WORLD HEALTH ORGANIZATION FOOD AND AGRICULTURE
ORGANIZATION
ORGANISATION MONDIALE DE LA SANTE ORGANISATION POUR L'ALIMENTATION
ET L'AGRICULTURE
VBC/DS/77.29
ORIGINAL: ENGLISH
DATA SHEETS ON PESTICIDES No. 29
MALATHION
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
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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:
1. GENERAL INFORMATION
1.1 COMMON NAME:
Malathion (ISO)
Identity: diethyl [(dimethoxyphosphino-thioyl)thio]butanedioate
Synonyms: OMS 1 Malathion
Carbofos TM 4049
Fyfanon Mercaptothion
Local synonyms:
1.2 SYNOPSIS
An organophosphorus insecticide and acaricide of moderate mammalian
toxicity, and brief to moderate persistence. It is an indirect
inhibitor of cholinesterase.
1.3 SELECTED PROPERTIES
1.3.1 Physical characteristics
Malathion is a clear amber liquid of m.p. 2.85°C and b.p. 156-157°C
at 0.7 mm Hg. It has a mercaptan-like odour. The technical product
is about 95% pure.
1.3.2 Solubility
Approximately 145 ppm in water at 25°C; it is miscible with most
organic solvents though of limited solubility in petroleum oils.
1.3.3 Stability
Malathion is rapidly hydrolysed at pH above 7.0 or below 5.0 but is
stable in aqueous solution buffered at pH 5.26. It is incompatible
with alkaline pesticides and is corrosive to iron.
1.3.4 Vapour pressure (volatility)
4 x 10-5 mm Hg at 30%.
1.4 AGRICULTURE, HORTICULTURE AND FORESTRY
1.4.1 Common formulations
Water dispersible powders 25%; dusts 4% emulsifiable concentrates
5-10 lb/US gall. Aerosols 2-20%. Pressurized spray bombs; fly baits,
dry baits.
1.4.2 Susceptible pests
Aphids, Mexican bean beetle, plum curculio, houseflies, mosquitos,
animal ectoparasites, stored product pests.
1.4.3 Use pattern
Wide range of agricultural and horticultural uses on all types of
vegetables and firm fruits. Should not be applied to crops with
sulfur or when sulfur residues are present.
1.4.4 Unintended effects
Because of unpleasant smell, certain crops develop taint and should
not be treated. Slightly phytotoxic to sweet cherries and, under
greenhouse conditions, to string beans, squash and cucumber.
1.5 PUBLIC HEALTH PROGRAMMES:
Used in malaria control as a residual insecticide applied indoors on
walls and roofs at 1-2 g/m2; usually applied as a 5% a.i.
suspension of a water dispersible powder. Also used extensively for
control of mosquito borne viral infections from ground and from air,
usually at the rate of 500 ml/ha or as a thermal fog using 2-4%
solution in diesel oil.
As a mosquito larvicide 2.5% suspension or emulsion is used. Also
recommended against DDT resistant fleas as 5% dust in the burrows of
the rodents.
1.6 HOUSEHOLD USE:
Malathion is an effective insecticide against most household pests
and has been used to control houseflies, cockroaches, mosquitos,
aphids, animal ectoparasites and human head and body lice. For
application to the body, it is used at a 1% concentration.
2. TOXICOLOGY AND RISKS
2.1 TOXICOLOGY - MAMMALS
2.1.1 Absorption route
Malathion may be absorbed by inhalation, from the gastrointestinal
tract or through the intact skin.
2.1.2 Mode of action
Cholinesterase inhibition after metabolism to malaoxon, the oxygen
analogue.
2.1.3 Excretion products
A lactating cow was dosed with 32P labelled malathion at 1.3 mg/kg
for three days. After seven days 77.2% of the dose was recovered,
69% in the urine, 8% in the faeces and 0.2% in milk. The principle
metabolite in early samples was the monoacid of malathion and in
later samples the diacid. Demethyl malathion, dimethyl phosphate and
O,O-dimethylphosphorothioate were also present. In the faeces 85% of
the labelled material was malathion and 12% malaoxon.
2.1.4 Toxicity, single dose
Oral LD50 rats (M) 1375 mg/kg
LD50 rats (F) 1000 mg/kg
Note: These values may be lower in some cases where more toxic
contaminants are present. For example, the oral LD50 for rats of
isomalathion is 89 mg/kg and this contaminant is sometimes present
to a significant degree.
Dermal LD50 rats of either sex >4444 mg/kg
(*sex not stated)
LD50 rabbits* 4100 mg/kg
Most susceptible species: the cow, oral LD50 560 mg/kg and
guinea-pig, oral LD50 570 mg/kg.
2.1.5 Toxicity, repeated doses
Oral: See para. 2.1.6
Inhalation: Rats and mice were exposed twice for two hours 45
minutes on day one and 7.5 hours on day two in Air near saturated
with 99% technical malathion. One out of 13 mice and none out of
seven rats died. All showed signs of discomfort but recovered
rapidly.
Cumulation of compound: Malathion is not cumulative in body
tissues.
Cumulation of effect: Due to its effect on cholinesterase,
continued exposure may reduce cholinesterase activity to hazardous
levels.
2.1.6 Dietary studies
Short-term: Ninety-five per cent. technical malathion was fed to
three groups of rats for 33 days at 100, 1000, and 5000 ppm. No sign
of toxicity was observed, nor any deaths, and food intake and weight
gain in the groups fed 100 and 1000 ppm were higher than in the
control group. Cholinesterase activity was not affected at 100 ppm.
Erythrocyte cholinesterase activity was 68% of normal at 1000 ppm,
22% of normal at 5000 ppm, and plasma cholinesterase was 78% of
normal at 5000 ppm. No inhibition of brain cholinesterase was found.
Long-term: Ninety per cent. technical malathion was fed as a 25%
wettable powder in the diet to groups of rats at 100, 1000 and 5000
ppm for two years. Mortality rate, growth response and food intake
were not influenced by any of these diets, except for slight growth
retardation At 5000 ppm. Terminal cholinesterase determinations
revealed 10-30% inhibition of cholinesterase activity in plasma,
erythrocytes and brain at 100 ppm. At 1000 ppm, 60-95% inhibition of
erythrocyte cholinesterase activity was observed. The 5000 ppm group
showed total inhibition of erythrocyte cholinesterase activity and
60-95% inhibition of plasma and brain activity.
2.1.7 Supplementary studies of toxicity
Carcinogenicity: There are no reports of an experiment primarily
concerned with carcinogenicity, however, no increased incidence of
cancers has been reported in several long-term dietary studies in
rats.
Reproduction and teratogenicity. Ninety-five per cent. technical
malathion was fed to 40 male and 40 female rats for five months at
240 mg/kg/day (4000 ppm in the diet). Growth was normal And no signs
of intoxication occurred. Ten weeks after the beginning of the
experiment 18 females and 12 males were used for breeding. The
average litter size from the treated females was smaller than the
controls and the number of newborn alive after 7 and 21 days was
about half the number in the control litters.
Mutagenicity: When tested both in a histidine reverse mutation
system (Ames test) and in a dominant lethal test, malathion was
found to be nonmutagenic.
Delayed neurotoxicity: No delayed neurotoxicity was observed in
experimental studies with hens.
2.1.8 Modification of toxicity:
Some impurities present in technical malathion may significantly
Potentiate its mammalian toxicity through the blocking of normal
enzymatic detoxifying pathways. Simultaneous administration of
malathion and ethyl p-nitrophenyl thionobenzene phosphate (EPN) has
resulted in a potentiation of the cholinesterase inhibitory effect
of malathion in the mouse, rat and dog. Similarly, potentiation was
observed after oral administration of malathion and fenitrothion
when one-half the LD50 doses were given. However, no potentiation
was seen when one-tenth the LD50 doses were given.
Newborn rats are more susceptible: for them the LD50 was 124 mg/kg
in a colony where the LD50 for adults was 925 mg/kg.
2.2 TOXICOLOGY - MAN
2.2.1 Absorption
Malathion may be Absorbed by inhalation, from the gastrointestinal
tract and though the intact skin.
2.2.2 Dangerous doses
Single: Ingestion of 4 g of malathion caused severe but non-fatal
poisoning in a child and a dose of 14 g had a similar effect on a
woman. However, fatalities have resulted from 5 g in a 75-year-old
man, 1.5 hours after ingestion. Another fatality resulted from an
estimated dose of 56 g of malathion.
Repeated: A dosage of 24 mg of malathion for 56 days produced a
maximal reduction of 25% in both plasma and erythrocyte
cholinesterase activity.
2.2.3 Observations of occupationally exposed workers
Until recently, extensive use of malathion has not,apparently led to
significant reduction of blood cholinesterase activity in
applicators, though small falls in SGOT, SGPT and serim aldolase
have been reported following heavy exposure with insufficient
protective clothing. In 1976 a large number of cases of intoxication
with five deaths occurred in a group of spraymen and mixers in a
malaria control programme. This was caused by the use of batches of
malathion with a higher toxicity than usual due to the presence of
contaminants (see section 2.1.4) and the neglect of safety
precautions, especially washing and changing clothes after work.
2.2.4 Observations on exposure of the general population
In a two-year study of the pesticide contamination of dietary
intake, malathion accounted for 80% of the calculated daily intake
of organophosphorus compounds. The average daily intake for this
two-year period was 0.009 mg malathion per day. (The joint FAO/WHO
meeting of Pesticides Residues in Food, 1972, gives a maximum
acceptable daily intake for malathion as 0.02 mg/kg/day.) No
problems have been encountered with correct usage of 1% formulations
of malathion as a dusting powder for control of body lice or as a
shampoo for control of head lice.
2.2.5 Observations of volunteers
Volunteers given 8 mg of malathion for 32 days and 16 mg of
malathion for 47 days, were unaffected, and their cholinesterase was
normal. 24 mg malathion taken for 56 days was followed by a
significant reduction of plasma cholinesterase activity after two
weeks. Three weeks after the cessation of administration maximum
reduction of 25% was observed. Ten volunteers ingesting 6 mg EPN And
16 mg malathion daily for 42 days showed a slight inhibition of both
plasma and erythrocyte cholinesterase.
2.2.6 Reported mishaps
Most cases of poisoning have involved gross misuse, usually
involving accidental or suicidal ingestion of this compound, or have
occurred in occupational use (see 2.2.3 above). The latter have
usually been due to lack of safety precautions which leads to
poisoning if men are exposed to a batch of malathion of higher
toxicity than normal due to the presence of contaminants.
2.3 TOXICITY TO NON-MAMMALIAN SPECIES
2.3.1 Fish
Toxic.
2.3.2 Birds.
Moderately toxic (mallards LD50 1485 mg/kg).
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)
Formulations of 2% or above, category 4; Formulations below 2%,
category 5
3.2 TRANSPORTATION AND STORAGE
Formulations in category 4 - Should be stored in clearly labelled
rigid and leakproof containers (it should be noted that malathion is
corrosive to some metals) under lock and key, safe from access by
unauthorized persons and children. No food or drink should be stored
in the same compartment.
Formulations in category 5 - Should be stored in clearly labelled
leakproof containers out of reach of children, away from food and
drink.
3.3 HANDLING
Formulations in category 4 - Protective clothing (see 4.1.3 on
part 4) should be used by all 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.
Formulations in category 5 - No facilities other than those needed
for handling of any chemical need be required.
3.4 DISPOSAL AND/OR DECONTAMINATION OF CONTAINER
All formulations - Container may be decontaminated (for method see
para. 4.3 on page 4). Decontaminated containers should not be used
for food and drink. Containers that are not decontaminated should be
burned or should be crushed an buried below topsoil. Care must be
taken to avoid subsequent contamination of water sources.
3.5 SELECTION, TRAINING AND MEDICAL SUPERVISION OF WORKERS
Formulations in category 4 - Pre-employment medical examination
for workers desirable. Workers suffering from active hepatic or
renal disease should be excluded from contact. Special account
should be taken of workers' mental ability to comprehend and follow
instructions. Training of workers to avoid contact essential.
Formulations in category 5 - Warning of workers to minimize
contact essential.
3.6 ADDITIONAL REGULATIONS RECOMMENDED IF DISTRIBUTED BY AIRCRAFT
All formulations - Pilot and loaders should have special training
application methods and 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
Formulations in category 4 - Minimum cautionary statement
Malathion is an organophosphorus compound that inhibits
cholinesterase. It is poisonous if swallowed. It may be absorbed
through the skin. Avoid skin contact and wear protective gloves and
clean clothing 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 and pralidoxime are
specific antidotes and artificial respiration may be needed.
Formulations in category 5 Minimum cautionary statement
This formulation contains malathion which is a toxic substance. Keep
the material out of the reach of children and well away from
foodstuffs animal feed and their containers.
3.8 RESIDUES IN FOOD
Maximum residue limits for malathion have been recommended by the
Joint FAO/WHO Meeting on Pesticide Residues.
4. PREVENTION OF POISONING IN MAN AND EMERGENCY AID
4.1 PRECAUTIONS IN USE
4.1.1 General
Malathion is an organophosphorus pesticide of moderate toxicity. It
is absorbed by inhalation, from the gastrointestinal tract or
through the intact skin.
4.1.2 Manufacture and formulation
TLV: (ACGIH) 15 mg/m3 (USSR) 0.5 mg/m3. Closed systems and
forced ventilation may be required to reduce as much as possible the
exposure of workers to the chemical.
4.1.3 Mixers and applicators
When opening the container and when mixing, care should be taken to
Avoid contact with the mouth and eyes. If necessary, a facial visor
and gloves should be worn. Mixing, if not mechanical should always
be carried out with a paddle of appropriate length. Splashes should
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. If contaminated, clothing should,be washed at
the end of a working day.
4.1.4 Other associated workers (including flagmen in aerial
operations)
Persons exposed to malathion and associated with its application
should observe the precautions described above in 4.1.3 under
"mixers and applicators".
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 malathion.
4.2 ENTRY OF PERSONS INTO TREATED AREAS
Unprotected persons should be kept out of treated areas for at least
one day.
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 ground waters. The empty container may
be decontaminated by rinsing two or three times with water and
scrubbing the sides. An additional rinse should be carried out with
5% sodium hydroxide solution which should remain in the container
overnight. Impermeable gauntlets should be worn during this work and
a soakage pit should be provided for the rinsings. Decontaminated
containers should not be used for food and drink. Spillage of
malathion 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, 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 respiratory failure, artificial respiration should be given.
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 moderate toxicity which is absorbed
through the intact skin as well as by inhalation, and from the
gastro-intestinal tract. It acts, after metabolic conversion to the
oxygen analogue, melaoxon, by cholinesterase inhibition. Continued
exposure may inhibit cholinesterase activity to hazard levels.
5.1.2 Symptoms and signs
Initial symptoms may include excessive sweating, headache, weakness,
giddiness, nausea, vomiting, stomach pains, blurred vision, slurred
speech, muscle twitching and hypersalivation. Advanced symptoms of
poisoning may include convulsions, coma loss of reflexes and
sphincter control; cardiac manifestations have been reported.
5.1.3 Laboratory
The most important laboratory finding is reduction in activity of
blood cholinesterase, though this has on occasions been less
pronounced than might be expected from the state of the patient.
Urinary levels of other extractable phosphates is also a useful
measure of exposure, though method may not be specific for
malathion.
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 and 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 4-6 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 as a guide
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. If the inhibitor and the
enzyme have been in contact for a prolonged period - more than 24
hours - the enzyme may become "aged" or irreversibly inhibited: in
this case, the clinical effect of the reactivator may be markedly
diminished.
Contraindicated - 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 anoxia may
give rise to permanent brain damage.
5.1.6 References of previously reported cases
Namba, T. et al. (1970) Arch. of Environmental Health, 21, 533
Sare, W. M. (1972) N. Z. Med. J., 75, 93
Crawley, W. J., jr & Johns, T. R. (1966) Arch. Neurol., 14, 611
Goldman, H. & Teitel, M. J. (1958) Pediat., 52, 76
5.2 SURVEILLANCE TESTS
Test Normal level* Action level* Symptomatic level*
Plasma cholinesterase 100% 50% Variable
Erythrocyte cholinesterase 100% 70% Usually 40%
*Expressed as percentage of pre-exposure activity.
Urinary levels of ether extractable organophosphates may also be
used to determine the degree of exposure.
5.3 LABORATORY METHODS
5.3.1 Detection and assay of compound
References are given only.
Unchanged malathion may be found in the faeces after ingestion.
Determination of blood cholinesterase should be used in cases of
suspected poisoning. For determining levels of malathion in animal
tissues, see Norris, A. et al. (1958) Agr. Food Chem., 6, 111.
Levels of detection by this method are fat, 1.0 ppm, liver 1.0 ppm,
meat 0.5 ppm, eggs 0.5 ppm and milk 0.02 ppm.
A method for plant material involving extraction with carbon
tetrachloride and measurement at 418 mu is given in: Analytical
Methods for Pesticides and Plant Growth Regulators Vol. VI Gas
Chromatographic analysis. Edited by G. Zweig, Academic Press, London
and New York 1972.
5.3.2 Other tests in cases of poisoning
Levels of cholinesterase in blood, particularly plasma, provide the
most useful diagnosis of poisoning, see:
Michel, N. O. (1949) J. Lab. Clin. Med., 35, 1564
Ellman, G. L. et al. (1961) Biochem. Pharmacol., (I) 88
Urinary levels of ether extractable organic phosphorus also give a
measure of exposure.
Mattson, A. M. & Sedlak, V. A. (1960) J. Agr. Food Chem., 8, 107
Enos, H. F. (1970) J. Agr. Food Chem., 18, 1174