FAO/PL:1968/M/9/1 WHO/FOOD ADD./69.35 1968 EVALUATIONS OF SOME PESTICIDE RESIDUES IN FOOD THE MONOGRAPHS Issued jointly by FAO and WHO The content of this document is the result of the deliberations of the Joint Meeting of the FAO Working Party of Experts and the WHO Expert Committee on Pesticide Residues, which met in Geneva, 9-16 December, 1968. FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS WORLD HEALTH ORGANIZATION Geneva, 1969 PARATHION-METHYL This pesticide was evaluated for acceptable daily intake under the heading "Methyl parathion" by the Joint Meeting of the FAO Committee on Pesticides in Agriculture and the WHO Expert Committee on Pesticide Residues (FAO/WHO, 1965). Since that time additional toxicological data have become available as well as data on its residues in food and their evaluation. The earlier monograph is now rendered obsolete and a completely revised monograph is presented in its entirety below. IDENTITY Chemical name OO-dimethyl O-(4-nitrophenyl) phosphorothioate (IUPAC) Synonyms Metaphos, Folidol M, E 605, Nitrox, Wofatox FormulaOther information on identity and properties Typical analyses of technical parathion-methyl are not available. In 1966 the world production of parathion-methyl was 31 700 metric tons (United States of America production, 14 800 metric tons). EVALUATION FOR ACCEPTABLE DAILY INTAKE Biochemical aspects When 32P-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 one to two hours. A high degree of absorption was found in the liver (Gar et al., 1958). Parathion-methyl is biologically similar to parathion and is metabolized to its oxygen analogue, paraoxon-methyl (Augustinsson and Jonsson, 1957). Parathion-methyl is an in vivo cholinesterase inhibitor (Williams et al., 1959). It also inhibits this enzyme in vitro, however it is weaker in this respect than its ethyl analogue, parathion (DuBois and Coon, 1952). The same is true of the respective oxygen metabolites; paraoxon-methyl being a weaker cholinesterase inhibitor than paraoxon. The in vitro molar I50 value for paraoxon-methyl, using rat brain cholinesterase, is 4 × 10-8 (Davison, 1955). Acute toxicity Animal Route LD50 (mg/kg References body-weight) Mouse oral 32.1 Ikeda, 1962 Mouse oral 150 Wills, 1968 Rat (M) oral 14 Wills, 1968 Rat (F) oral 24 Wills, 1968 Rat oral 17.2 Hagan, 1958 Rat (F) oral 9.7-14.8 Deichmann et al., 1952 Rat i.p. 3.5 DuBois and Coon, 1952 Rabbit oral 420 Wills, 1968 (in oil) Rabbit oral 1 270 Wills, 1968 (undiluted) Short-term studies Dog. Pairs of dogs, comprising one male and one female, were fed parathion-methyl for 12 weeks at dietary levels of 5, 20, and 50 ppm along with four dogs used as controls. In the 20 and 50 ppm group, erythrocyte cholinesterase began to be significantly depressed soon after commencement of the test-diet. The same was true of plasma cholinesterase in the 50 ppm group. Maximum extent of depression was attained at the end of the 12-week period but recovery was complete within four to eight weeks after withdrawal of parathion-methyl. Depression of plasma cholinesterase was questionable at the 20 ppm level and no significant anti-cholinesterase activity was found in the 5 pm group. Long-term studies No information available. Special studies (a) Reproduction A three-generation reproduction study using 10 male and 20 female rats per dose level for each generation, at 0, 10 and 30 ppm parathion-methyl, and comprising two litters per generation, revealed no consistent effect on the number of live or stillbirths, birth weights, physical structure of newborn, litter size, weanling weights or percentage survival to weaning. Sporadic effects included lower weanling survival rate in F1a, F1b and F2a generations at 30 ppm, and in F3a generation at 10 ppm, increased stillbirth rate in F1b and F3a generations at 30 ppm, and F3a at 10 ppm; reduced mean weanling rate in F2a generation at 30 ppm, and F1b generation at 10 ppm. Reduced reproductive performance in F1a, F1b, F2aand F3b generations at 30 ppm, was the only parameter consistently affected. No such activity was evident at the 10 ppm level (Woodard Research Corp., 1966). (b) Teratogenicity Some cases of foetal deaths and malformations have been reported in Japan and may possibly be related to the use of organo-phosphorus insecticides in the field (Ogi and Hamada, 1965). Parathion was identified in a human term foetus whose mother had used it to commit suicide (Le Breton et al., 1963). No significant developmental defects in rats whose mothers had been injected intraperitoneally with methyl parathion was observed (Fish, 1966). Because of these reports, the effect on organogenesis in the rat and mouse was studied by injecting intraperitoneally parathion-methyl suspended in a 0.5 per cent aqueous solution of sodium carboxymethyl cellulose once on day 12 of gestation in rats and once on day 10 in mice. The dosage was 5 to 15 mg/kg of body-weight in rats and 20 to 60 mg/kg in mice. The animals were killed near term on day 21 in rats and on day 18 in mice. The foetuses were examined for intrauterine death, external malformations, internal abnormalities and skeletal abnormalities. All animals of both species showed signs of toxicity about 30 minutes after administration of parathion-methyl. Ataxia and paralysis of voluntary muscles were followed by hypersecretion of saliva and tears, urinary incontinence, tremor and general convulsions. Some died and the others recovered by the next day. Food and water intake in rats was noted to be lower for three or four days and there was a weight loss. No external or internal malformations were found. In mice lethality, teratogenicity and suppression of growth were noted in the group treated with the higher dosage. The only malformation was cleft palate, the incidence of which was 0.71 per cent when used as control groups in other experiments. Retardation of ossification of the caudal vertebrae and increased incidence of cervical rib occurred in the group treated with the higher dose (Tanimura et al., 1967). Observations in man Normal levels of red blood cell and plasma cholinesterase were established in 12 human subjects, five of these subjects were given parathion-methyl in dose levels of 3 mg/day for 28 days, 3.5 mg/day for 28 days and 4.0 mg/day for 43 days. No anti-cholinesterase activity in plasma on red blood cells, and no side effects, were observed (Moeller and Rider, 1961) In a second study, three groups of five subjects were given parathion-methyl, The first group received 4.5 mg/day for 30 days followed by 5.0 mg/day for 29 days; the second group, 5.5 mg/day for 28 days, then 6.0 mg/day for 29 days; and the third group, 6.5 mg/day for 35 days, then 7.0 mg/day for 24 days. The maximum depression of cholinesterase occurred in plasma and was approximately 15 per cent of the control values established beforehand (Moeller and Rider, 1962). In a continuing experiment, groups of five subjects were given parathion-methyl for 30 days at daily doses of 7 mg, 7.5 mg, 8 mg and 9 mg. Plasma and red blood cell cholinesterase activities were within 20 per cent of the pre-established control values (Moeller and Rider, 1963). Comments Teratogenic effects were observed in mice only after parenteral administration, on the other hand the reproduction studies in rats showed some disturbance of the physiology of the reproductive process. It was therefore found necessary to re-evaluate this compound by using a higher safety factor and to change the acceptable daily intake to a temporary acceptable daily intake. TOXICOLOGICAL EVALUATION Estimate of temporary acceptable intake 0-0.001 mg/kg body-weight RESIDUES IN FOOD AND THEIR EVALUATION Use pattern Pre-harvest treatments Parathion-methyl is a broad spectrum insecticide and is mainly used on cotton, but is also used on food crops including fruits and vegetables. For plant protection one, two or more applications are usually made, the pre-harvest interval varying from 14 to 21 days and is usually twice as long under greenhouse conditions. Reported application rates are given in the following table: Application rate* Crop (g/ha) Fruit 240-720 Vegetables 240-720 Grain crops (rice) 240-720 Hay, forage crops, and cotton 100-1 000 * USDA, 1967. Prescribed pre-harvest intervals in various countries are given below: Pre-harvest Country interval (days) Austria 21 Belgium 14 Denmark 14 Finland 21 France 15 Germany (B.R.D.) 14 Germany (D.D.R.) 14/21 Italy 14 Netherlands, field conditions 21 Netherlands, greenhouses 28 Norway 14 Portugal 21 Switzerland 21 United States of America 5/21 Post-harvest treatments Parathion-methyl is not used on stored food. Other uses Parathion-methyl has a very limited use for public health purposes. Residues resulting from supervised trials Many residue studies based on chemical methods, gas-liquid chromatography and cholinesterase inhibition, are available. Residue data include the oxon derivative. Parathion-methyl penetrates through the cuticula but is usually not translocated within plants (Shipp, 1963; Stobwasser, 1967). Only in the case of carrots was it found that parathion-methyl penetrates into the oil-cells and undergoes very slow degradation (Engst, 1966). On the surfaces of plants parathion-methyl residues decrease very rapidly as shown by Shipp (1963) on cotton. The half-life is usually one to two days: Pre-harvest Residue (ppm) Crop Country interval Author Half-life (days) range average (days) Cabbage- Germany 3 0.1-0.7 0.3 III 1-1.5 lettuce 7 <0.15 <0.15 (field) Cabbage- Germany 4 0.76 II 2-3 lettuce 7 0.65 (greenhouse) 14 0.06 Tomatoes Germany 3 0.06 I (field) 7 n.d. Tomatoes Germany 3 n.d.-0.15 0.08 I 1.5 (greenhouse) 7 n.d. <0.03 3 0.07 II 7 0.04 Apples Italy 10 0.05 I Apples Germany 3 0.38-0.54 II 2.5-5 7 0.11-0.12 14 0.05-0.15 Cabbage United 14 <1 <1 Hoelscher States of 1968 America Peas United 5 1.1 Hoelscher States of 1968 America Sorghum United 3 0.36 Dorough 1-1.5 grain States of 6 n.d. 1966 America (continued) Pre-harvest Residue (ppm) Crop Country interval Author Half-life (days) range average (days) Cotton United 3 2.5-18.0 8.7 Shipp 1 States of 7 1.0-11.8 5.3 1963 America 12 0.8-9.9 4.2 References: I Institut für Pflanzenschutz, Hohenheim, Germany II Professor Maier-Bode, Bonn, Germany III Biologisches Institut der Farbenfabriken Bayer, A.G. Leverkusen, Germany Disappearance of residues is dependent on climatic factors, rain appreciably reducing the residues (Hightower, 1958). Fate of residues General comments Under ultra-violet irradiation parathion-methyl is isomerized to the S-methyl compound (I) (Metcalf, 1953). Ultra-violet degradation under field conditions has not been reported. Upon heating to 140-160°C, parathion-methyl is transformed to the S-methyl compound (I) (McPherson, 1956; Metcalf, 1953). Thermal decomposition under field conditions is not reported.
Parathion-methyl is quickly hydrolyzed by 1 N NaOH (K/min = 0.69), but no information of hydrolysis under field conditions is available (Metcalf, 1953). No data on metabolism in humans has been reported. Dietary intake for all organo-phosphates in the United States of America in 1966/1967 was 0.00025 mg/kg body-weight/day, the three-year average being 0.00013 mg/kg body-weight/day (Duggan, 1968). In soils Parathion-methyl is quickly broken down in soil. In silty loam, 30 days after application of 0.1 ppm active ingredient, only three per cent was recovered. Disappearance of residues is accelerated by moisture and micro-organisms (Lichtenstein and Schulz, 1964). Trials on leaching in soil showed that in soil-water, parathion-methyl has a persistence of two weeks and in lake-water of three months, but no conclusions can be drawn concerning leaching of parathion-methyl from soil to rivers (Lichtenstein et al., 1966). In plants Three metabolites of parathion-methyl were found in cottonleaves, one of which was identified as paraoxon-methyl (II) (Shipp, 1963; Coffin, 1964). After application of parathion-methyl to lettuce only small amounts of residues of paraoxon-methyl were found (one to four per cent of the total active residue) (Möllhoff, 1968). Under normal field conditions, paraoxon-methyl constitutes only a small fraction (a few per cent) of the residue of the parent compound (Möllhoff, 1968). Under elevated temperatures (72-112°F) and high humidity (40-62 per cent) the total residue on cotton after an interval of 72 hours consisted of 14.5 per cent parathion-methyl and 38.5 per cent paraoxon-methyl (Shipp, 1963). However, the oxon, when formed, is subject to rapid hydrolysis in and on the plant (Möllhoff, 1968). The classical colorimetric method by Averell et al. (1948) is able to determine the parent compound plus the oxon. When applying GLC methods the parent compound and the oxon can be determined separately. Probably, breakdown of parathion-methyl in higher plants is the same as the breakdown of parathion, but more rapid. The products of hydrolysis - phosphorothioate and 4-nitro-phenol - are only present in small amounts and seem to have no toxicological importance.
In animals In warm-blooded animals parathion-methyl is transformed enzymatically to paraoxon-methyl (II) (Augustinsson and Jonsson, 1957). In storage and processing Although residues of parathion-methyl in vegetables and fruit are found near the peel, washing does not markedly reduce residues (Stobwasser, 1967). During ensilage of pea plants the active ingredient is quickly broken down because of the weak acids present. One month after ensilage, residues up to 0.2 ppm were found; after another month no residues could be detected (Heinisch, 1966). Evidence of residues in food in commerce or at consumption Samples of beef, butter and rice were examined for the presence of organo-phosphorus compounds by the non-specific cholinesterase inhibition method. Calculated as diasinon or parathion-methyl, residues in all samples were 0.1 ppm or less (data from Woodstock Agricultural Research Centre, Sittingbourne, 1967). Methods of residue analysis The residue analysis of parathion-methyl is usually carried out colorimetrically, the results including metabolites and all other substances with the nitrophenyl-group (Averell, 1948). Specific detection of parathion-methyl is possible by paper and thin-layer chromatography (Abbott, 1965; Getz, 1963). Gas-liquid chromatographic analysis is carried out with microcoulometric (Nelson, 1966), electron capture (Giuffrida, 1966; Egan, 1964; Möllhoff, 1967) and phosphorus detectors (Möllhoff, 1967), The sensitivity of the colorimetric method is 0.05 ppm, and of the gas-liquid chromatography methods, 0.01-0.02 ppm. The latter can be used as a referee method. RECOMMENDATIONS FOR TOLERANCES AND PRACTICAL RESIDUE LIMITS National tolerances Country Crop Tolerance* (ppm) Germany (Fed. Rep.) Fruit, vegetables 0.5 Netherlands General 0.5 Switzerland Fruit 0.75 United States of America Fruit, vegetables 1.0 * Total of parathion and parathion-methyl. Appraisal Parathion-methyl is a broad spectrum insecticide which is widely and heavily used in many countries for many purposes. It is especially valuable in the control of cotton insects and is frequently used in combination with DDT, endrin, toxaphene, and parathion. Although its use involves an acute toxic hazard, the residues are relatively non-persistent and rapidly disappear from treated crops. Residues actually found after reasonable waiting periods rarely exceed 1 ppm and are composed of the parent compound and paraoxon-methyl plus three metabolites which have been identified and can be detected by currently available analytical methods. However, additional information on the occurrence of paraoxon-methyl and the three metabolites is required. Further, the residue data which is available comes mostly from experimental trials in the Federal Republic of Germany with some data originating in the United States of America. Residue data from other countries are required. Sensitive methods of residue analysis are available for the parent compound and the oxygen analogue. The meeting considered the possibility of combining tolerances for parathion and parathion-methyl but rejected this concept since modern analytical techniques allow for the differentiation of the two compounds and their metabolites, Accordingly, recommendations for tolerances of parathion-methyl include paraoxon-methyl. If a tolerance is required for rice, residue data should be furnished. Recommendations Temporary tolerances The following temporary tolerances, to be in effect until 1972, are to apply to raw agricultural products moving in commerce unless otherwise indicated. In the case of fruit and vegetables the tolerances should be applied as soon as practicable after harvest and in any event prior to actual retail to the public. In the case of commodities entering international trade, the tolerances should be applied by the importing country at the point of entry or as soon as practicable thereafter. Vegetables 1 ppm Cole crops, cucurbits, fruit 0.2 ppm Cotton-seed oil (as processed) 0.05 ppm The above figures are for parathion-methyl and its oxygen analogue. Further work or information Required before 30 June 1972 1. Quantitative data on the occurrence of paraoxon-methyl and the other metabolites in plants and animal products. 2. Data from countries other than the Federal Republic of Germany and the United States of America on the required rates and frequencies of application, pre-harvest intervals, and the resultant residues. 3. If a tolerance is required on rice, data on the required rates and frequencies of application, pre-harvest intervals, and the resultant residues. 4. Further data on residue levels in raw agricultural products moving in commerce. 5. Data on residue levels in total diet studies. 6. Comparative evaluation of available methods for regulatory purposes. 7. Oral studies on teratogenicity and reproduction in species other than rats and mice, preferably in subhuman primates. Desirable Collaborative studies to establish a referee method. REFERENCES Abbott, D. C., Crosby, N. T. and Thomson, J. (1965) Use of thin-layer and semipreparative gas-liquid chromatography in the detection, determination and identification of organophosphorous pesticide residues. Proc. Soc. Anal. Chem. Conf., pp. 121-133 Augustinsson, K.-B. and Jonsson, G. (1957) Acta chem. scand., 11:275. See Schrader: Die Entwicklung neuer insektizider Phosphorsäureester, 444 pp. (1962). Verlag Chemie, Weinheim Averell, P. R. and Norris, M. V. (1948) Estimation of small amounts of O,O-di-ethyl O-(p-nitrophenyl) thiophosphate. Anal. Chem., 20: 753-756 Coffin, D. E. (1964) Residues of parathion, methylparathion, EPN, and their oxons in Canadian fruits and vegetables. Res. Rev., 7: 61-73 Davison, A. N. (1955) Return of cholinesterase activity in the rat after inhibition by organophosphorus compounds. 2. A comparative study of true and pseudo cholinesterase. Biochem. J., 60: 339-346 Deichmann, W. B., Pugliese, W. and Cassidy, J. (1952) Effects of dimethyl and diethyl paranitrophenyl thiophosphate on experimental animals. A.M.A. Arch. industr. Hyg., 5: 44-51 Dorough, H. W., Randolph, N. M. and Wimbish, G. H. (1966) Residual nature of certain organophosphorus insecticides in grain sorghum and coastal Bermuda grass. Bull. environm. Contam. Toxicol., 1: 46-58 DuBois, K. P. and Coon, J. M. (1952) Toxicology of organicphosphorus-containing insecticides to mammals. A.M.A. Arch industr. Hyg., 6: 9-13 Duggan, R. E. (1968) Residues in food and feed. Pesticides Monitoring J., 2: 2-12 Egan, H., Hammond, E. W. and Thomson, J. (1964) The analysis of organo-phosphorous pesticide residues by gas chromatography. Analyst, 89: 175-178 Engst, R., Seidler, H. and Härtig, M. (1966) Nahrung, 10: 413-425 FAO/WHO. (1965) Evaluation of the toxicity of pesticide residues in food, FAO Mtg. Rept. PL/1965/10/1; WHO/Food Add/27.65 Fish, S. A. (1966) Organophosphorus cholinesterase inhibitors and fetal development. Amer. J. Obstet. Gynec., 96: 1148-1154 Gar, K. A., Sazonova, N. A., Fadeev, Y. N., Vladimirova, I. L. and Golubeva, Z. Z. (1958) Incorporation and excretion of dimethyl 4-nitrophenyl thiophosphate in guinea pigs. Org. Insektofungitsidy i Gerbitsidy, pp. 93-105 [Chem. Abstr., 54: 15688e (1960)] Getz, M. E. (1963) The determination of organophosphate pesticides and their residues by paper chromatography. Res. Rev., 2: 9-25 Giuffrida, L., Ives, N. F. and Bostwick, D. C. (1966) J. Assoc. Off. Anal. Chemists, 49: 8-21 Hagan, E. C. (1958) Personal communication cited in Williams at al., 1959, q.v. Heinisch, E. (1966) Nachrichtenblatt dt. Pflanzenschutzdienst, 20: 57-64 Hightower, B. G. and Martin, D. F. (1958) Effects of certain climatic factors on the toxicities of several organic phosphorus insecticides. J. Econ, Entomol., 51: 669-671 Hoelscher, C. E., Wolfenbarger, D. A. and Foster, N. E. (1968) Parathion and methyl parathion residues on cabbage and southern peas. J. Econ. Entomol., 61: 56-58 Ikeda, Y. (1962) "Report to the Japan Academy of Sciences" Le Breton, R., Leyrie, J. and Garat, J. (1963) Intoxication materno-foetale aigüe par dérivé organo-phosphoré. Ann. Méd. lég., 43: 258-261 Lichtenstein, E. P. et al. (1966) Toxicity and fate of insecticide residues in water. Insecticide residues in water after direct applications or by leaching of agricultural soil. Arch. environm. Hlth, 12: 199-212 Lichtenstein, E. P. and Schulz, K. R. (1964) Effect of moisture and micro-organisms on the persistence of some organophosphorus insecticides in soil, with special emphasis on parathion. J. Econ, Entomol., 57: 618-627 McPherson, J. B. and Johnson, G. A. (1956) Thermal decomposition of some phosphorothioate insecticides. J. Agr. Food Chem., 4: 42-49 Metcalf, R. L. and March, R. B. (1953) The isomerization of organic thionophosphate insecticides. J. Econ. Entomol., 46: 228-294 Moeller, H. C. and Rider, J. A. (1961) Studies on the anti-cholinesterase effect of parathion and methyl parathion in humans. Fed. Proc., 20: 434 Moeller, H. C. and Rider, J. A. (1962) Threshold of incipient toxicity to Systox and methyl parathion. Fed. Proc., 21: 451 Moeller, H. C. and Rider, J. A. (1963) Further studies on the toxicity of Systox and methyl parathion. Fed. Proc., 22: 189 Möllhoff, E. (1968) Beitrag zur Frage der Rückstände und ihrer Bestimmung in Pflanzen nach Anwendung von Präparaten der E 605- und Agritox-Reihe. Pflanzenschutz-Nachrichten "Bayer", 21: 331-358 Möllhoff, E. (1967) Gas chromatographic determination of residues of E 605 products and Agritox in plants and soil samples. Pflanzenschutz-Nachrichten "Bayer", 20: 557-574 Nelson, R. C. (1966) Screening procedure for organothiophosphate pesticide residues on fruits and vegetables by microcoulometric gas chromatography. J. Assoc. Off. Anal. Chemists, 49: 763-766 Ogi, D. and Hamada, A. (1965) Case reports on fetal deaths and malformations of extremities probably related to the insecticide poisoning. Cited in Tanimura et al., 1967, q.v. Shipp, O. E., Lindquist, D. A. and Brazzel, J. R. (1963) Characteristics of residues of methyl parathion applied to field cotton. J. Econ. Entomol., 56: 793-798 Stobwasser, H., Rademacher, B. and Lange, E. (1967) Effect of post harvest factors on occurrence of pesticide residues in fruit, vegetables, etc. Res. Rev., 22: 45-112 Tanimura, T., Katsuya, T. and Nishimura, H. (1967) Embryotoxicity of acute exposure to methyl parathion in rats and mice. Arch. environm. Hlth., 15: 609-613 USDA. (1967) Summary of Registered Agricultural Pesticide Chemical Uses. Williams, M. W., Fuyat, H. N. and Fitzhugh, O. G. (1959) The subacute toxicity of four organic phosphates to dogs. Toxicol. appl. Pharmacol., 1: 1-7 Wills, H. (1968) To Coulston, F., Division of Pharmacology, Albany Medical College, Albany, N.Y. Unpublished report Woodard Research Corp. (1966) Methyl parathion. Three-generation reproduction study in the rat. Unpublished report
See Also: Toxicological Abbreviations Parathion-methyl (WHO Pesticide Residues Series 2) Parathion-methyl (WHO Pesticide Residues Series 5) Parathion-methyl (Pesticide residues in food: 1978 evaluations) Parathion-methyl (Pesticide residues in food: 1979 evaluations) Parathion-methyl (Pesticide residues in food: 1980 evaluations) Parathion-methyl (Pesticide residues in food: 1984 evaluations) Parathion-methyl (Pesticide residues in food: 1995 evaluations Part II Toxicological & Environmental)