6  Dietary studies

    Short-term: Groups of 10 male rats were fed levels of 1, 5, 25 and
    125 mg dimethoate/kg diet for 15 weeks. At the highest
    concentration, a slight fall in the rate of gain of weight was
    observed as well as mild symptoms of poisoning. In the group fed
    25 mg/kg and at higher concentrations, a significant reduction in
    plasma and erythrocyte cholinesterase activity was observed, while
    in the animals fed 5 mg/kg, a reduction of 20% in plasma
    cholinesterase activity only was found. In another study, groups of
    20 male rats were fed for 6-12 months with various levels of
    laboratory grade dimethoate. At 800 mg/kg intoxication developed
    within a few days. No toxic effects were seen at 50 mg/kg. Marked
    inhibition of erythrocyte cholinesterase occurred at 50 mg/kg but at
    10 mg/kg and below neither erythrocyte nor plasma cholinesterase
    showed significant inhibition. The maximum no-effect level
    corresponded to 0.5-0.8 mg/kg body weight.

    Dogs were fed levels of 2, 10 and 50 mg/kg diet for 13 weeks. The
    erythrocyte cholinesterase activity was only slightly reduced at
    50 mg/kg.

    Long-term: See below.

    2.1.7  Supplementary studies of toxicity

    Carcinogenicity: Dimethoate was administered in feed to groups of
    50 male and 50 female rats for 80 weeks, followed by 35 weeks of
    observation. Initial doses were not well tolerated; therefore, they
    were reduced during the study. The "time-weighted average doses" for
    rats were 155 and 310 mg/kg diet for males and 192 and 384 mg/kg for
    females. All surviving rats were killed between 113 and 115 weeks.
    Similarly, dimethoate was administered in feed to groups of 50 male
    and 50 female mice at concentrations of 250 and 500 mg/kg diet for
    80 weeks. High-dose males were returned to the control diet at 60
    weeks, and low-dose males at 69 weeks. All surviving mice were
    killed between 93 and 94 weeks. In both studies, tremor and
    hyperexcitability, indications of dimethoate toxicity, were observed
    in the treated animals. Pathologic evaluation revealed no
    statistically significant increase in tumours associated with
    dimethoate treatment in either species.

    Reproduction and teratogenicity: A three-generation reproduction
    study was conducted on mice at levels of 5, 15 and 50 mg/kg diet,
    with two litters produced by generation. No effect was seen on
    fertility, lactation or survival of the pups to weaning, gross
    appearance, weight of major organs, or gross and microscopic
    appearance of tissues.

    In another study, a dietary level of 60 mg/kg produced reduced
    mating success and longer reproduction time. At birth, litter size
    and weight were not reduced, but pup mortality increased
    significantly with treatment. Growth rate of the pup was generally
    lower also. No teratogenic or other adverse effects on organs were
    observed.

    2.1.8  Modification of toxicity

    Acute oral toxicity of dimethoate was not potentiated by any of 17
    other insecticides. There is a theoretical possibility that the
    toxicity of some batches might vary due to the presence of
    impurities.

    2.2  TOXICOLOGY - MAN

    2.2.1  Absorption

    See 2.1.1.

    2.2.2  Dangerous doses

    Single: Not known. Studies on the degradation of dimethoate by 11
    specimens of human liver have been conducted. On the basis of
    comparisons with other species for which data both on toxicity and
    on degradation in liver are known, it is predicted that the acute
    oral LD50 of dimethoate to humans is about 30 mg/kg.

    Repeated: Not known.

    2.2.3  Observations of occupationally exposed workers

    No reports.

    2.2.4  Observations on exposure of the general population

    No reports.

    2.2.5  Observations of volunteers

    Twenty subjects ingested 2.5 mg of dimethoate in aqueous solution,
    corresponding to about 0.04 mg/kg body weight daily for four weeks.
    No toxic effect, or significant change in the blood cholinesterase
    activity were observed. Similar results were found in two subjects

    who ingested daily during 21 days, 0.13 mg/kg and 0.26 dimethoate
    mg/kg body weight respectively. Thirty-six male and female
    volunteers were given daily oral doses of dimethoate of 5, 15, 30,
    46 and 60 mg for periods of 14 to 57 days. There was no effect on
    the blood cholinesterase levels with intakes of 5 and 15 mg daily,
    but effects were noted at 30 mg and above.

    2.2.6  Reported mishaps

    Human cases of poisoning have occurred due to ingestion.

    2.3  TOXICITY TO NON-MAMMALIAN SPECIES

    2.3.1  Fish

    Toxic to fish but disappears from water rapidly under field
    conditions.

    2.3.2  Birds

    Toxic to birds.

    2.3.3  Other species

    Toxic to bees.

    3.  FOR REGULATORY AUTHORITIES RECOMMENDATIONS ON REGULATION OF
        COMPOUND

    3.1  RECOMMENDED RESTRICTIONS ON AVAILABILITY

    (For definition of categories, see introduction).

    Liquid formulations 10% and above category 3, below 10% category 4.
    Solid formulations above 25% category 3, all other formulations
    category 4.

    3.2  TRANSPORTATION AND STORAGE

    All formulations - Should be transported in clearly labelled rigid
    or leakproof containers and stored in these containers, under lock
    and key, secure from access by unauthorized persons and children.
    No food or drink should be transported or stored in the same
    compartment.

    3.3  HANDLING

    All formulations - Protective clothing should be provided for
    those handling the compound. Adequate washing facilities should be
    available at all times during handling and should be close to the
    site of handling. Eating, drinking and smoking should be prohibited
    during handling and before washing after handling.

    3.4  DISPOSAL AND/OR DECONTAMINATION OF CONTAINER

    All formulations - Containers may be decontaminated (for method,
    see part 4). Decontaminated containers should not be used for food
    and drink. If not decontaminated, containers should be burned or
    crushed and buried below topsoil. Care must be taken to avoid
    subsequent contamination of water sources.

    3.5  SELECTION, TRAINING AND MEDICAL SUPERVISION OF WORKERS

    All formulations - Pre-employment medical examination of workers
    desirable. Workers suffering from active hepatic or renal disease
    should be excluded from contact. Pre-employment and periodic
    cholinesterase tests for workers desirable. Special account should
    be taken of the workers' mental ability to comprehend and follow
    instructions. Training of workers in techniques to avoid contact
    essential.

    3.6  ADDITIONAL REGULATIONS RECOMMENDED IF DISTRIBUTED BY AIRCRAFT

    All formulations - Pilots and loaders should have special training
    in application methods and recognition of early symptoms of
    poisoning, and must wear a suitable respirator. Flagmen, if used,
    should wear overalls and be located well away from the dropping
    zone.

    3.7  LABELLING

    All formulations - Minimum cautionary statement: Dimethoate is an
    organophosphorus compound which inhibits cholinesterase. It is
    poisonous if swallowed. It may be absorbed through the skin or
    inhaled as dusts or mists. Avoid skin contact; wear protective
    gloves, clean protective clothing and a dust mask when handling the
    material. Wash thoroughly with soap and water after using. Keep the
    material out of reach of children, and well away from foodstuffs,
    animal feed and their containers. If poisoning occurs, call a
    physician. Atropine is a specific antidote and artificial
    respiration may be needed.

    3.8  RESIDUES IN FOOD:

    Maximum residue limits for dimethoate have been recommended by the
    Joint FAO/WHO Meeting on Pesticide Residues. These are subject to
    change at annual reviews.

    4.  PREVENTION OF POISONING IN MAN AND EMERGENCY AID

    4.1  PRECAUTIONS IN USE

    4.1.1  General

    Dimethoate is an organophosphorus pesticide of moderate toxicity. It
    penetrates the intact skin and is also absorbed by inhalation of
    dust and spray mists, and from the intestinal tract. Most
    concentrated formulations should be handled only by trained
    personnel wearing protective clothing.

    Manufacture and formulation - T.L.V. No information. Closed
    systems of forced ventilation may be required to reduce as much as
    possible the exposure of workers to the chemical. Protective
    equipment for the skin and respiratory protection may be desirable.

    4.1.3  Mixers and applicators

    When opening the container and when mixing, protective impermeable
    boots, clean overalls, gloves and a visor should be worn. Mixing, if
    not mechanical, should always be carried out with a paddle of
    appropriate length. When spraying tall crops or during aerial
    application, a face mask should be worn, as well as an impermeable
    hat, clothing, boots and gloves. The applicator should avoid contact
    with the mouth. Particular care is needed when equipment is being
    washed after use. All protective clothing should be washed
    immediately after use, including the insides of gloves. Splashes
    must be washed immediately from the skin or eyes with large
    quantities of water. After washing, eyes that have been splashed
    should be examined by a doctor. Before eating, drinking or smoking,
    hands and other exposed skin should be washed.

    4.1.4  Other associated workers (including flagmen in aerial
           operations)

    Persons exposed to dimethoate and associated with its application
    should wear protective clothing and observe the precautions
    described in 4.1.3 under "Mixers and applicators".

    4.1.5  Other populations likely to be affected

    With good agricultural and manufacturing practice subject to 4.2
    below, other populations should not be exposed to hazardous amounts
    of dimethoate.

    4.2  ENTRY OF PERSONS INTO UNTREATED AREAS:

    Unprotected persons should be kept out of tall crops for four days;
    and out of other crops for 24 hours.

    4.3  DECONTAMINATION OF SPILLAGE AND CONTAINERS:

    Residues in containers should be emptied in a diluted form into a
    deep pit taking care to avoid contamination of ground waters. The
    empty container should be filled with 5% sodium hydroxide solution
    which should remain in the container overnight. Then containers may
    be further decontaminated by rinsing two or three times with water
    and scrubbing the sides. Impermeable gauntlets should be worn during
    the work and a soakage pit should be provided for the rinsing.
    Decontaminated containers should not be used for food and drink.
    Spillage of dimethoate and its formulations should be removed by
    washing with 5% sodium hydroxide solution and then rinsing with
    large quantities of water.

    4.4  EMERGENCY AID

    4.4.1  Early symptoms of poisoning

    These may include excessive sweating, headache, weakness, giddiness,
    nausea, vomiting, stomach pains, blurred vision, slurred speech and
    muscle twitching. Later there may be convlusions, coma, loss of
    reflexes and loss of sphincter control.

    4.4.2  Treatment before person is seen by a physician, if these
           symptoms appear following exposure

    The person should stop work immediately, remove contaminated
    clothing and wash the affected skin with soap and water, if
    available, and flush the area with large quantities of water. If
    swallowed, vomiting should be induced, if the person is conscious.
    In the event of collapse, artificial respiration should be given,
    bearing in mind that if mouth-to-mouth resuscitation is used, vomit
    may contain toxic amounts of dimethoate.

    5.  FOR MEDICAL AND LABORATORY PERSONNEL

    5.1  MEDICAL DIAGNOSIS AND TREATMENT IN CASES OF POISONING

    5.1.1  General information

    Dimethoate is an organophosphorus pesticide of moderate toxicity
    which is readily absorbed from the gastrointestinal tract.
    Absorption may occur through the dermal route and by inhalation. It
    is converted in vivo to the oxygen analogue which then inhibits
    acetylcholinesterase. Continued exposure to low amounts may inhibit
    acetylcholinesterase to dangerous levels.

    5.1.2  Symptoms and signs

    Initial symptoms of poisoning may include excessive sweating,
    headache, weakness, giddiness, nausea, vomiting, stomach pains,
    blurred vision, slurred speech and muscle twitching. More advanced
    symptoms of poisoning may be convulsions, coma, loss of reflexes and
    loss of sphincter control.

    5.1.3  Laboratory

    The most important laboratory finding is reduction in activity of
    blood cholinesterases. Urinary levels of organic phosphorus
    containing metabolites may also be used as a measure of exposure.
    Neither method is specific for dimethoate.

    5.1.4  Treatment

    If the pesticide has been ingested, unless the patient is vomiting,
    rapid gastric lavage should be performed using 5% sodium
    bicarbonate, if available. For skin contact, the skin should be
    washed with soap and water. If the compound has entered the eyes,
    they should be washed with isotonic saline or water. Persons without
    signs of respiratory inefficiency but with manifest peripheral
    symptoms should be treated with 2-4 mg of atropine sulfate by
    intravenous injection. More atropine may be given as needed. Persons
    with severe intoxication, with respiratory difficulties convulsions
    and unconsciousness, should immediately be given atropine and a
    reactivator. In such severe cases 4-6 mg of atropine sulfate should
    be given initially followed by repeated doses of 2 mg at 5-10 minute
    intervals. The patient's condition including respiration, blood
    pressure, pulse frequency, salivation and convulsions should be
    carefully observed to further administration of atropine. If the
    patient is cyanotic, artificial respiration should be given at the
    same time as atropine sulfate. The airways should be kept free and
    artificial respiration should be applied, if required, preferably by

    mechanical means. If necessary, intubation should be performed.
    Contraindications are morphine, barbiturates, phenothiazine,
    tranquillizers and central stimulants of all kinds. Pralidoxime and
    toxogonin are not regarded as effective antidotes in dimethoate
    poisoning.


    5.1.5  Prognosis

    If the acute toxic effect is survived and adequate artificial
    respiration has been given, if needed, the chances of complete
    recovery are good. However, in very severe cases, particularly if
    artificial respiration has been inadequate, prolonged anoxia may
    give rise to permanent brain damage.

    5.1.6  References of previous reported cases

    None.

    5.2  SURVEILLANCE TESTS


    Test                          Normal level*  Action level*  Symptomatic level

    Plasma cholinesterase              100%         50%           Variable

    Erythrocyte cholinesterase         100%         70%         Usually <   40%


    Urinary levels of ether extractable organic phosphorus may also be
    used to determine the degree of exposure.

    5.3  LABORATORY METHODS

    References only are given.

    5.3.1  Detection and assay of compound

    Residues of dimethoate may be determined by GLC (Abbott et al.,
    1970) or colorimetrically, see: Chillwell & Beecham (1960), de
    Pietri-Tonelli et al. (1965) and Van Middlem & Waites (1964).

                  
    *Expressed as percentage of pre-exposure activity.

    5.3.2  Other tests in cases of poisoning

    Levels of cholinesterase in blood provide the most useful diagnosis
    of poisoning. See: Michel, N. O. (1949) J. Lab. Clin. Med., 34,
    1564-1568. Ellman, G. L. et al. (1961) Biochem. Pharmacol., 7,
    88-95. Urinary levels of dimethyl phosphate and phosphorothionate
    (Shafik & Emos, 1969) can also be used to determine exposure.
See Also:
        Dimethoate (EHC 90, 1989)
        Dimethoate (PIM 388)