WORLD HEALTH ORGANIZATION FOOD AND AGRICULTURE
ORGANISATION MONDIALE DE LA SANTE ORGANISATION POUR L'ALIMENTATION
DATA SHEETS ON PESTICIDES No. 7 Rev.1
Primary use: Insecticide
Secondary uses: Acaricide
Chemical group: Organophosphorus compound
Data sheet No. 7, Rev.1 (8/78)
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.
1. GENERAL INFORMATION
1.1 COMMON NAME: Parathion-methyl (ISO)
1.1.1 Identity: O,O-dimethyl O-(4-nitrophenyl) phosphorothioate
1.1.2 Synonyms: OMS-213
methyl parathion - USA
metaphos - USSR
1.2 SYNOPSIS: An organophosphorus pesticide of high mammalian
toxicity. It may be absorbed through the skin, by inhalation and
via the gastro-intestinal tract. It is active upon metabolism.
1.3 SELECTED PROPERTIES
1.3.1 Physical characteristics - When pure a white crystalline solid;
m.p. 35-36°C. The technical material is a tan to brown liquid
with a pungent garlic-like odour; m.p. about 29°C.
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
The issue of this document does not constitute formal publication.
It should not be reviewed, abstracted or quoted without the
agreement of the Food and Agriculture Organization of the
United Nations or of the World Health Organization.
Ce document ne constitue pas une publication. Il ne doit faire l'objet
d'aucun compte rendu ou résumé ni d'aucune citation sans l'autorisation
de l'Organisation des Nations Unies pour l'Alimentation et l'Agriculture
ou de l'Organisation Mondiale de la Santé.
1.3.2 Solubility - Water at 25°C very slightly soluble (50-60 mg/l),
soluble in most organic solvents, very slightly soluble in
petroleum and mineral oils.
1.3.3 Stability - Stable for several days in neutral aqueous
suspensions, rapidly hydrolysed in alkaline media at a rate 4.3
times greater than parathion. Isomerizes readily to the OS-
dimethyl isomer on heating. Compatible with most other
1.3.4 Vapour pressure (volatility): 0.14 mg/m3 at 20°C (cf. parathion
1.4 AGRICULTURE, HORTICULTURE AND FORESTRY
1.4.1 Common formulations - Solutions up to 800 g/l dispersible
powders, usually 200-400 g/kg; emulsifiable concentrates 200-800
g/l; dusts 25 and 50 g/kg. There are FAO specifications for the
technical material and for these formulations.
1.4.2 Susceptible pests - Broad-spectrum contact and stomach
insecticide with some fumigant action. Effective against most
insects and phytophagous mites.
1.4.3 Use pattern - Pre-harvest treatment of a wide range of grain,
vegetable and fruit crops, and ornamentals. Main use is on
cotton. Application rates are in range 0.2-2 kg/hectare, or 250-
750 mg a.i./kg. The pre-harvest interval, depending on crop, is
usually 14-21 days, but twice as long in glasshouses. Also used
as a mosquito larvicide on irrigated pastures. Not used on stored
1.4.4 Unintended effects - Although generally non-phytotoxic, injury to
alfalfa under certain weather conditions and to some sorghum
hybrids, particularly lighter seeded varieties, has been
reported. Toxic to wildlife but hazard reduced by brief
1.5 PUBLIC HEALTH PROGRAMMES - Very limited use as a mosquito
1.6 HOUSEHOLD USE - Parathion-methyl is too toxic for-household use.
2. TOXICOLOGY AND RISKS
2.1 TOXICOLOGY - MAMMALS
2.1.1 Absorption route - Absorbed by the skin as well as by inhalation
and from the gastrointestinal tract.
2.1.2 Mode of action - Cholinesterase inhibition after conversion to
the oxygen analogue paraoxon-methyl.
2.1.3 Excretion products - Parathion-methyl is biologically similar to
parathion and is metabolized to its oxygen analogue paraoxon-
methyl. This is more quickly broken down than paraoxon and is
excreted as p-nitrophenol and methyl and dimethyl esters of
phosphoric and/or thiophosphoric acid.
2.1.4 Toxicity, single dose
Oral: LD50 rat (M) 14 mg/kg
LD50 rat (F) 24 mg/kg
Dermal: LD50 rat (M) 67 mg/kg
LD50 rat (F) 67 mg/kg
Most susceptible species: rat oral LD50 14-24 mg/kg.
2.1.5 Toxicity, repeated doses:
Oral: See dietary studies
Dermal: No information
Inhalation: No information
Cumulation of compound: Parathion-methyl does not accumulate in
body tissue. When 37-P labelled parathion-methyl was
administered orally to guinea-pigs, the phosphorus was found to
enter the organs almost immediately and the maximum tissue level
was attained in 1-2 h.
Cumulation of effect: Repeated exposure to sublethal amounts
may reduce cholinesterase activity to hazard levels.
2.1.6 Dietary studies
Short-term: In dogs which were fed 5, 20 or 50 mg/kg diet
(0.25, 1.0, or 2.5 (mg/kg)/day) of parathion-methyl for 12 weeks,
the no-effect level with respect to plasma cholinesterase was 5
mg/kg (0.25 (mg/kg)/day) and possibly 20 mg/kg (1.0 mg/kg)/day).
Erythrocyte cholinesterase inhibition reached a maximum at 50
mg/kg at the end of the 12-week period but recovery was complete
4-8 weeks after withdrawal of the parathion-methyl.
Long-term: No information.
2.1.7 Supplementary studies of toxicity
Mouse: Seven-week dietary administration of 3 dosage levels to
male mice did not produce dominant lethal effects.
Rat: In a three-generation reproduction study there was reduced
reproductive performance at a dietary level of 30 mg/kg (1-5
(mg/kg)/day) but there was no such effect at 10 mg/kg (0.5
(mg/kg)/day). There is no information on cholinesterase activity
levels in this study.
Mouse: Teratogenic effects were observed in the offspring of
mice injected once on day 10 of gestation with 60 mg/kg of
parathion-methyl. Retardation of ossification of the caudal
vertebrae, increased incidence of cervical rib and cleft palate
were observed. A similar effect was not seen in rats injected
with 15 mg/kg on day 12 of gestation.
2.1.8 Modifications of toxicity: No information.
2.2 TOXICOLOGY - MAN
2.2.1 Absorption - See 2.1.1
2.2.2 Dangerous doses
Single: The dangerous dose is not known; based upon animal
studies and the relatively good occupational safety record it is
assumed to be higher than for parathion, since parathion-methyl
is more rapidly hydrolysed.
Repeated: See "Observations on volunteers".
2.2.3 Observations on occupationally exposed workers - See 2.2.6
2.2.4 Observations on exposure of the general population - Based upon
information on the dietary intake of all organophosphorus
pesticides in one country it can be assumed that the general
public will not be exposed to hazardous amounts of parathion-
2.2.5 Observations on volunteers - No reduction in plasma or
erythrocyte cholinesterase activity and no side effects were
observed in human subjects given a maximum of 4.0 mg/day for 43
days. In subjects given a maximum of 6.5 mg/day for 35 days,
followed by 7.0 mg/day for 24 days, the maximum in plasma
cholinesterase activity was approximately 15% reduction. When
doses of 7, 7.5, 8 and 9 mg/day of parathion-methyl were given
for 30 days the plasma and erythrocyte cholinesterase activities
were within 20'/ of the control values.
2.2.6 Reported mishaps - Unlike its ethyl analogue parathion,
parathion-methyl has not been associated with a large number of
cases of intoxication and death. All confirmed cases of human
poisoning have had substantial exposure. Most of the reported
fatal cases appear to be suicide.
2.3 TOXICOLOGY, OTHER SPECIES
The entries in these sections are intended to draw attention to
special risks and to give warnings of any needs for special
2.3.1 Fish: Toxic to fish (LC50 for bluegills and rainbow trout in
range 5-10 mg/l).
2.3.2 Birds: Toxic to birds.
2.3.3 Other species - Toxic to bees.
3. FOR REGULATORY AUTHORITIES
3.1 RECOMMENDED RESTRICTIONS ON AVAILABILITY
All formulations in category 3 (for definition of categories
3.2 TRANSPORTATION AND STORAGE
United Nations Classification 6.1 No. 1893 - Should be
transported or stored in clearly labelled rigid and leakproof
containers, and away from containers of food and drink. Storage
should be under lock and key and secure from access by
unauthorized persons and children.
All formulations - Full protective clothing (see Part 4) should
be provided for all handling of 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
3.4 DISPOSAL AND/OR DECONTAMINATION OF CONTAINERS
All formulations - Container must either be burned or crushed
and buried below topsoil. Care must be taken to avoid subsequent
contamination of water sources. Decontamination of containers in
order to use them for other purposes should not be permitted.
3.5 SELECTION AND TRAINING AND MEDICAL SUPERVISION OF WORKERS
All formulations - Pre-employment medical examination of workers
necessary. Workers suffering from active hepatic or renal
disease should be excluded from contact. Pre-employment and
periodic cholinesterase test for workers desirable. A periodic
urinary p-nitrophenol test may be used as an alternative.
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 early symptoms of poisoning,
and must wear a suitable respirator. Use of flagmen not
recommended. Flagmen, if used, should wear protective clothing
and be located well away from the dropping zone.
Minimum cautionary statement - All formulations.
"Parathion-methyl is an organophosphorus compound which inhibits
cholinesterase. it is a very toxic substance. Contact with the
skin, inhalation of dust or spray, or swallowing may be fatal.
Wear protective gloves, clean protective clothing, and a
respirator of the organic-vapour type when handling this
material. Bathe immediately after work.
"Ensure that containers are stored under lock and key. Empty
containers must be disposed of in such a way as to prevent all
possibility of accidental contact with them. Keep the material
out of reach of children and well away from foodstuffs, animal
feed and their containers.
"In case of contact, immediately remove contaminated clothing and
wash the skin thoroughly with soap and water; for eyes, flush
with water for 15 minutes.
"If poisoning occurs, call a physician. Atropine and pralidoxime
are specific antidotes and repeated doses may be necessary.
Artificial respiration may be needed."
3.8 RESIDUES IN FOOD
3.8.1 Maximum residue levels - The Joint FAO/WHO Meeting on Pesticide
Residues has recommended maximum residue levels.
4. PREVENTION OF POISONING IN MAN AND EMERGENCY AID
4.1. PRECAUTIONS IN USE
4.1.1 General - Parathion-methyl is a highly toxic organophosphorus
pesticide. it penetrates the intact skin and is also absorbed by
inhalation and via the gastro-intestinal tract. Most
formulations should be handled by trained personnel wearing
4.1.2 Manufacture and formulation
T.L.V. - 0.2 mg/m3. Levels o f 0.1 mg/m3 have been established
in Hungary and Romania. Closed systems and forced ventilation
may be required to reduce as much as possible the exposure of
workers to the chemical. Formulation should not be attempted
without advice from the manufacturer.
4.1.3 Mixers and applicators - When opening the container and when
mixing, protective impermeable boots, clean overalls, gloves and
respirator should be worn. Beware of possible positive pressure
build up, especially with liquid formulations in metal containers
with inverted pour spouts. Mixing, if not mechanical, should
always be carried out with a paddle of appropriate length. When
spraying tall crops or during aerial application, a respirator
should be worn as well as an impermeable hood, clothing, boots
and gloves. The applicator should avoid working in spray mist
and 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 inside of
the gloves. Splashes must be washed immediately from the skin or
eyes with large quantities of water. 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 parathion-methyl and associated with its
application should wear protective clothing and observe the
precautions described above in 4.1.3 under "Mixers and
4.1.5 Other populations likely to be affected - With good agricultural
practice subject to 4.2 below, other populations should not be
exposed to hazardous amounts of parathionmethyl.
4.2 ENTRY OF PERSON INTO TREATED AREAS - Unprotected persons should
be kept out of treated areas for several days. Periods of 6-21
days, according to crops, have been recommended.
4.3 SAFE DISPOSAL Of CONTAINERS AND SPILLAGE - Residues in containers
should be emptied in a diluted form into a deep pit taking care
to avoid contamination of ground waters. Decontamination of
containers in order to use them for other purposes should not be
permitted. Spillage of parathion-methyl 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 - Early symptoms of poisoning may
include excessive sweating, headache, weakness, giddiness,
nausea, vomiting, stomach pains, blurred vision, slurred speech,
and muscle twitching. Later there may be convulsions, 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 parathion-methyl.
5. FOR MEDICAL AND LABORATORY PERSONNEL
5.1 MEDICAL DIAGNOSIS AND TREATMENT IN CASES OF POISONING
5.1.1 General information - An organophosphorus pesticide of high acute
toxicity which is easily absorbed through the intact skin as well
as by inhalation and via the gastrointestinal tract. It is
converted in vivo to the oxygen analogue paraoxon-methyl which
then inhibits acetyl cholinesterase. Continued exposure to low
amounts may reduce blood cholinesterase 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
5.1.3 Laboratory - The most important laboratory finding is reduction
in activity of blood cholinesterases. Other findings are raised
urinary levels of p-nitrophenol.
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.
Persons without signs of respiratory inefficiency but with
manifest peripheral symptoms should be treated with 2-4 mg of
atropine sulfate and additionally with 1000-2000 mg of
pralidoxime chloride or 250 mg of toxogonin (adult dose) by slow
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 4-8 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 as a
guide to further administration of atropine. If the patient is
cyanotic, artificial respiration should be given at the same time
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.
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 hypoxia may give rise to permanent brain damage.
5.1.6 References of previously reported cases - Case histories and
general methods for treatment are given in: Hayes, W. J., jr
(1963) Clinical Handbook on Economic Poisons, United States
Public Health Service,No. 476, revised. See also "Safe use of
pesticides in public health" (1967) Wld Hlth Org. techn. Rep.
Ser., No. 356, pp. 58-59.
5.2 SURVEILLANCE TESTS
Test Normal Action Symptomatic
level level level
Plasma cholinesterase 100% 50% variable
Erythrocyte cholinesterase 100% 70% usually < 40%
5.3 LABORATORY METHODS
References are given only.
5.3.1 Detection and assay of compound - It is unlikely that unchanged
parathion-methyl will be detectable in human tissue unless there
has been gross exposure. Determination of the activities of
blood cholinesterases and p-nitrophenol in the urine (see 5.3.2
below) should be used in cases of suspected poisoning. For
parathion-methyl in foodstuffs, determination can be made by
using the colorimetric method of Averell & Norris (1948).
Specific detection of parathion-methyl is possible by paper and
thin-layer chromatography, see: Abbott et al. (1965) and Getz
(1965). There are also a number of gas-liquid chromatographic
methods, see: Nelson (1966), Egan et al. (1964) and Guiffrida et
al. (1966). A multi-detection method is also reported by Bowman
& Beroza (1968).
5.3.2 Other tests in cases of poisoning
Levels of cholinesterase in the blood, particularly plasma,
provide the most useful diagnosis of poisoning, see: Michel, N.
0. (1949) J. Lab. Clin. Med., 34, 1564; Ellman, G. L.,
Courtney, K. D., Andres, V., jr & Featherstone, R. M. (1961)
Biochem. Pharmacol., 7, 88.
Urinary levels of p-nitrophenol are also an index of poisoning:
p-nitrophenol may be determined colorimetrically (Elliott et al.,
1960) or by gas chromatography (Crammer, 1970).
Levels of phosphate esters in urine may also be determined in
order to give an indication of the extent of exposure by the
method of Shafik & Enos (1969) as modified by Shafik et al.
Abbot, D. C., Crosby, N. T. & Thomson, J. (1965) Proc. Soc. Anal.
Chem. Conf., p. 121
Averell, P. R. & Norris, M. V. (1948) Anal. Chem., 20, 753
Bowman, M. C. & Beroza, M. (1968) (1948) Anal. Chem., 20, 753
Cranmer, M. (1970) Bull. Environm. Contamin. Toxicol., 5, 329
Egan, H., Hammon, E. W. & Thomson, J. (1964) Analyst., 89, 175
Elliott, J. W., Walker, K. D., Penick, A. E. & Durham, W. F. (1960) J.
Agr. Fd. Chem., 8, Ill
Getz, M. E. (1963) Res. Rev., 2, 9
Guiffrida, L., Ives, N. F. & Bostwick, D. C. (1966) J. Assoc. Offic.
Anal. Chem., 49, 8
Shafik, M. T. & Enos, H. F. (1969) J. Agr. Fd. Chem., 17, 1186
Shafik, M. T., Bradway, D., Biros, F. J. & Enos, H. F. (1970) J. Agr.
Fd. Chem., 18, 1174