FAO Meeting Report No. PL/1965/10/1
    WHO/Food Add./27.65


    The content of this document is the result of the deliberations of the
    Joint Meeting of the FAO Committee on Pesticides in Agriculture and
    the WHO Expert Committee on Pesticide Residues, which met in Rome,
    15-22 March 19651

    Food and Agriculture Organization of the United Nations
    World Health Organization

    1 Report of the second joint meeting of the FAO Committee on
    Pesticides in Agriculture and the WHO Expert Committee on Pesticide
    Residues, FAO Meeting Report No. PL/1965/10; WHO/Food Add./26.65


    Chemical names

    phosphorothioate; diethyl 2-isopropyl-6-methyl-4-pyrimidinyl
    phosphorothionate; O, O-diethyl-O-(4-methyl-2-

    Empirical formula


    Structural formula



    Biochemical aspects

         Diazinon can be broken down to diazoxon and
    tetraethylmonothiopyrophosphate which are very potent cholinesterase
    inhibitors (Schrader, 1963). Experiments carried out with diazinon
    labelled with 32P in cow (Robbins et al., 1957) and a goat (Vigne et
    al., 1957) have shown that the 32P is rapidly eliminated in the
    urine, since only a small proportion of the radioactivity can be
    detected after 24 hours in the blood, faeces and milk. The urinary
    elimination products in the cow have been studied by a combination of
    paper chromatography and measurement of radioactivity; they largely
    consist of metabolites which include diethyl thiophosphate and diethyl
    phosphate. Similar results have been observed in a dog (Miller, 1963).

         In experiments in which guinea-pigs received 32P-labelled
    diazinon either orally or subcutaneously, it was concluded that
    diazinon is efficiently absorbed through the intestine and eliminated
    with the urine. The caecum might have a role in the metabolism of the
    compound, even if it is injected subcutaneously (Kaplanis et al.,
    1962). Technical diazinon contains 1-3% of dipyrimidyl ester which has
    a slight insecticidal activity (oral LD50 to mice approximately 325
    mg/kg). A by-product is diethyl thiophosphate which has no
    insecticidal properties (oral LD50 to mice 750 mg/kg) (Gysin &
    Margot, 1958).

         Hereford cattle were sprayed weekly with 1-1.5 gallons of a spray
    containing 0.0-0.1% diazinon for 16 weeks and samples of omental fat
    were examined 1-14 days after the last application. Residues of
    diazinon were found at 1 and 7 days but not after 14 days (Claborn et
    al., 1963).

    Acute toxicity
    Animal                Route            LD50 mg/kg     References

    Mouse, male           Oral                   82       Bruce et al., 1955
                   (technical product)

    Mouse                 Oral               77-122.5     Gasser, 1953
                     (as emulsion or

    Mouse            Intraperitoneal             65       Klotzche, 1955
                   (technical product)

    Rat, male             Oral              100-150       Gasser, 1953
                   (technical product)

    Rat, male             Oral                  108       Gaines, 1960

    Rat, female           Oral                   76       Gaines, 1960

    Guinea-pig            Oral                  320       Gasser, 1953

    Rabbit                Oral                  143       Gasser, 1953

    Turkey                Oral                    6.81    Hazleton Laboratories, 1954

    Chicken               Oral                   40.8     Hazleton Laboratories, 1954

    Goose                 Oral                   14.7     Hazleton Laboratories, 1954
             Sheep and cattle. In calves oral doses of 1 mg/kg produced
    signs of toxicity and 10 mg/kg is a lethal dose. In steers and sheep
    doses up to 25 and 30 mg/kg respectively produced toxic signs but not
    death. Doses of 10 and 20 mg/kg respectively were non-toxic (Radeleff,

         Man. One man swallowed a quantity of diazinon equivalent to 30
    mg/kg body-weight without any detriment to health (Gassmann, 1957),
    and another man took an amount equivalent to 250 mg/kg body-weight and
    recovered after treatment (Bockel, 1957).

    Short-term studies

         Mouse. One batch of 10 animals was subjected to the daily oral
    administration of approximately 2.2 mg/kg as a 20% emulsion; 2 of the
    animals were still alive after 330 days (Gasser, 1953).

         Rat. The daily oral dose of diazinon (as an emulsion) which can
    be tolerated by 50% of the animals for at least 30 days is
    approximately 55 mg for males and approximately 77 mg for females
    (Gasser, 1953).

         Two groups, each of 10 male rats, were fed for 4 weeks on diets
    containing respectively 100 and 1000 ppm of technical diazinon (85%
    pure). No toxic symptoms or pathological lesions were noted in the
    experimental groups, except a slight inhibition of growth at the
    higher concentration, i.e. 1000 ppm. A distinct inhibition of the
    cholinesterase activity of the brain and particularly of the
    erythrocytes was seen at the higher concentration, whereas at the
    concentration of 100 ppm, only the cholinesterase activity of the
    erythrocytes was inhibited to any significant extent. No significant
    inhibition of the plasma cholinesterase activity was found at either
    of the 2 concentrations tested (Bruce et al., 1955).

         Seven groups of young rats, 5 males and 5 females, were fed for 6
    months diets containing 1, 2, 4, 8 and 16 ppm of diazinon (in the form
    of a freshly made solution in olive oil or in the form of oil
    containing the insecticide as a residue). As compared with 2 control
    groups of 10 animals each, no influence on growth-rate was found. The
    cholinesterase activity of the erythrocytes was significantly
    inhibited while that of the plasma remained practically unchanged. No
    differences in toxicity were detected among rats given freshly-made
    solutions and those given the insecticide as a residue (Mélis et al.,

         Groups of male rats were fed for 16 weeks diets containing
    respectively 1, 5, 25 and 125 ppm of diazinon. At the end of this
    treatment, there was no significant inhibition of the plasma or brain
    cholinesterase activity in the animals fed 25 ppm, whereas the
    cholinesterase activity of the erythrocytes was decreased by 46%. The
    concentrations of 5 and 1 ppm did not bring about any significant
    changes (Edson & Noakes, 1960).

         Groups of 30 rats (15 of each sex) were given 0.5, 1, 2 or 4 ppm
    diazinon in their diets for 90 days. No weight or growth changes were
    observed. Cholinesterase activity in the plasma was inhibited at 4 ppm
    (Hazleton Laboratories, 1956).

         Dog. Six dogs, 2 males and 4 females, received, on 6 days per
    week for 43 weeks, oral doses of diazinon as a water-dispersible
    powder in capsules. In animals receiving doses of up to 6.5 mg/kg
    body-weight (on the average of 4.3 mg/kg), no visible symptoms were
    noted, but as compared with 2 controls, 1 male and 1 female, there was
    a very distinct inhibition of the cholinesterase activity of the plasm

    and erythrocytes. With doses of 9.3 mg/kg body-weight or above, toxic
    effects were observed in addition to inhibition of cholinesterase
    activity (loss of appetite, slight loss of weight, excitation or
    depression, trembling) (Bruce et al., 1955).

         Three groups of 2 dogs each, one male and one female, received
    for 90 days diets containing 0.25, 0.75 and 75 ppm of diazinon and
    were compared with 5 controls. The erythrocyte cholinesterase activity
    was inhibited only at the concentration of 75 ppm whereas inhibition
    of plasm cholinesterase activity was also observed at the
    concentration of 0.75 ppm. However, this inhibition was small and the
    activity returned to normal after an additional period of 6 weeks,
    despite continuation of the treatment. At the concentration of 0.25
    ppm no significant inhibition of blood cholinesterase activity was
    observed (Williams et al., 1959).

         Groups of 2 male and 2 female dogs received diazinon as daily
    oral doses of either 0.02, 0.04 or 0.08 mg/kg/day for 31 days.
    Inhibition of plasma cholinesterase, was obvious in the latter group
    after 3 days. In the group receiving 0.04 mg/kg/day only a moderate
    decrease was observed (Hazleton Laboratories, 1956).

         Monkeys. In an experiment which is still in progress, groups of
    3 males and 3 female monkeys were given daily oral doses of 0.05, 0.5,
    5 or 10 mg/kg of diazinon. Doses of 10 mg/kg/day for 3 weeks produced
    general signs of sickness. Plasm cholinesterase activity decreased at
    doses of 0.5-1 mg/kg/day or more. Red blood cells cholinesterase
    decreased with doses of 5 mg/kg/day or more (Geigy, 1964).

         Cattle. Cows were given orally 1.06, 5.3 or 10.6 mg/kg/day for
    3 weeks and steers received 1.06 or 5.3 mg/kg/day for 2 weeks. Blood
    cholinesterase was inhibited in all groups. No diazinon residues were
    found in the cows' milk. In steers diazinon was found in the fat
    tissue (Rai & Roan, 1960).

         Fifteen calves were given 10, 25, 40 or 80 ppm of diazinon in the
    diet, starting at 1 week of age for 14 weeks. Blood cholinesterase
    inhibition was obvious in all the groups, with a dose-response
    relationship. However, it did not appear before weaning. One calf
    given 40 ppm died after 12 weeks of treatment, and the death was
    considered "indirectly" related to the latter. In animals killed after
    the end of the treatment no pathological changes were found (Geigy,

    Long-term studies

         Rat. Two groups of 40 weanling rats each, 20 males and 20
    females, and 1 group of 26 male rats were fed for 72 weeks on diets
    containing 10, 100 and 1000 ppm of diazinon incorporated as a 25%
    water-dispersible powder. At the concentration of 1000 ppm some
    inhibition of growth was observed, while at all concentrations some of
    the animals developed respiratory troubles and skin lesions. These

    symptoms, nevertheless, cannot be regarded as significant, since they
    also appeared in some of the controls. The authors carried out
    autopsies on a number of the animals, but did not find any macroscopic
    or histological lesions. Nor did they find any significant difference
    in food consumption and mortality rate as between the treated and
    control batches (Bruce et al., 1955).

    Comments on experimental studies reported

         The information concerning the long-term toxicity effects of
    diazinon is inadequate. On the other hand, the influence of the
    repeated ingestion of diazinon on the blood cholinesterase activity of
    the rat and the dog has been well studied.


    Level causing no significant toxicological effect

         Rat: 2 ppm in the diet, equivalent to 0.10 mg/kg/day.

         Dog: 0.02 mg/kg/day.

    Estimate of acceptable daily intake for man

         0-0.0002 mg/kg/day.

    Comment and further work required

         The acceptable daily intake figure for diazinon arrived at by
    applying the customary safety factor to the no-effect level in the
    most sensitive species (i.e. the dog), is much lower than any other
    acceptable daily intake figure for organo-phosphorus compounds in the
    present series, thus suggesting a high toxicity of this compound. This
    did not seem to be borne out by user experience in the field but the
    Committee emphasized the lack of definite information on the toxicity
    of diazinon to man.


    Bockel, P. (1957) Dtsch. med. Wschr., 1230

    Bruce, R. B., Howard, J. W. & Elsea, J. R. (1955) J. Agric. Food
    Chem., 3, 1017

    Claborn, H. V., Mann, H. D., Younger, R. L., & Radeleff, R. D. (1963)
    J. Econ. Ent., 56, 858

    Edson, E. F. & Noakes, D. N. (1960) Toxicol. Appl. Pharmacol., 2,

    Gaines, Th. B. (1960) Toxicol. Appl. Pharmacol., 2, 88

    Gasser, R. (1953) Ztschr. f. Naturforsch., 8b, 225

    Gassmann, R. (1957) Praxis, 46 (18) 393; (19), 416

    Geigy (1963) Unpublished data

    Geigy (1964) Unpublished data

    Gysin, H. & Margot, A. (1958) J. Agr. Food Chem., 6, 900

    Hazleton Laboratories (1954) Unpublished data

    Hazleton Laboratories (1956) Unpublished data

    Kaplanis, J. N., Louloudes, S. J. & Roan, C. C. (1962) Trans. Kansas
    Acad. Sci., 65, 70

    Klotzche, C. (1955) Arzneimittel-Forsch., 5, 436

    Mélis, R., Montanelli, P. & Mélis, G. (1959) Ann. Sanità pubbl.,
    20, 5

    Miller, C. R. (1963) N.Z. Vet. J., 11 (Received from Geigy
    Agricultural Chemicals (1964))

    Radeleff, R. D. (1958) Advanc. vet. Sci., 4, 265

    Rai, L. & Roan, C. C. (1960) Submitted P. P. 232

    Robbins, W. E., Hopkins, T. L. & Eddy, G. W. (1957) J. Agr. Food
    Chem., 5, 509

    Schrader, J. (1963) Die Entwicklung neuer insektizider
    Phosphorsäure-Ester, Verlag Chemie Weinheim

    Vigne, J. P., Chouteau, J., Tabau, R. L., Rancien, P. & Karamanian, A.
    (1957) Bull. Acad. vét. Fr., 30, 84

    Williams, M. W. Fuyat, H. N. & Fitzhugh, O. G. (1959) Toxicol. Appl.
    Pharmacol., 1, 1

    See Also:
       Toxicological Abbreviations
       Diazinon (EHC 198, 1998)
       Diazinon (ICSC)
       Diazinon (FAO/PL:CP/15)
       Diazinon (FAO/PL:1967/M/11/1)
       Diazinon (FAO/PL:1968/M/9/1)
       Diazinon (AGP:1970/M/12/1)
       Diazinon (WHO Pesticide Residues Series 5)
       Diazinon (Pesticide residues in food: 1979 evaluations)
       Diazinon (Pesticide residues in food: 1993 evaluations Part II Toxicology)
       Diazinon (JMPR Evaluations 2001 Part II Toxicological)