WORLD HEALTH ORGANIZATION FOOD AND AGRICULTURE
ORGANISATION MONDIALE DE LA SANTE ORGANISATION POUR L'ALIMENTATION
DATA SHEETS ON PESTICIDES No. 42
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.
Primary use: Insecticide
Secondary use: Acaricide
Chemical group: Organophosphorus compound
Date issued: March 1980
1. GENERAL INFORMATION
1.1 COMMON NAME:
O,O-dimethyl S-[2-(methylamino)-2-oxoethyl] phosphorodithioate
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.
Water: 25 g/l at 21°C. Soluble in most organic solvents except
saturated hydrocarbons such as hexane.
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
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
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
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
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
2.2 TOXICOLOGY - MAN
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
2.2.4 Observations on exposure of the general population
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
Toxic to fish but disappears from water rapidly under field
Toxic to birds.
2.3.3 Other species
Toxic to bees.
3. FOR REGULATORY AUTHORITIES RECOMMENDATIONS ON REGULATION OF
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
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
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
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
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
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
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
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.
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.
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
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
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.