UNITED NATIONS ENVIRONMENT PROGRAMME INTERNATIONAL LABOUR ORGANISATION WORLD HEALTH ORGANIZATION INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY ENVIRONMENTAL HEALTH CRITERIA 197 Demeton-S-methyl The issue of this document does not constitute formal publication. It should not be reviewed, abstracted, or quoted without the written permission of the Manager, International Programme on Chemical Safety, WHO, Geneva, Switzerland. This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organisation, or the World Health Organization. Environmental Health Criteria 197 First draft prepared by Dr. A. Moretto, Institute of Occupational Medicine, University of Padua, Italy Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organisation, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals. World Health Organization Geneva, 1997 The International Programme on Chemical Safety (IPCS) is a joint venture of the United Nations Environment Programme, the International Labour Organisation, and the World Health Organization. The main objective of the IPCS is to carry out and disseminate evaluations of the effects of chemicals on human health and the quality of the environment. Supporting activities include the development of epidemiological, experimental laboratory, and risk-assessment methods that could produce internationally comparable results, and the development of manpower in the field of toxicology. Other activities carried out by the IPCS include the development of know-how for coping with chemical accidents, coordination of laboratory testing and epidemiological studies, and promotion of research on the mechanisms of the biological action of chemicals. WHO Library Cataloguing in Publication Data Demeton-S-Methyl. (Environmental health criteria ; 197) 1.Insecticides, Organophosphate - toxicity 2.Insecticides, Organophosphate - adverse effects 3.Environmental exposure 4.Occupational exposure I.International Programme on Chemical Safety II.Series ISBN 92 4 157197 7 (NLM Classification: WA 240) ISSN 0250-863X The World Health Organization welcomes requests for permission to reproduce or translate its publications, in part or in full. Applications and enquiries should be addressed to the Office of Publications, World Health Organization, Geneva, Switzerland, which will be glad to provide the latest information on any changes made to the text, plans for new editions, and reprints and translations already available. (c) World Health Organization 1997 Publications of the World Health Organization enjoy copyright protection in accordance with the provisions of Protocol 2 of the Universal Copyright Convention. All rights reserved. The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or of certain manufacturers' products does not imply that they are endorsed or recommended by the World Health Organization in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. CONTENTS ENVIRONMENTAL HEALTH CRITERIA FOR DEMETON-S-METHYL PREAMBLE ABBREVIATIONS 1. SUMMARY AND EVALUATION, CONCLUSIONS AND RECOMMENDATIONS 1.1. Summary and evaluation 1.1.1. Identity, physical and chemical properties, and analytical methods 1.1.2. Sources of human and environmental exposure 1.1.3. Environmental transport, distribution and transformation 1.1.4. Environmental levels and human exposure 1.1.5. Kinetics and metabolism 1.1.6. Effects on laboratory animals and in vitro test systems 1.1.6.1 Single exposure 1.1.6.2 Short-term exposure 1.1.6.3 Long-term exposure 1.1.6.4 Skin and eye irritation and sensitization 1.1.6.5 Reproduction, embryotoxicity and teratogenicity 1.1.6.6 Mutagenicity and related end-points 1.1.6.7 Delayed neurotoxicity 1.1.6.8 Toxicity of metabolites 1.1.7. Mechanism of toxicity - mode of action 1.1.8. Effects on humans 1.1.9. Effects on other organisms in the laboratory and field 1.2. Conclusions 1.3. Recommendations 2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS 2.1. Identity 2.2. Physical and chemical properties 2.3. Conversion factors 2.4. Analytical methods 2.5. Formation of derivatives during storage 3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE 3.1. Natural occurrences 3.2. Man-made sources 3.2.1. Production 3.2.2. Uses 4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION 4.1. Transport and distribution between media 4.2. Abiotic and biotic transformation 4.2.1. Hydrolytic degradation 4.2.2. Photodegradation 4.2.3. Degradation in soil 4.2.4. Biodegradation in plants 5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE 5.1. General population exposure 5.2. Occupational exposure during manufacture, formulation or use 6. KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS 6.1. Absorption, distribution and excretion 6.2. Metabolic transformation 7. EFFECTS ON EXPERIMENTAL ANIMALS AND IN VITRO TEST SYSTEMS 7.1. Single exposure 7.1.1. Oral 7.1.2. Inhalation 7.1.3. Dermal 7.2. Short-term exposure 7.2.1. Rat 7.2.2. Dog 7.3. Long-term exposure 7.3.1. Mouse 7.3.2. Rat 7.4. Skin and eye irritation and sensitization 7.4.1. Skin and eye irritation 7.4.2. Skin sensitization 7.5. Reproduction, embryotoxicity and teratogenicity 7.5.1. Reproduction 7.5.2. Embryotoxicity and teratogenicity 7.5.2.1 Rat 7.5.2.2 Rabbit 7.6. Mutagenicity and related end-points 7.6.1. DNA damage and repair 7.6.2. Mutation 7.6.3. Chromosomal effects 7.7. Delayed neurotoxicity 7.8. Toxicity of metabolites 7.9. Mechanism of toxicity - mode of action 7.10. Potentiation 8. EFFECTS ON HUMANS 8.1. General population exposure 8.2. Occupational exposure 8.2.1. Acute poisoning 8.2.2. Effects of short- and long-term exposure 9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD 9.1. Aquatic organisms 9.1.1. Algae 9.1.2. Invertebrates 9.1.3. Fish 9.2. Terrestrial organisms 9.2.1. Soil microorganisms 9.2.2. Invertebrates 9.2.3. Birds 9.2.4. Effects in field 10. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT 10.1. Evaluation of human health risks 10.2. Evaluation of effects on the environment 10.2.1. Aquatic organisms 10.2.1.1 Acute risk 10.2.1.2 Chronic risk 10.2.2. Terrestrial organisms 10.2.2.1 Birds 10.2.2.2 Mammals 10.2.2.3 Bees 10.2.2.4 Earthworms 11. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT 12. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES REFERENCES RÉSUMÉ ET ÉVALUATION, CONCLUSIONS ET RECOMMANDATIONS RÉSUMEN Y EVALUACION, CONCLUSIONES Y RECOMENDACIONES NOTE TO READERS OF THE CRITERIA MONOGRAPHS Every effort has been made to present information in the criteria monographs as accurately as possible without unduly delaying their publication. In the interest of all users of the Environmental Health Criteria monographs, readers are requested to communicate any errors that may have occurred to the Director of the International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland, in order that they may be included in corrigenda. * * * A detailed data profile and a legal file can be obtained from the International Register of Potentially Toxic Chemicals, Case postale 356, 1219 Châtelaine, Geneva, Switzerland (telephone no. + 41 22 - 9799111, fax no. + 41 22 - 7973460, E-mail irptc@unep.ch). Environmental Health Criteria PREAMBLE Objectives In 1973 the WHO Environmental Health Criteria Programme was initiated with the following objectives: (i) to assess information on the relationship between exposure to environmental pollutants and human health, and to provide guidelines for setting exposure limits; (ii) to identify new or potential pollutants; (iii) to identify gaps in knowledge concerning the health effects of pollutants; (iv) to promote the harmonization of toxicological and epidemiological methods in order to have internationally comparable results. The first Environmental Health Criteria (EHC) monograph, on mercury, was published in 1976 and since that time an ever-increasing number of assessments of chemicals and of physical effects have been produced. In addition, many EHC monographs have been devoted to evaluating toxicological methodology, e.g., for genetic, neurotoxic, teratogenic and nephrotoxic effects. 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Content The layout of EHC monographs for chemicals is outlined below. * Summary - a review of the salient facts and the risk evaluation of the chemical * Identity - physical and chemical properties, analytical methods * Sources of exposure * Environmental transport, distribution and transformation * Environmental levels and human exposure * Kinetics and metabolism in laboratory animals and humans * Effects on laboratory mammals and in vitro test systems * Effects on humans * Effects on other organisms in the laboratory and field * Evaluation of human health risks and effects on the environment * Conclusions and recommendations for protection of human health and the environment * Further research * Previous evaluations by international bodies, e.g., IARC, JECFA, JMPR Selection of chemicals Since the inception of the EHC Programme, the IPCS has organized meetings of scientists to establish lists of priority chemicals for subsequent evaluation. Such meetings have been held in: Ispra, Italy, 1980; Oxford, United Kingdom, 1984; Berlin, Germany, 1987; and North Carolina, USA, 1995. The selection of chemicals has been based on the following criteria: the existence of scientific evidence that the substance presents a hazard to human health and/or the environment; the possible use, persistence, accumulation or degradation of the substance shows that there may be significant human or environmental exposure; the size and nature of populations at risk (both human and other species) and risks for environment; international concern, i.e. the substance is of major interest to several countries; adequate data on the hazards are available. If an EHC monograph is proposed for a chemical not on the priority list, the IPCS Secretariat consults with the Cooperating Organizations and all the Participating Institutions before embarking on the preparation of the monograph. 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It is accepted that the following criteria should initiate the updating of an EHC monograph: new data are available that would substantially change the evaluation; there is public concern for health or environmental effects of the agent because of greater exposure; an appreciable time period has elapsed since the last evaluation. All Participating Institutions are informed, through the EHC progress report, of the authors and institutions proposed for the drafting of the documents. A comprehensive file of all comments received on drafts of each EHC monograph is maintained and is available on request. The Chairpersons of Task Groups are briefed before each meeting on their role and responsibility in ensuring that these rules are followed. WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR DEMETON-S-METHYL Members Dr P.J. Abbott, Australia and New Zealand Food Authority (ANZFA), Canberra, Australia Dr K. Barabas, Department of Public Health, Albert Szent-Gyorgyi, University Medical School, Szeged, Hungary Dr A.L. Black, Woden, ACT, Australia Professor J.F. Borzelleca, Pharmacology and Toxicology, Richmond, Virginia, USA Dr P.J. Campbell, Pesticides Safety Directorate, Ministry of Agriculture, Fisheries and Food, Kings Pool, York, United Kingdom Professor L.G. Costa, Department of Environmental Health, University of Washington, Seattle, USA Dr S. Dobson, Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, United Kingdom Dr I. Dewhurst, Mammalian Toxicology Branch, Pesticides Safety Directorate, Ministry of Agriculture, Fisheries and Food, Kings Pool, York, United Kingdom Dr V. Drevenkar, Institute for Medical Research and Occupational Health, Zagreb, Croatia Dr W. Erickson, Environmental Fate and Effects Division, US Environmental Protection Agency, Washington, D.C., USA Dr A. Finizio, Group of Ecotoxicology, Institute of Agricultural Entomology, University of Milan, Milan, Italy Mr K. Garvey, Office of Pesticide Programs (7501C), US Environmental Protection Agency, Washington, D.C., USA Dr A.B. Kocialski, Health Effects Division, Office of Pesticide Programs, US Environmental Protection Agency, Washington, D.C., USA Dr A. Moretto, Institute of Occupational Medicine, University of Padua, Padua, Italy Professor O. Pelkonen, Department of Pharmacology and Toxicology, University of Oulu, Oulu, Finland Dr D. Ray, Medical Research Council Toxicology Unit, University of Leicester, Leicester, United Kingdom Dr J.H.M. Temmink, Department of Toxicology, Wageningen Agricultural University, Wageningen, The Netherlands Observers Dr J.W. Adcock, AgrEvo UK Limited, Chesterford Park, Saffron, Waldon, Essex, United Kingdom Mr D. Arnold, Environmental Sciences, AgrEvo UK Ltd., Chesterford Park, Saffron Waldon, Essex, United Kingdom Dr E. Bellet, CCII, Overland Park, Kansas, USA Mr Jan Chart, AMVAC Chemical Corporation, Newport Beach, California, USA Dr H. Egli, Novartis Crop Protection AG, Basel, Switzerland Dr P. Harvey, AgrEvo UK Ltd., Chesterford Park, Saffron Walden, Essex, United Kingdom Dr G. Krinke, Novartis Crop Protection AG, Basel, Switzerland Dr A. McReath, DowElanco Limited, Letcombe Regis, Wantage, Oxford, United Kingdom Dr H. Scheffler, Novartis Crop Protection AG, Basel, Switzerland Dr A.E. Smith, Novartis Crop Protection AG, Basel, Switzerland Secretariat Dr L. Harrison, Health and Safety Executive, Bootle, Merseyside, United Kingdom Dr J.L. Herrman, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland Dr P.G. Jenkins, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland Dr D. McGregor, Unit of Carcinogen Identification and Evaluation, International Agency for Research on Cancer, Lyon, France Dr R. Plestina, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland Dr E. Smith, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland Dr P. Toft, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland IPCS TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR DEMETON-S-METHYL The Core Assessment Group (CAG) of the Joint Meeting on Pesticides (JMP) met at the Institute for Environment and Health, Leicester, United Kingdom, from 3 to 8 March 1997. Dr L.L. Smith welcomed the participants on behalf of the Institute, and Dr R. Plestina on behalf of the three IPCS cooperating organizations (UNEP/ILO/WHO). The CAG reviewed and revised the draft monograph and made an evaluation of the risks for human health and the environment from exposure to demeton-S-methyl. The first draft of the monograph was prepared by Dr A. Moretto, Institute of Occupational Medicine, University of Padua, Italy. Extensive scientific comments were received following circulation of the first draft to the IPCS contact points for Environmental Health Criteria monographs and these comments were incorporated into the second draft by the Secretariat. Dr R. Plestina and Dr P.G. Jenkins, both members of the IPCS Central Unit, were responsible for the overall scientific content and technical editing, respectively. The efforts of all who helped in the preparation and finalization of the monograph are gratefully acknowledged. ABBREVIATIONS ACGIH American Conference of Governmental Industrial Hygienists AChE acetylcholinesterase ADI acceptable daily intake a.i. active ingredient BuChE butyrylcholinesterase b.w. body weight ChE cholinesterase EC50 median effective concentration GLC gas-liquid chromatography HID highest ineffective dose I50 concentration inhibiting 50% of the enzyme activity i.p. intraperitoneal administration i.v. intravenous administration JMPR Joint Meeting on Pesticide Residues Kd sorption coefficient LC50 median lethal concentration LD50 median lethal dose LED lowest effective dose LOEC lowest-observed-effect concentration MRL maximum residue level NT not tested NTE neuropathy target esterase NOAEL no-observed-adverse-effect level NOEC no-observed-effect concentration PEC predicted environmental concentration RBC red blood cell s.c. subcutaneous SCE sister chromatid exchange STS standard type of soil TER toxicity-exposure ratio TLC thin layer chromatography TLV threshold limit value TWA time-weighted average 1. SUMMARY AND EVALUATION, CONCLUSIONS AND RECOMMENDATIONS 1.1 Summary and evaluation 1.1.1 Identity, physical and chemical properties, and analytical methods Demeton-S-methyl, a pale yellow oily liquid with a penetrating odour, is a systemic and contact organophosphate insecticide and acaricide used to control Acarina, Thysanoptera, Hymenoptera and Homoptera in fruits, cereals, ornamentals and vegetables. It has a vapour pressure of 63.8 mPa at 20°C, is readily soluble in most organic solvents, has a high water solubility of 3.3 g/litre at room temperature and an octanol-water partition coefficient (log Pow) of 1.32. Demeton-S-methyl is stable in non-aqueous solvents. Residual and environmental analyses are performed by extraction with an organic solvent, followed by oxidation to the corresponding sulfone. Measurement is then performed by gas chromatography, using a phosphorus-specific detector. 1.1.2 Sources of human and environmental exposure Prior to 1957, methyl-demeton was marketed as a mixture of demeton-S-methyl and demeton-O-methyl isomers. Demeton-S-methyl has been in use since 1957. It is formulated as an emulsifiable concentrate and used as a spray on cereals, fruits, ornamentals and vegetables. It is being replaced by oxydemeton-methyl, which is a plant, soil and mammalian metabolite of demeton-S-methyl. 1.1.3 Environmental transport, distribution and transformation Hydrolytic degradation of demeton-S-methyl depends on the pH of the solution; at 22°C the half-life is 63 days at pH 4, 56 days at pH 7 and 8 days at pH 9. In soil, biodegradation is the primary route of degradation. The half-life of demeton-S-methyl in soil is about 4 h. However, after 24 h, oxydemeton-methyl still represents 20-30% of the applied dose of demeton-S-methyl. The sorption coefficient (Kd) of demeton-S-methyl in soil is 0.68 to 2.66, depending on the soil composition. Photolysis is not one of the major mechanisms of degradation of demeton-S-methyl in the environment. Metabolism in spring wheat is rapid and similar to that in soil and mammals. 1.1.4 Environmental levels and human exposure Primary exposure for the general human population is from residues of demeton-S-methyl on food crops. The Joint FAO/WHO Meeting on Pesticide Residues (JMPR) recommended acceptable daily intake (ADI) is 0.0003 mg/kg body weight. This is a group ADI for demeton-S-methyl, oxydemeton-methyl and demeton-S-methyl-sulfone, since the routine analytical methods do not discriminate between these three compounds. Excessive dermal exposure and absorption of demeton-S-methyl has caused cholinergic toxicity in workers inadequately protected during packaging of the concentrate formulation. When volunteers engaged in a simulated spray activity with a mixture of demeton-S-methyl and demeton-O-methyl (30 and 70%, respectively) were exposed to 8.8-27 mg/m3 of the two active ingredients combined, they experienced no adverse effects on plasma or erythrocyte cholinesterase activities. 1.1.5 Kinetics and metabolism Demeton-S-methyl is rapidly and almost completely absorbed from the intestinal tract of rats and is uniformly (except for high concentration in erythrocytes) distributed to body tissues. It is rapidly metabolized and excreted via the urine. Blood concentration decreases with an initial half-life of about 2 h. About 1% of the oral dose is present in the body 24 h after treatment. The main metabolic pathway of demeton-S-methyl in rats is the oxidation of the side chain leading to the formation of the corresponding sulfoxide (oxydemeton-methyl) and, to a lesser extent, after further oxidation, to the sulfone. Another important metabolic route is O-demethylation. 1.1.6 Effects on laboratory animals and in vitro test systems 1.1.6.1 Single exposure Demeton-S-methyl causes cholinergic toxicity. The LD50 values for mammals range from 7 to 100 mg/kg body weight, depending on the route of administration and species. 1.1.6.2 Short-term exposure An early dietary study showed that rats fed demeton-S-methyl at 50 mg/kg diet had substantially reduced brain and erythrocyte cholinesterase activity after 26 weeks of exposure. Cholinergic signs were observed in rats fed 200 mg/kg diet during the first 5 weeks of exposure. In a one-year dietary study on dogs, a no-observed-adverse-effect level (NOAEL) of 1 mg/kg diet (equal to 0.036 mg/kg body weight per day) was established, based on effects on brain cholinesterase. 1.1.6.3 Long-term exposure Mice were fed diets containing 0, 1, 15 or 75 mg/kg demeton-S- methyl for 21 months. The NOAEL was found to be 1 mg/kg diet (equal to 0.24 mg/kg body weight per day) based on inhibition of brain cholinesterase. In rats fed diets containing 0, 1, 7 or 50 mg/kg demeton-S- methyl, the NOAEL, based on inhibition of brain cholinesterase, was 1 mg/kg diet (equal to 0.05 mg/kg body weight per day). No increased tumour incidence was found in either species. 1.1.6.4 Skin and eye irritation and sensitization Demeton-S-methyl is a mild skin and eye irritant. Positive results were obtained with the Magnusson and Klingman maximization test in guinea-pigs. However, the Buehler epidermal patch test gave no indication of skin sensitization, suggesting that sensitization should not be a problem in the practical use of demeton-S-methyl. 1.1.6.5 Reproduction, embryotoxicity and teratogenicity In a two-generation dietary rat study, demeton-S-methyl caused reduced viability and body weight of pups (F1b generation only) at a dose level of 5 mg/kg diet. The NOAEL was 1 mg/kg diet, equal to 0.07 mg/kg body weight per day. Demeton-S-methyl was neither embryotoxic nor teratogenic in rats and rabbits. 1.1.6.6 Mutagenicity and related end-points Demeton-S-methyl induces point mutations in vitro. Chromosomal effects have been demonstrated in vivo with commercial formulations only. The available information is insufficient to permit an adequate assessment of the genotoxic potential of demeton-S-methyl. 1.1.6.7 Delayed neurotoxicity Demeton-S-methyl caused neither delayed polyneuropathy nor inhibition of neuropathy target esterase (NTE) when tested in hens at a level equal to the oral LD50. 1.1.6.8 Toxicity of metabolites Two plant and mammalian metabolites of demeton-S-methyl (i.e. oxydemeton-methyl and demeton-S-methylsulfone) are also commercial pesticides and have been extensively studied. It has been reported that the toxicological profile of these two compounds does not significantly differ, either quantitatively or qualitatively, from that of demeton-S-methyl. 1.1.7 Mechanism of toxicity - mode of action Demeton-S-methyl is a direct cholinesterase inhibitor, and the toxicity it causes is related to inhibition of acetylcholinesterase (AChE) at nerve terminals. AChE inhibited by demeton-S-methyl reactivates spontaneously with an in vitro half-life of about 1.3 h, as expected for dimethyl phosphorylated AChE. 1.1.8 Effects on humans A few cases of acute intoxication with cholinergic syndrome, following suicide attempts, have been reported. Surviving patients, including a pregnant woman, did not show delayed effects. Following careless occupational exposure during packaging of the commercial formulation, some workers developed cholinergic toxicity which required pharmacological treatment. Absorption of demeton-S-methyl was probably through the skin. Similarly, improper working conditions may have caused excessive dermal absorption during application of demeton-S-methyl in cotton fields. 1.1.9 Effects on other organisms in the laboratory and field The 96-h EC50s for green algae range from 8 to 37 mg/litre. The LC50s for a range of aquatic invertebrates range from 0.004 to 1.3 mg/litre. The toxicity for fish varies, with 96-h LC50 ranging from 0.59 mg/litre for the rainbow trout to about 40 mg/litre for the golden orfe, the goldfish and the carp. The acute oral LD50 for the Japanese quail and the canary is 10-50 mg/kg body weight. In starlings, a single oral dose of 2 mg/kg body weight caused 20% inhibition of brain AChE 3 h after treatment. The LC50 of demeton-S-methyl in soil for earthworms is 66 mg/kg for 14 days. The acute oral and contact LD50 for demeton-S-methyl are 0.21 and 0.6 µg/bee respectively. When used on winter wheat at the suggested rate, demeton-S-methyl significantly reduced the number of crop foliage invertebrates (mainly Empididae flies) but not the number of soil surface entomophagous invertebrates. 1.2 Conclusions Demeton-S-methyl is a highly toxic (class Ib of the WHO classification) (WHO, 1996) organophosphorus ester insecticide. The mechanism of toxicity is that of AChE inhibition at nerve terminals. Exposure of the general population results mainly from residues present in crop commodities. With good work practices, hygienic measures and safety precautions, the use of demeton-S-methyl during manufacture or application should not cause adverse effects. Effects due to chronic exposure are unlikely to occur. Demeton-S-methyl does not persist in the environment and is not accumulated by organisms. It has high acute toxicity to aquatic invertebrates and is toxic to fish and birds, leading to high or moderate risk factors for these organisms. However, significant field kills of organisms have not been reported for the compound. Precautions should be taken to minimize exposure of non-target organisms (e.g., do not spray over water bodies, minimize exposure by spray drift). 1.3 Recommendations For the health and welfare of workers and the general population, the handling and application of demeton-S-methyl should only be entrusted to supervised and well-trained operators who follow the required safety measures and good application practices. 2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS 2.1 Identity Chemical formula: C6H15O3PS2 Chemical structure: O " CH3CH2SCH2CH2SP(OCH3)2 Relative molecular mass: 230.3 Common name: demeton-S-methyl CAS chemical name: S-[2-(ethylthio)ethyl] O,O-dimethyl phosphorothioate IUPAC name: S-2-ethylthioethyl O,O-dimethyl phosphorothioate CAS registry number:
919-86-8 RTECS number: TG1750000 Common synonyms and AI3-24963; BAY 18436; Bayer 18 436; trade names: Bayer 25/154; Demetox; DEP 836 349; Duratox; ethanethiol, 2-(ethylthio)-S-ester with O,O-dimethyl phosphorothioate; HSDB 6410; Isometasystox; Isomethylsystox; Metaisoseptox; Metaisosystox; Metasystox (I); metasystox forte; Metasystox I; Metasystox J; Metasystox 55; methyl demeton thioester; methyl isosystox; methyl-mercaptofos teolery; methyl-mercaptofos teolovy (USSR); methylmercaptofostiol (USSR); Mifatox; O,O-dimethyl S-(2-(ethylthio) ethylphosphorothioate; O,O-dimethyl S-ethylmercaptoethyl thiophosphate; O,O-dimethyl 2-ethylmercaptoethyl thiophosphate, thiolo isomer; phosphorothioic acid, O,O-dimethyl S-(2-(ethylthio)ethyl) ester; phosphorothioic acid, S-(2-(ethylthio)ethyl) O,O-dimethyl ester; S-(2-(ethylthio)ethyl); dimethyl phosphorothiolate; S-(2-(Ethylthio)ethyl) O,O-dimethyl phosphorothioate (8CI)(9CI); S-(2-(ethylthio)ethyl) O,O-dimethyl phosphorothioate; S-(2-ethylthio)ethyl) O,O-dimethyl phosphorothioate; S-(2-ethylthioetyl)0,0-dimethyl phosphorothioate; S-2-Ethylthioethyl- dimethyl phosphorothioate; USP 2 571 989; 2-Ethylthioethyl dimethyl phosphorothioate. Formulations: EC (250 or 500 g a.i./litre), DSM (Campbell), Metasystox55 (Bayer), Mepatox (FCC), EC (580 g/litre). Purity: >90% Impurities: O,O,S-trimethylthiophosphate (maximum of 1.5%) O-methyl-S-2-(ethylmercapto)-ethylthioph osphate (maximum of 3.0%) 2-ethylthioethylmercaptan max 0.8% bis(2-ethylthioethyl)-disulfide (maximum of 0.8%) Various ionic components (sulfonium compounds, organic salts) (total maximum of 2.5%) Oligomeric alkyl(thio) phosphates (maximum of 1.0%) Water (maximum of 0.1%) 2.2 Physical and chemical properties Some relevant physical and chemical properties are summarized in Table 1. Table 1. Some chemical and physical properties of demeton-S-methyl Physical state: oily liquid Colour: pale yellow Odour: penetrating, reminiscent of leeks Boiling point 74°C at 6.65 Pa (0.05 mmHg) 92°C at 26.6 Pa (0.20 mmHg) 102°C at 53.2 Pa (0.40 mmHg) 118°C at 133 Pa (1.00 mmHg) Vapour pressure: 21.3 mPa (1.6 × 10-4 mmHg) at 10°C 63.8 mPa (4.8 × 10-4 mmHg) at 20°C 193 mPa (1.45 × 10-3 mmHg) at 30°C 400 mPa (3.8 × 10-3 mmHg) at 40°C Relative density at 20°C: 1.21 n-Octanol/water partition coefficient: log Pow = 1.32 Solubility in water: 3.3 g/litre (at room temperature) Solubility in organic readily soluble in most organic solvents: solvents; limited solubility in petroleum ether Stability: hydrolysed by alkali and oxidized to the sulfoxide (oxydemeton-methyl) and sulfone (demeton-S-methylsulfone) Half-life in water: 11 days at 37°C Half-lives at 22°C: 63 days at pH 4 56 days at pH 7 8 days at pH 9 2.3 Conversion factors 1 ppm = 9.42 mg/m3 (at 25°C) 1 mg/m3 = 0.106 ppm 2.4 Analytical methods Analytical methods for the determination of residues of the demeton-S-methyl group (i.e. demeton-S-methyl, its sulfoxide and its sulfone) are either identical or very similar. Originally, colorimetric methods (i.e. determination of total phosphorus) were used (FAO/WHO, 1993). Current methods are based on GLC. In principle, these methods involve an oxidation step, using potassium permanganate, to produce demeton-S-methylsulfone, which is then determined with a thermoionic emission detector. A GLC method (Wagner & Thornton, 1977) is suitable for determining residues in plants, soil and water. The method is based on the principle of oxidation described above, with variations depending on the sample to be analysed. Before the oxidation step, maceration with acetone is used for samples with a high fat or oil content. The macerate and water samples are then extracted with chloroform or dichloromethane. Since the analytical method involves the oxidation to the sulfone, the determination is of demeton-S-methylsulfone, from which the demeton-S-methyl residue can be calculated. The method was used to determine residues in a wide range of crops with a minimum recovery above 80%. The limit of determination depends upon the sample and generally lies between 0.01 and 0.2 mg/kg. Another method, based on similar principles, has been described for oxydemeton-methyl residues in plant and animal tissues and in soil (Thornton et al., 1977). An alternative GLC method for sulfides (including demeton-S- methyl), sulfoxides and sulfones has been proposed by Hill et al. (1984), who reported that the use of acetone as a co-solvent during potassium permanganate oxidation causes unpredictable (from negligible to complete) loss of demeton-S-methyl. These authors used ethyl acetate for extraction of organophosphorus compounds from fruit and vegetable samples (Anonymous, 1977). The extracts were cleaned-up by chromatography on a column of activated charcoal, magnesium oxide and Celite. The compounds were eluted from the column using a mixture of ethyl acetate, acetone and toluene. A mean recovery of >68% (mainly >80%) was found for a number of organophosphorus sulfides, sulfoxides and sulfones, that of demeton-S-methyl being 82.4±6.6% (mean ± SD, n=10) when determined in lettuce. Wilkins et al. (1985) reported the characterization of nearly 90 organophosphorus sulfides, sulfoxides and sulfones by gas chromatography and mass spectrometry. Using similar experimental conditions with three different mass spectrometers, the spectra produced from a given compound were almost identical. A TLC method for 10 different organophosphorus insecticides (including demeton-S-methyl and demeton-S-methylsulfone) was reported by Funk et al. (1989). This method has a limit of detection of 4-10 ng/spot. 2.5 Formation of derivatives during storage Storage of "pure" demeton-S-methyl in the dark at room temperature leads to the formation of sulfonium derivatives. This was found to be associated with increased intravenous toxicity but not oral toxicity (Heath & Vandekar, 1957). Hecht (1960) reported that the oral toxicity in rat did not change if 4- or 24-hour-old aqueous solutions of demeton-S-methyl were used. The concentration in a water suspension of 1 g a.i./litre decreased by about 50 and 75% after 7 and 28 days of storage (apparently at 37°C), respectively (Hecht, 1960) (see also section 7.1). 3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE 3.1 Natural occurrences Demeton-S-methyl does not occur as a natural product. 3.2 Man-made sources 3.2.1 Production In 1954, a reaction mixture containing demeton-S-methyl and demeton-O-methyl (O-2-ethylthioethyl O,O-dimethyl phosphorothioate) was introduced by Farbenfabrik Bayer AG (now Bayer AG) with the common name of demeton-methyl. An improved manufacturing process led to the introduction, in 1957, of demeton-S-methyl by Bayer AG. Information on the global production is not available. It is formulated as an emulsifiable concentrate. 3.2.2 Uses Demeton-S-methyl is a systemic and contact insecticide and acaricide used to control Acarina, Thysanoptera, Hymenoptera and Homoptera on cereals, fruits, ornamentals and vegetables. It is applied as an emulsifiable concentrate formulation mainly as a spray and usually at a concentration of 0.025% a.i. (FAO/WHO, 1974). It has been reported that most national registrations for demeton-S-methyl should be transferred during the next few years to oxydemeton-methyl, which has similar use and is applied at similar rates (FAO/WHO, 1993). 4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION AND TRANSFORMATION 4.1 Transport and distribution between media The sorption behaviour of demeton-S-methyl and three other organophosphorus pesticides in natural pond sediments was tested by Froebe et al. (1989). Two different sediments were used, containing 9.3 and 6.2% organic matter, at pH 5.9 and 6.5, and a specific surface area of 23.7 and 16.7 m2/g, respectively. The semi-quantitative mineralogical composition was similar. For the four pesticides, their sorption coefficients (Kd) followed the same sequence as their lipophilicity (expressed as n-octanol/water coefficient). The sorption efficiency for demeton-S-methyl was higher in the sediment with a lower content of organic matter (Kd of 0.689 and 2.66, respectively). 4.2 Abiotic and biotic transformation 4.2.1 Hydrolytic degradation In water at 22°C demeton-S-methyl has a half-life of 63 days at pH 4, 56 days at pH 7, and 8 days at pH 9. The main reactions are demethylation in acid medium and hydrolysis of the phosphorus-ester bond in basic medium (Krohn, 1984) (see also section 2.2). 4.2.2 Photodegradation Demeton-S-methyl in aqueous solution does not absorb any light at 247 nm or longer wavelengths. Therefore, no direct photo-degradation of demeton-S-methyl in the environment is to be expected (Hellpointner, 1990). When solutions of demeton-S-methyl in water (3.6-3.7 mg/litre) were irradiated for 8 h with a high-pressure mercury vapour lamp, no photodegradation was detected. However, when solutions were fortified with humic acid (10 mg/litre), the half-life of photodegradation was 8 h (the degradation products were not identified). This suggests that degradation by sensitized or indirect photolysis may also occur in the environment (Wilmes, 1984). 4.2.3 Degradation in soil The metabolism of demeton-S-methyl in soil is shown in Fig. 1. The ability of microbial organisms to biodegrade demeton-S-methyl sulfoxide was evaluated in a laboratory test and under aerobic conditions using various species (Nocardia, Arthrobacter, Corynebacter, Brevibacterium, Bacillus and Pseudomanas) and strains (Ziegler et al., 1980). Almost all the organisms were able to degrade the insecticide. The amount of insecticide degraded ranged between 65% (Arthrobacter roseoparaffineus) and 99% (Pseudomonas putida) after 14 days of incubation. The biodegradation process led to theproduction of several metabolites. In particular, P. putida and Nocardia sp. during growth were able to metabolize almost completely 2 mmol/litre of demeton-S-methyl sulfoxide (99% and 98% respectively) within 13 days. Three major metabolites, i.e., O-demethyl-demeton-S- methyl, demeton-S-methyl sulfoxide and bis[2-ethylsulfinyl)ether] disulfide, were found to be produced by Pseudomonas, whereas Nocardia showed different pathways leading to the formation of different metabolites, i.e., 2-(ethylsulfonyl)ethane sulfonic acid, demeton-S-methyl sulfone and bis[2-(ethylthio)ethyl] disulfide. In sterile controls about 48% of the parent compound remained after 20 days of incubation. In a laboratory study, biodegradation of demeton-S-methyl was investigated in two different standardized soils (S1, and S2) with different characteristics particularly in terms of organic matter content and cation exchange capability (Wagner et al., 1985). The study was conducted under aerobic and anaerobic conditions with sterile controls and using 14C-labelled demeton-S-methyl. One day after the start of the test no parent compound could be detected. After 63 days under aerobic conditions 54% (S1) and 34% (S2) of the 14C activity applied was eliminated as 14CO2, indicating a higher activity in the soil with higher organic matter content and cation exchange capability. Various metabolites were isolated and identified (Fig. 1). Under anaerobic conditions 0.5% (S1) and 1.1% (S2) of the 14C activity applied was eliminated as 14CO2, and there was a predominance of the metabolites O-demethyl-demeton-S-methyl and demeton-S-methyl sulfoxide. The half-lives of demeton-S-methyl were approximately 5 h in the non-sterile soil and 70 h in the sterile test control. 4.2.4 Biodegradation in plants The metabolic behaviour of ethylene-1-14C demeton-S-methyl in spring wheat (Schirokko variety) was investigated in a greenhouse test (Wagner & Oehlmann, 1987). The distribution of the 14C radioactivity in the wheat matrix was determined at 3, 14, 42 and 60 days after application of demeton-S-methyl during crop stage 0 (flowering), and the isolated biotransformation products were analysed by spectroscopy. The composition of the applied spray was 241.5 µCi ethylene-1-14C demeton-S-methyl (corresponding to about 0.5 kg a.i./hectare as opposed to a use rate of 0.15 kg a.i./hectare) in 2 ml benzene plus 1 drop emulsifier Np10 plus 20 ml water. The majority of the applied radioactivity was found in the wheat straw (about 85% of total radioactivity corresponding to 10.3 mg a.i. equivalents/kg, 60 days after application), while the amount in the kernels at harvest was substantially smaller (0.7 mg a.i. equivalents/kg). About 0.5 mg a.i. equivalents/kg could not be extracted with water and organic solvents, and 24% of the 14C activity could not be extracted from the straw using solvents of different polarity. Only a minor portion of the a.i. was detected 3 days after application. Identified metabolites were O,O-dimethyl-S-[2-(ethylsulfonyl)-ethyl]-thiophosphate (11.7% of recovered radioactivity at 60 days), O,O-dimethyl-S-[2- (ethylsulfonyl)-ethyl]-thiophosphate (9.8% of recovered radioactivity at 60 days), 2-ethylsulfonyl-ethanesulfonic acid (8.2% of recovered radioactivity at 60 days), S-(2-(ethylsulfonyl)-ethyl)-thiophosphate (5.2% of recovered radioactivity at 60 days), 1-(ethylsulfinyl)-2- (methylsulfinyl)-ethane (8.9% of recovered radioactivity at 60 days), and 2-ethylsulfinylethanol (5.1% of recovered radioactivity at 60 days). Additional biotransformation products occurred to a minor extent (<0.2%) in the kernels only. 5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE 5.1 General population exposure Data on residues in crops resulting from the use of demeton-S- methyl have been summarized by FAO/WHO (1974). Maximum residue limits, varying from 0.01 to 1 mg/kg, have been recommended for a range of commodities. These residue limits were previously expressed as demeton-S-methyl but now as oxydemeton-methyl, and are common for demeton-S-methyl, oxydemeton-methyl and demeton-S-methylsulfone (see section 2.4). More updated values referring to the use of oxydemeton-methyl have been reported by FAO/WHO (1993). Data on residue levels in meat and milk from cows, and in chickens, eggs and fish have been reported for oxydemeton-methyl in the same document but not for demeton-S-methyl. Some data are also available for demeton-S-methylsulfone (FAO/WHO, 1993). The JMPR recommended a group ADI of 0-0.003 mg/kg body weight for demeton-S-methyl, oxydemeton-methyl and demeton-S-methyl sulfone (FAO/WHO, 1990). 5.2 Occupational exposure during manufacture, formulation or use When Metasystox (30% demeton-S-methyl, 70% demeton-O-methyl) was sprayed with a hand-held nebulizer, the concentration of the two active ingredients combined was 8.8-27 mg/m3 of ambient air (Klimmer & Pfaff, 1955; see also section 8.2.2). The ACGIH proposed a very conservative TLV/TWA for methyl-demeton of 0.5 mg/m3 with the "skin" notation (ACGIH, 1993). The "skin" notation indicates that dermal absorption is likely to occur and therefore adequate protective equipment should be used. 6. KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS 6.1 Absorption, distribution and excretion Absorption, distribution and excretion of (ethylene-1-14C)- demeton-S-methyl (93% radiochemical purity) were studied in SD SPF rats. Male rats (n=5 per group) were given a single oral dose of 0.1, 0.5, 5 or 10 mg/kg body weight or a single intravenous dose of 0.5 or 1 mg/kg body weight of radio-labelled compound. Female rats (n=5) received a single oral dose of 0.5 mg/kg body weight. Total radioactivity recovery was 90-100% in each animal. The authors reported that kinetic parameters did not change with dose or sex; therefore, only data on male rats given 5 mg/kg orally have been reported. Absorption after oral administration was rapid (peak blood concentration was reached within one hour) and almost complete (98-99% of the administered radioactivity was, in fact, eliminated through the urine). The blood concentration decreased with a half-life of about 2 h during the first 6 h and then with a half-life of about 6 h for the next 48 h. The half-life thereafter was even longer. The radioactivity associated with erythrocytes accounted for almost all of the blood radioactivity found 24 h or more after dosing. The half-life of urinary elimination was 2-3 h during the first 24 h and 1.5 days thereafter. Elimination through faeces and exhaled air accounted for 0.5-2% and about 0.2% of the applied dose, respectively. Except for erythrocytes, radioactivity was distributed rather uniformly in various body tissues and organs. At 2, 24 and 48 h after dosing, the radioactivity remaining in the body was about 60%, 1% and 0.5% of the administered dose, respectively. At 10 days, radioactivity was almost undetectable in most organs except in the erythrocytes. In a separate experiment, whole-body autoradiography indicated some localized accumulation of radioactivity in the pineal gland, thyroid and some glands of the genital tract (Cowper's gland, seminal vesicle, accessory genital gland). When the labelled compound (0.5 mg/kg body weight) was administered into the duodenum of rats with cannulated bile ducts, it was shown that about 3% of the radioactivity was excreted into the bile in the first 24 h (Weber et al., 1978). 6.2 Metabolic transformation The proposed metabolic pathway of demeton-S-methyl in rats is shown in Fig. 2. This was derived from the analysis of urine samples of SD rats given a single oral dose of 5 or 10 mg/kg of (ethylene-1-14C)-demeton-S-methyl. Urine samples were collected for 8 or 24 h after dosing and there was a 92% or 96% recovery, respectively, of the applied dose. The main metabolic route was oxidation of the side chain leading to the formation of the corresponding sulfoxide oxydemeton-methyl, and to a lesser extent, after further oxidation, the sulfone; O-demethylation was also an important route. Neither glucuronide nor sulfate conjugates were found (Ecker, 1978; Ecker & Cölln, 1983).
7. EFFECTS ON EXPERIMENTAL ANIMALS AND IN VITRO TEST SYSTEMS 7.1 Single exposure Demeton-S-methyl causes cholinergic toxicity. The acute toxicity data are reported in Table 2. Heath & Vandekar (1957) and Vandekar (1958) showed a significant increase in intravenous but not oral toxicity after storage of demeton-S-methyl ("pure") at room temperature in the dark. This was associated with the formation of sulfonium derivatives, which have a lower oral toxicity, possibly because of poor absorption. Dilution with water also increased the intravenous toxicity, and this was again associated with the formation of sulfonium derivatives. The maximum toxicity of a 1 mg/ml suspension kept at 35°C for one day was about 30 times the initial toxicity (the LD50 decreased from about 60 to about 2 mg/kg in rats) (Heath & Vandekar, 1957). Similar results were obtained with commercial Metasystox (70% demeton-O- methyl, 30% demeton-S-methyl) (see also section 2.2.) 7.1.1 Oral Following oral dosing with demeton-S-methyl performed on a very small number of animals (one per dose level), one rabbit given 50 mg/kg died within 2 h, while the dose of 20 mg/kg caused symptoms but the animal recovered. A cat given 10 mg/kg died after 2 days, while the dose of 5 mg/kg caused signs that were reversible and animals recovered (few details given) (Hecht, 1955). Single oral doses (180 mg/kg) of metasystox, containing 25% demeton-S-methyl, were administered to seven male buffalo calves (10-12 months of age). Signs of poisoning appeared 15 to 35 min later. Two calves were treated repeatedly with atropine (1.5 mg/kg body weight, i.v.), D-tubocurarine (0.1 mg/kg body weight, i.v.) and glucose (3-4 mg/kg body weight, i.v.). Another two calves were treated repeatedly with atropine (1.5 mg/kg body weight, i.v.), gallamine (1 mg/kg body weight, i.v.) and glucose (3-4 mg/kg body weight, i.v.) Three calves were not treated. All calves displayed typical cholinergic signs. Treatments delayed but did not prevent death, which occurred in about 2 h in calves not treated with antidotes and in about 23 h in calves treated with antidotes (Mitra et al., 1978). 7.1.2 Inhalation The LC50 (4 h of exposure) for Wistar rats was found to be 310 and 210 mg/m3 for males and females, respectively (Flucke & Pauluhn, 1983). Table 2. LD50 of demeton-S-methyl for various species and different routes of administration Species Sex Observation Route Purity LD50 Vehicle References (strain) period (mg/kg (days) body weight) Mouse M 3 oral ? 17 ? Klimmer & Pfaff (1955) Mouse ? 7 i.v. ? 7 water Hecht (1960) Rat F 1 oral "pure" 63 ? Heath & Vandekar (1957) Vandekar (1958) Rat M 3 days minimum oral ? 40 ? Klimmer & Pfaff (1955) Rat ? ? oral ? 35 ? Hecht (1955) Rat M 14 oral 25% formulation 33 water Klimmer (1964) (Wistar) Rat M ? oral 86-89% 57-64 Lutrol 9 Klimmer (1964) (Sprague- Dawley) Rat M & F 14 oral 50% formulation 64-65 water Flucke & Pauluhn (1983) (Wistar) Rat M 7 oral 50% formulation 129 water Edson (1960) (Wistar) Rat M 14 oral 90% formulation 44 cremophor EL Flucke & Kimmerle (1977) (Wistar) Table 2. (con't) Species Sex Observation Route Purity LD50 Vehicle References (strain) period (mg/kg (days) body weight) Rat F 10 oral ? 80 ethanol (20%) DuBois & Doull (1955) (Sprague- and DuBois & Plzak (1962) Dawley) propylene glycol (80%) Rat M & F 14 i.p. ? 7.5 ethanol (20%) DuBois & Doull (1955) (Sprague- and propylene DuBois & Plzak (1962) Dawley) glycol (80%) Rat ? ? i.p. formulation 10 water Hecht (1960) Rat ? ? dermal ? 85 ? Edson (1960) Rat M 14 dermal 50% formulation 71 none Flucke & Pauluhn (1983) (Wistar) Rat F 14 dermal 50% formulation 45 none Flucke & Pauluhn (1983) (Wistar) Rat ? 7 dermal technical 100-200 none Hecht (1960) Rat ? 7 dermal 25% formulation about 10 none Hecht (1960) Rat F 1 i.v. "pure" 65 ? Heath & Vandekar (1957) Vandekar (1958) Table 2. (con't) Species Sex Observation Route Purity LD50 Vehicle References (strain) period (mg/kg (days) body weight) Guinea- M 10 oral ? 110 ethanol (20%) Du Bois & Doull (1955) pig and propylene glycol (80%) Guinea- M 14 i.p. ? 12.5 ethanol (20%) Du Bois & Plzak (1962) pig and propylene glycol (80%) When rats (n=2) and mice (n=4) were exposed for 1 h to 1, 2.5 or 5 g/m3 of demeton-S-methyl (alcohol solution), all mice died whereas rats displayed signs but recovered (few experimental details given) (Hecht, 1955). Rats (n=20) were exposed for 8 h to nebulized demeton-S-methyl (0.5 g/m3 of air), which was obtained from a 25% emulsifiable commercial formulation diluted 1:250 (0.1% final concentration of active ingredient). None of the animals died or showed overt cholinergic signs. Erythrocyte cholinesterase activity, determined in three animals immediately after the end of exposure, was reduced by 70%. When purified active ingredient was used, lower erythrocyte cholinesterase inhibition (60%) was found under the same experimental conditions. This was paralleled by an increased rat LD50 (from 10 to 27.5 mg/kg i.p.) (Hecht, 1960). 7.1.3 Dermal Doses of 20 or 100 mg/kg demeton-S-methyl applied to the shaved skin of two cats caused death. A dose of 10 mg/kg caused mild signs; very few details were given (Hecht, 1955). 7.2 Short-term exposure 7.2.1 Rat Groups of six male rats (strain not specified) were fed demeton-S-methyl at levels of 0, 50, 100 or 200 mg/kg diet in the diet (equivalent to 0, 5, 10 and 20 mg/kg body weight per day, respectively) for 6 months. Cholinergic signs (slight tremors and fasciculations) were observed at the highest dose-level during the first 5 weeks of the study. Brain and erythrocyte cholinesterase activities were reduced at 50 mg/kg diet (by about 80 and 88%, respectively), 100 mg/kg diet (by about 85 and 92%) and 200 mg/kg diet (by about 90 and 94%) groups. Body weight gain was depressed at 100 and 200 mg/kg diet. Gross microscopic examination of tissues (liver, kidney and adrenals) showed no treatment-related changes (Vandekar, 1958). Groups of 30-day-old female Holtzman rats were fed dietary levels of 0, 1, 5 or 25 mg/kg diet of demeton-S-methyl (purity not reported) for 1 week. At termination, serum, liver and brain acetylcholinesterase (n=3) and liver and serum aliesterase (n=3) with diethylsuccinate and tributyrin as substrates activities were measured. Interpolated dietary levels producing 50% inhibition were 15-28 mg/kg diet for acetylcholinesterase, 4-6 mg/kg diet for liver aliesterase and about 25 mg/kg diet for serum aliesterase, equivalent to 1.5-2.8, 0.4-0.6 and 2.5 mg/kg body weight per day, respectively (Su et al., 1971). 7.2.2 Dog In a one-year study, pure-bred beagle dogs (n=6 animals of each sex per group) were fed 0, 1, 10 or 100 mg a.i./kg diet (day 1-36) or 50 mg a.i./kg diet (day 37-termination) of demeton-S-methyl (52.2% in xylene). Haematological, blood biochemical and urinalysis parameters were determined during pretest period and at months 1, 2, 3, 4, 5, 6, 8, 10, 12. Hearing tests and ophthalmoscopic examinations were performed once in the pretest period and at months 3, 6 and 12 of treatment. At termination animals were killed for pathology, and determination of organ weights, brain cholinesterase activity, hepatic cytochrome P-450 and triglyceride contents and N-demethylase activity were carried out. All animals survived the study. Diarrhoea and vomiting were observed in all animals, most frequently in the high-dose group: these animals also showed reduced food consumption before the dose was reduced to 50 mg/kg diet. The mean daily compound intake was found to be 0.036, 0.36 and 4.6/1.5 mg/kg body weight at 1, 10 and 100/50 mg/kg diet, respectively. Body weight was similar in all groups. No alterations were observed in the hearing test and ophthalmoscopic examination. Haematological, blood chemistry (excluding cholinesterase activities) and urinalysis parameters and organ weights at termination were not significantly altered by any of the treatments. Hepatic biochemical parameters were not altered by any of the treatments. No treatment-related gross pathology alterations were found. However, multifocal slight/moderate atrophy and/or hypertrophy of proximal renal tubules was demonstrated in three males and three females of the high-dose group. Plasma cholinesterase activity was reduced as compared to controls by 20-30% and 5-20% in males and females, respectively, at 10 mg/kg diet, and by 45-65% (males) and 50-70% (females) at 50 mg/kg diet. Erythrocyte cholinesterase activity was reduced by 25-35% and 30-45% in males and females, respectively, at 10 mg/kg diet. A higher inhibition was found at 50 mg/kg diet, where inhibition was 80-90% and 55-65% in males and females, respectively. Brain cholinesterase activity was reduced by 25% in males at 10 mg/kg diet and by 64% (males) and 15% (females) at 50 mg/kg diet. Based on effects on brain cholinesterase activity, the NOAEL was 1 mg/kg diet, equal to 0.036 mg/kg body weight per day (Bathe, 1983). 7.3 Long-term exposure 7.3.1 Mouse A long-term carcinogenicity study was conducted in NMRI mice (70 animals of each sex per group) that were given demeton-S-methyl (about 50% in xylene) mixed into the feed with approximately 10 mg/kg diet of groundnut oil at concentrations of 0, 1, 15 or 75 mg a.i./kg diet, or xylene (75 mg/kg diet). Groups were subdivided into two subgroups: one (n=20) was terminated at 12 months, the second one was terminated at 21 months. Animals in the high-dose group had a lower (significantly lower during the first 4 weeks only) food consumption and a reduced (about 10% throughout the study in males only, and at the beginning in females) body weight. The mean daily intake of demeton-S-methyl was (males/females): 0.24/0.29, 3.47/4.18, 17.81/20.0 mg/kg body weight at 1, 15, 75 mg/kg diet, respectively. Clinical signs due to cholinesterase inhibition were not observed. Mortality at 21 months was (males/females) 16/30, 13/34, 17/35, 16/35 and 16/32% in the control, low-, mid-, high-dose and xylene groups, respectively. Haematological and clinical chemical parameters were not affected by the treatment except plasma urea (lower than control in the high-dose males) and plasma and erythrocyte cholinesterase activities. Plasma cholinesterase activity was significantly decreased in mid- (by 63-74%) and high-dose (by 91-97%) groups. Erythrocyte cholinesterase activity was only slightly reduced in the mid- and high-dose groups. Brain cholinesterase activity (n=10 per group) was reduced in high-dose groups (in males by 70% and in females by 59%) and in the mid-dose group (in males by 44% and in females by 38%). Histological examination did not reveal an increased incidence of neoplastic and non-neoplastic lesions in treated groups. Based on inhibition of brain cholinesterase, the NOAEL was 1 mg/kg diet, equal to 0.24 mg/kg body weight per day (Schmidt & Bomhard, 1988). 7.3.2 Rat Wistar rats (60 animals of each sex per group) were given demeton-S-methyl (about 50% in xylene mixed into the feed with approximately 10 mg/kg of groundnut oil) at concentrations of 0, 1, 7 or 50 mg a.i./kg diet, or 50 mg/kg diet of xylene. Groups were subdivided into two subgroups; one (n=10) was terminated at 12 months, the second one was terminated at 24 months. Hair loss (up to 50% of females) and diarrhoea (up to 50% of males) were observed significantly more frequently in the animals of the high-dose group. Body weight was reduced in mid-dose males (by 5-10%) and in both males (by 10-20%) and females (by 5%) in the high-dose group. Food consumption was similar in all groups. The mean daily intake of demeton-S-methyl was (males/females): 0.05/0.06, 0.31/0.41, 2.59/3.09 mg/kg body weight at 1, 7 and 50 mg/kg diet, respectively. Mortality at 24 months was (males/females) 12/26, 10/20, 10/20, 4/26 and 8/24% in the control, low-, mid-, high-dose and xylene groups, respectively. Haematological and clinical chemical parameters, except for cholinesterases, measured at months 6, 12, 18 and 24, were unaffected by the treatments. Plasma and erythrocyte cholinesterase activities, measured at months 3, 6, 12 and 24, were significantly decreased in groups given 7 mg/kg diet (plasma cholinesterase by 30-56%, erythrocyte cholinesterase by 12-31%) or 50 mg/kg diet (plasma cholinesterase by 75-92%, erythrocyte cholinesterase by 20-44%). Brain cholinesterase activity (measured at 12 and 24 months) was reduced in the high-dose group (by 67-75%) and in the mid-dose group (by 15-47%). A statistically significant decrease in brain cholinesterase activity (33%) was observed in males given 1 mg/kg diet at 24 months but not at 12 months. The toxicological significance of this finding is not clear; it should also be noted that brain cholinesterase activity at 7 mg/kg diet was higher than at 1 mg/kg diet and that plasma cholinesterase and erythrocyte cholinesterase activities were not inhibited at either of these dose levels. Histological examination did not reveal an increased incidence of neoplastic lesions in treated groups. Increased incidence of retinal atrophy (78% of males, 92% of females as compared to 36-63% and 61-70% in the other groups) and keratitis (44% of males, 22% of females as compared to 4-12% and 0-2%, respectively, in the other groups) was observed at 50 mg/kg diet. The retinal atrophy mainly affected the fundus, unilaterally or bilaterally, and occurred in either the inner corneal layer, the inner and outer layers, or all layers of the retina. Based on inhibition of brain cholinesterase, the NOAEL was considered to be 1 mg/kg diet, equal to 0.05 mg/kg body weight per day (Schmidt & Westen, 1988). 7.4 Skin and eye irritation and sensitization 7.4.1 Skin and eye irritation Demeton-S-methyl (commercial formulation, 50% of active ingredient) was applied (0.5 ml) to the shaved skin of three New Zealand white rabbits on a 2.5 × 2.5 cm cellulose dressing for 4 h. Mild erythema and oedema were observed, which generally disappeared after 3 days (Flucke & Pauluhn, 1983). A 52.5% solution of demeton-S-methyl in xylene produced slight skin and eye irritancy in New Zealand white rabbits, but this was attributed to the solvent (Thyssen, 1981). A commercial formulation of demeton-S-methyl (50%) was instilled (0.1 ml) either undiluted or as a 0.5% aqueous dilution into the conjunctival sac of one eye of groups (n=3) of New Zealand white rabbits. Treated eyes were washed with physiological solution after 24 h. No signs of eye irritation were observed in rabbits treated with the 0.5% aqueous solution. The undiluted formulation caused severe lacrimation and miosis on application. Mild corneal opacity and discrete redness and oedema of conjunctivae were observed and recovered in about 7 days (Flucke & Pauluhn, 1983). 7.4.2 Skin sensitization The skin-sensitizing potential of demeton-S-methyl was assessed by the Magnusson and Kligman maximization test with Freund's adjuvant on guinea-pigs (n=20, Bor:SPF, DHPW strain). The concentrations of demeton-S-methyl (96.3% purity, average of three determinations) used were: 0.1% for the intra-dermal induction, 10% for the topical induction and the first challenge, and 1% for the second challenge. All twenty animals reacted positively to the 1st challenge (controls 4/10), and 16 reacted positively to the 2nd challenge (controls 3/10). The results indicate that demeton-S-methyl has a skin-sensitizing potential (Heimann, 1987a). In another study, the Buehler epidermal patch test was used on guinea-pigs (n=12, Bor: SPF, DHPW strain). The concentrations of demeton-S-methyl (95.6% purity, average of three determinations) used were 10% for topical induction (once a week for 3 weeks) and the first challenge, and 20% for the second challenge. The results indicate that demeton-S-methyl does not have a skin-sensitizing potential under these conditions (Heimann, 1987b). It is concluded that demeton-S-methyl might have some skin-sensitizing potential, but this is of no relevance in practice. 7.5 Reproduction, embryotoxicity and teratogenicity 7.5.1 Reproduction A standard two-generation study (two litters/generation) was conducted in SPF rats (BOR:WISW) (10 males and 20 females) that were given demeton-S-methyl at 0, 1, 5 or 25 mg/kg diet (Eiben et al., 1984). The compound was used as a pre-mix in xylene (about 50%). Rats in an extra control group were given xylene at 25 mg/kg diet. In the F0 generation, none of the animals died. No treatment- related signs were observed in any animal. Body weight gain was reduced in males (by 10%) and in some females at 25 mg/kg diet. Food intake was also reduced (by 7%) in high-dose males. Fertility index was not affected by treatment. At 25 mg/kg diet, the viability of pups was reduced; it was 89% and 85% in first and second matings, respectively. Lactation index was also reduced in the high-dose group (85-92%), the control value being 98-99%. Body weight at birth was comparable in all groups, but body weight gain was significantly reduced (by 8-10%) in pups fed 25 mg/kg diet. In the F1b generation, one female was found dead in the 5-mg/kg diet group and one in the 25-mg/kg diet group; one male and one female in one of the 25-mg/kg diet litters also died. Autopsy did not reveal treatment-related alterations. No treatment-related signs were observed in any animal. Body weight gain was reduced at times in low-dose males and consistently in mid- and high-dose and xylene- treated males when compared to untreated animals. When compared to xylene-treated animals (which were 5-15% lighter than untreated animals), however, only males of the high-dose group had a significantly reduced (by about 15%) body weight gain. Females of the high-dose and xylene groups had a reduced (by about 10%) body weight, compared to controls, the former being at times lighter weight than the latter. Fertility index was not significantly affected. The number of pups born was reduced in the high-dose group and the viability of pups was also reduced in the mid- and high-dose groups in a dose-related manner (82-88% and 47-67% of controls, respectively). No compound-related malformation was found in animals of any of the treatment groups. Demeton-S-methyl intake, as calculated in the F0 generation, was found to be (female data in parentheses): 0.07 (0.08), 0.32 (0.39) and 1.71 (1.90) mg/kg body weight per day in the low-, mid- and high-dose groups, respectively, and xylene intake was 1.66 (1.96) mg/kg body weight per day. Based on the viability of pups and body weight in the F1b generation, the NOAEL was 1 mg/kg diet, equal to 0.07 mg/kg body weight per day (Eiben et al., 1984). 7.5.2 Embroytoxicity and teratogenicity 7.5.2.1 Rat Groups (n=25) of fertilized female rats (BAY:FB 30 strain) were given daily (0, 0.3, 1 or 3 mg/kg body weight orally) demeton-S-methyl (from a 52.6% solution in xylene) dissolved in corn oil from day 6 to 15 of gestation. At day 20 of gestation, pups were delivered by caesarean section. Fetuses were weighed, sexed, inspected for external abnormalities and examined for visceral and bone malformations. No alteration of physical appearance or behaviour was observed in any group. All animals survived until the caesarean section. Body weight gain was reduced (by 13%) in the high-dose group. The numbers of live fetuses and resorptions, fetal weight, number of fetuses with malformations and number of implants were comparable in all groups. No treatment-related visceral or skeletal abnormalities were observed (Renhof, 1985). 7.5.2.2 Rabbit A formulation of demeton-S-methyl (52.2% a.i. in xylene) was administered by gavage to mated chinchilla hybrid rabbits (n=15-16) on gestation days 6 to 18 at dose levels of 0, 3, 6 and 12 mg/kg body weight per day. Caesarean sections were performed on gestation day 28. There were no mortalities. In the high-dose group, diarrhoea was observed in all animals after 4 to 10 days of treatment. Beginning 1 to 2 h after dosing, it persisted for 6 to 24 h. In the high-dose group, mean food consumption was decreased by 7% during gestation days 6 to 18 and by 17% during gestation days 19 to 24 when compared to controls. This was associated with decreased mean body weight gain (-7%). There were no abortions and no relevant differences between test and control groups in the numbers of implantations per dam, pre- implantation losses, post-implantation losses, resorptions, living and dead fetuses or sex ratios. A decrease in mean fetal body weight, compared to the mean control weight, of 6.6% was observed in the high-dose group. There was no treatment-related increase in gross, skeletal or visceral malformations (Becker, 1983). 7.6 Mutagenicity and related end-points A summary of the studies conducted to assess mutagenicity of demeton-S-methyl is given in Table 3. 7.6.1 DNA damage and repair Demeton-S-methyl did not induce DNA damage in the Pol test on Escherichia coli either with or without metabolic activation. 7.6.2 Mutation Increased mutation rates were observed in the Ames test and in the mouse lymphoma forward mutation assay both with and without metabolic activation. 7.6.3 Chromosomal effects In in vivo tests, no SCEs were found in the bone marrow of Chinese hamsters treated with high doses of demeton-S-methyl. Bone marrow micronucleus and dominant lethal tests on mice treated with demeton-S-methyl gave negative results. Chromosomal aberrations were found in the bone marrow of Syrian hamsters treated with a commercial formulation of demeton-S-methyl. It is concluded that the available information is insufficient to permit an adequate assessment of the genotoxic potential of demeton-S-methyl. 7.7 Delayed neurotoxicity Adult Leghorn hens (n=20) were given two doses of 100 mg a.i./kg body weight (approximately equal to the LD50) of demeton-S-methyl (51.2% in xylene) by gavage. The second dose was given 21 days after the first one. Positive control animals (n=5) received tri- ortho-cresyl phosphate (TOCP) (375 mg/kg body weight by gavage). Animals were pretreated with atropine (100 mg/kg body weight intramuscularly 10 min before the dose of demeton-S-methyl and 50 mg/kg body weight subcutaneously 6 h later). Surviving animals received atropine (30 mg/kg s.c.) 24, 30 and 48 h later. At the second dose, atropine treatment was suspended after 24 h. Hens treated with demeton-S-methyl had signs of cholinergic toxicity. The recovery started on day 3 and by day 8 all treated animals, except for one, were free of signs. After the second dose, the recovery started on day 2 and by day 5 all treated animals were free of signs. One animal died after the second treatment and surviving animals did not develop neurological deficits. TOCP-treated animals showed locomotor Table 3. Studies on mutagenicity of demeton-S-methyl Test Organism Purity Results LED or HIDa Reference -S9 +S9 Microorganisms Pol assay Escherichia coli p3478, 93% - - 10 000 µg/plate Herbold (1983a) W3110 Reverse Salmonella typhimurium unknown + n.t. 5 µg/plate Hanna & Dyer (1975) mutation TA1530, TA1535, his C117, his G46 E. coli WP2, WP2 uvra, unknown + n.t. 5 µg/plate Hanna & Dyer (1975) CM561, CM571, CM611, WP67, WP12, S. typhimurium TA98, TA100, 50.2% ++ 300 µg/plate Herbold (1979) TA1535, TA1537 (formulation) S. typhimurium TA98, TA100, >98% ++ 20 µg/plate Herbold (1980a) TA1535, TA1537 Saccharomyces cerevisiae 53.1% -- 1062 µg/ml Hoorn (1982) S138 S211ý (formulations in xylene) Insects Recessive Drosophila melanogaster unknown + 80 mg/kg diet Hanna & Dyer (1975) lethal Table 3. (con't) Test Organism Purity Results LED or HIDa Reference -S9 +S9 Cultured mammalian cells Mutation, tk Mouse lymphoma L5178Y cells 94% ++ 50 µg/ml Cifone (1984) locus Mammals in vivo Bone marrow NMRI mouse >98% - 2 × 5 mg/kg b.w. Herbold (1980b) micronucleus oral SCE in bone Chinese hamster 94% - 20 mg/kg b.w. oral Herbold (1983b) marrow Chromosomal Syrian hamster, female 50% + 2 mg/kg b.w. i.p. Dzwonkowska & Hübner (1986) aberration (commercial) Dominant NMRO mouse >98% - 5 mg/kg b.w. oral Herbold (1980c) lethal a LED = lowest effective dose; HID = highest ineffective dose impairment beginning on day 10. Histological examination showed moderate axonal degeneration in peripheral nerves and medulla in TOCP-treated animals but not in demeton-S-methyl-treated or solvent control animals (Flucke & Kaliner, 1988). Neuropathy target esterase (NTE), the target for organophosphate- induced delayed neuropathy, was not inhibited in hen brain and spinal cord 1, 2 and 7 days after treatment with demeton-S-methyl (80 mg a.i./kg body weight) by gavage. Positive controls (TOCP at 100 mg/kg body weight) showed NTE inhibition (> 90%) in both brain and spinal cord (Flucke & Eben, 1988). 7.8 Toxicity of metabolites Two plant and mammalian metabolites of demeton-S-methyl (namely oxydemeton-methyl and demeton-S-methylsulfone) have been studied extensively, since they are also the active ingredient of commercial pesticides. According to the JMPR, the toxicity of the two compounds does not differ substantially, either qualitatively or quantitatively, from that of demeton-S-methyl (FAO/WHO, 1990). 7.9 Mechanism of toxicity - mode of action Demeton-S-methyl is a direct cholinesterase inhibitor and it causes signs and symptoms of the cholinergic syndrome. The in vitro I50 (30 min, 37°C) for sheep erythrocyte cholinesterase was 6.5 × 10-5 mol/litre. The I50 of oxydemeton-methyl and demeton-S-methylsulfone were of the same order of magnitude (2.7 × 10-5 and 4.3 × 10-5 mol/litre, respectively). The half-life of recovery of acetylcholinesterase activity after inhibition by demeton-S-methyl was 1.3 h, as expected from a dimethyl phosphorylated acetylcholinesterase (Heath & Vandekar, 1957). The 1973 JMPR (FAO/WHO, 1974) reported that rat brain cholinesterase was more sensitive to in vitro inhibition by demeton- S-methyl than by oxydemeton-methyl (I50 values of 9.52 × 10-5 and 1.43 × 10-3 mol/litre, respectively; time of incubation, temperature and pH not reported). It was also reported that demeton-S-methyl was a more potent inhibitor of human serum cholinesterase (no details given). Data on in vivo inhibition of plasma cholinesterase and erythrocyte and brain cholinesterase are reported in section 7.2 and 7.3. 7.10 Potentiation Male Wistar rats (160-180 g body weight) were given trichlorfon (98.6% purity) and demeton-S-methyl (90% purity) in combination by gavage. The amount of compound in the mixture was proportional to its oral LD50 (302 mg/kg body weight for trichlorfon and 44 mg/kg body weight for demeton-S-methyl) in order to obtain equitoxic doses. The resulting toxicity was additive (LD50 of the mixture was 223 mg/kg body weight against expected 173 mg/kg body weight) (Flucke & Kimmerle, 1977). Similarly, an additive effect was obtained when demeton-S-methyl was given in combination with phenamiphos (ethyl-4-(methylthio) m-tolyl-isopropyl-phosphoroamidate). The experimental LD50 of the mixture was 55 mg/kg body weight while that estimated from the individual LD50s was 50 mg/kg body weight (Kimmerle, 1972). 8. EFFECTS ON HUMANS 8.1 General population exposure A 31-year old woman attempted suicide with an unknown amount ("1 or 2 mouthfuls") of Metasystox I (25% demeton-S-methyl). On admission to hospital she was comatose, sweating and salivating, and had pin-point pupils. She stayed in the Intensive Care Unit for 15 days where she was treated with atropine (up to 97 mg per day, 550 mg in 14 days). Oximes (2-PAM) were only given on day 1 (0.5 + 0.25 + 0.25 g) and there was no apparent clinical improvement. On admission, plasma and erythrocyte cholinesterase activities were less than 10% of normal control values. The patient was discharged on the 30th day with normal plasma cholinesterase values; erythrocyte cholinesterase activity was still below the normal values (about 65%), but had recovered a month later (Barr, 1966). A case of acute poisoning in a 5-month pregnant woman has been described (Carrington da Costa et al., 1982). A 41-year-old woman attempted suicide by ingesting Metasystox, which resulted in an estimated intake of 12 g methyl demeton (it should be noted that in the report Metasystox was said to contain methyl-demeton and not oxydemeton-methyl as the commercial name implies). On admission, 3.5 h after poisoning, blood cholinesterase (it was not specified whether pseudo- or acetylcholinesterase) was 10% of normal values. The patient became comatose 12 h after admission. She was treated with atropine, obidoxime and haemoperfusion 72 h after hospitalization. Artificial ventilation was required on the 4th day of hospitalization. The patient was discharged after 24 days and 4 months later she delivered a healthy female child. In a fatal suicidal case with a commercial formulation of demeton-S-methyl, the concentration of the compound was measured in several organs. It was estimated that death occurred about 6 h after poisoning. Highest demeton-S-methyl levels were found in the proximal small intestine (166 mg/kg of tissue); in brain, kidney and muscles, the levels were 30-80 mg/kg; lowest concentrations were found in liver (7 mg/kg tissue) and blood (7-16 mg/litre). Metabolites were not measured (Schludecker & Aderjan, 1988). In the United Kingdom in 1991 there were five cases of poisoning by demeton-S-methyl and in 1994 three cases, none of which were fatal (personal communication by G. Volans, Medical Toxicology Unit, London, to the IPCS, dated 15 January 1996). Hegazy (1965) reported three cases of suspected poisoning with demeton-S-methyl in children (6-14 years of age) exposed in a recently sprayed field and 1 case after ingestion of contaminated feed. Symptoms were mild and the patients recovered fully. Serum cholinesterase determinations gave ambiguous results. 8.2 Occupational exposure 8.2.1 Acute poisoning A worker inadvertently exposed to demeton-S-methyl (no details) was monitored for about 100 days. Plasma cholinesterase activity was always within the normal values of the laboratory. However, if the activity on day 40 and 100 is considered as the normal value for this worker, then a 30% inhibition occurred 2-3 days after exposure. The activity recovered with an half-life of about 10 days. Erythrocyte cholinesterase activity was below the normal value for about 40 days. When calculated on the activity of day 100, a 60% inhibition was found up to day 10. The activity recovered with a half-life of about 35 days (Lewis et al., 1981). Two workers in a chemical packaging company, whose job it was to fill a concentrate of demeton-S-methyl (500 g/litre of xylene) into one-litre containers using a weight-triggered bottle-filling machine for 3 to 4 h, were admitted to hospital because of organophosphate poisoning and were treated with atropine. Cholinesterase measurement made 14 days after exposure (1 worker) showed inhibited erythrocyte cholinesterase, but the plasma cholinesterase activity was within the normal range. In the second worker, whose symptoms lasted for 3 days, erythrocyte cholinesterase activity was below the normal value 5 weeks after exposure. These workers wore gloves, overalls and boots, but frequent spillage was reported. Normal clothing had been left under the filling apparatus and apparently was contaminated. The filling system was changed by housing the filling machine in a fume cupboard and by providing new protective garments and changing room facilities. One more worker was admitted to the hospital and treated with atropine because of organophosphate poisoning. This occurred on the morning after a day spent fitting the infill seal and screwing on the tops of cans. He had reduced erythrocyte and plasma cholinesterase (erythrocyte > plasma) activities for several days after exposure. Another worker showed a sharp drop in erythrocyte and plasma cholinesterase activities associated with abdominal cramps, which resolved in about 5 days. Another worker had depressed erythrocyte and plasma cholinesterases activities without complaining of any adverse effect. It was concluded that absorption of demeton-S-methyl was through the skin because of penetration of the protective clothing used or because this clothing was not worn properly. It should be noted that the active ingredient was dissolved in xylene, which is known to attack rubber and plastics, making the penetration of gloves possible (Jones, 1982). 8.2.2 Effects of short- and long-term exposure Three volunteers without any protective equipment were exposed for two consecutive days to Metasystox (30% demeton-S-methyl, 70% demeton-O-methyl) while spraying with a hand-held nebulizer. Exposure lasted for 3 and 6 h on the first and second days, respectively. The concentrations of the active ingredient (isomers not separated) were 8.8-27 mg/m3 of ambient air. Plasma and erythrocyte cholinesterase activities measured up to 14 days after exposure did not show significant decreases when compared to pre-exposure values (Klimmer & Pfaff, 1955). Volunteers, wearing overalls but not mask protection, were exposed to metasystox (30% demeton-S-methyl, 70% demeton-O-methyl) while spraying in a greenhouse. They used the splash method (0.03-0.05% a.i.), the low volume (0.5% a.i.) or the high volume spray method (0.05% a.i.) for 5 to 25 min. There was no effect on plasma or erythrocyte cholinesterase activity measured after exposure (Klimmer & Pfaff, 1958). Six workers engaged in hop cultivation using Metasystox I (reported to contain demeton-O-methyl instead of demeton-S-methyl as the commercial name implies) were monitored. They sprayed up to 2400 litres of a 0.1% solution (in water) of the insecticide in one day. Protective clothing and masks were not always used. No significant inhibition of blood acetyl cholinesterase was observed at the end of exposure or 1 or 2 days later. One subject, who was exposed twice, showed a 29% decrease in blood acetyl cholinesterase after the second exposure. No signs or symptoms were observed in these workers (Winkler & Arent, 1970). The medical department of a company reported no adverse effect in workers employed in the formulation of demeton-S-methyl from 1967 to 1984 (Faul, 1984). Agricultural workers exposed to demeton-S-methyl for 3 consecutive days were monitored. Pre-and post-exposure urinary levels of the metabolite dimethyl phosphorothiolate potassium salt (DMPThK), and plasma and whole blood cholinesterase activities were measured. Exposed subjects were divided into three groups according to their job, i.e. mixers (n=7), sprayers (n=6) and others (n=7) not directly involved in handling the pesticide. Higher levels of DMPThK were found in mixers, with a medium value of 83 µg/litre and a range of 0-822 µg/litre (neither corrected for creatinine nor for urine volume). Sprayers had a mean value of 30 µg/litre (limit of detection) and a range of 0-208 µg/litre; the other subject had a mean value of 30 µg/litre and a range of 0-100 µg/litre. Whole blood cholinesterase activity was not affected by the exposure, while plasma cholinesterase activity was slightly (about 10%, statistically significant) reduced when compared to pre-exposure levels in mixers. However, no correlation was found between DMPThK levels and plasma cholinesterase activity (Vasilic et al., 1987). Hegazy (1965) reported a study of 121 spraymen exposed to Meta-isosystox during spraying of cotton fields in Egypt. The spraymen applied Meta-isosystox at a rate of 0.5 litres of concentrate in 400 litres of spray per 4200 m2 mainly by hand-operated knapsack sprayers, or in a few cases by high-pressure motor-powered sprayers with a large tank connected by a long hose to a multi-nozzled spray-boom. Sprayers were not involved in chemical mixing. Workmen washed exposed body parts with soap and water after spraying. Working clothes were removed at the end of the day, but the clothes may not have been washed before re-use. Not all workers used protective clothing and masks. None of the workers were re-exposed to Meta-isosystox after the onset of symptoms. Serum cholinesterase activity estimates were performed within 24 h after the onset of symptoms in some patients and after the cessation of symptoms in some others. In most cases they were repeated 2-3 times at various intervals up to 40 days from the onset of symptoms. In general, the serum cholinesterase activity in spraymen underwent a marked initial fall, followed by a rise to above-normal levels after about 30-40 days. Signs and symptoms of toxicity in spraymen occurred after 1-18 days of exposure, with a mean time of 3 days. These consisted of gastrointestinal disturbances (58% of total), dizziness (23%), persistent general weakness and fatigue (19%), respiratory manifestations (16%), headache (16%), sweating, salivation or lacrimation (12%), tremors of outstretched hands, intention tremors, ataxia (4.1%), exaggerated superficial and deep reflexes (5.8%), hiccough (2.5%), muscular fasciculations (2.5%), or had apparently resolved (30%), at the first determination of serum cholinesterase activity (Hegazy, 1965). 9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD 9.1 Aquatic organisms 9.1.1 Algae Table 4 reports the results obtained when different formulations of demeton-S-methyl were tested on the green alga Scenedesmus subspicatus (Heimbach, 1985a, 1990b). 9.1.2 Invertebrates The acute toxicity of demeton-S-methyl for molluscs and crustaceans is given in Table 5. The lowest concentration tested on the water flea Daphnia magna (10 µg a.i./litre) caused toxic effects (Heimbach, 1985b,c). A 21-day exposure test on water flea reproduction produced a NOEC of > 5.6 µg a.i./litre (Heimbach, 1990d). A study performed with a 27.3% emulsifiable concentrate formulation (Heimbach, 1985c) yielded a NOEC of 3.7 µg/litre (equal to 1 µg a.i./litre) and a LOEC of 11.7 µg/litre (equal to 3.1 µg a.i./litre). Using a commercial formulation of metasystox, the 96-h LC50 for the lammellibranch mollusc Paphia laterisulca was 2 µg/litre (Akarte et al., 1986) and that for the freshwater prawn Donax cuneatus was 4 µg/litre (Muley et al., 1987). 9.1.3 Fish Table 6 indicates the toxicity of demeton-S-methyl for fish. 9.2 Terrestrial organisms 9.2.1 Soil microorganisms In a study conducted in silty sand soil or loamy silt soil with doses of demeton-S-methyl up to 5 times those recommended (2 litres/hectare of a 27% emulsifiable concentrate formulation), no influence on soil respiration or nitrification in soil was found (Anderson, 1989; Blumenstock, 1989). 9.2.2 Invertebrates When demeton-S-methyl was mixed with an artificial soil where earthworms (Eisenia foetida) were kept for 14 days, the LC50 was 241 mg/kg of dry substrate of the commercial formulation (a 25% emulsifiable concentrate), corresponding to 60 mg a.i./kg (Heimbach, 1990a). Table 4. Effect of demeton-S-methyl on the green alga Scenedesmus subspicatusa Specification of test EC50 (mg a.i./litre) NOEC LOEC Duration Conditions substance (mg/litre) (mg/litre) (h) Increase of Growth rate biomass Technical 97.3% purity 8 22 1 3 96 pH 7.8 - 8.5 23 °C EC formulationb 37 >100 18 32 96 pH 7.6 - 10.4 (27.3% a.i.) 23 °C Pre-solutionc 13 37 1 10 96 pH 7.7 - 8.5 xylene (53.7% a.i.) 22 °C a From: Heinbach (1985a, 1990b) b Tests performed with the blank formulation gave the same results as the highest tested concentration. c No test with the blank pre-solution was performed. Table 5. Acute toxicity of demeton-S-methyl for molluscs and crustaceans Species Specification of test Temperature LC50 Duration of Reference substance (°C) (mg/litre) exposure (h) Mollusc commercial formulation ? 0.0042 96 Akarte et al. (1986) (Paphia laterisulca) Water flea (Daphnia magna) technical (96.7%) 20 ± 1 >0.1 24 Heimbach (1985b) 0.023 48 pre-solution in xylene 20 ± 1 >0.1 24 Heimbach (1985c) (53.7%) formulation 0.022 48 Clam (Donax cuneatus) commercial formulation 25-28.5 0.0064 96 Muley et al. (1987) Prawn (Macrobrachium lamerrii) commercial formulation 27 ± 2 1.3 72 Mary et al. (1986) Table 6. Toxicity of demeton-S-methyl for fish (96-h exposure) Species Mass and length Temperature LC50 (mg/litre) Reference (°C) Rainbow trout (Onchorhyncus mykiss) 4.0 - 5.5 cm 16 4.5 Grau (1985a) 1.0 - 1.5 g (52.7% of a.i.) 6.4 ± 1.0 cm 15±2 0.59 Grau (1990c) 3.0 ± 0.6 g (69.5% of a.i.) 6.9 ± 1.1 cm 15±2 6.44 Grau (1990a) 3.5 ± 1.6 g (27.3% of a.i.) Golden orfe (Leuciscus idus melamotus) 6.0 - 7.5 cm 21 43 Grau (1985b) 2.5 - 4.2 g (52.7% of a.i.) 6.4 ± 0.6 cm 21±2 23.2 Grau (1990b) 2.5 ± 0.6 g (27.3% of a.i.) Goldfish (Carassius auratus) 6 cm 18 20-40 Hermann (1974a) 1.5 g (28.1% of a.i.) Carp (Cyprinus carpio) 6 cm 18 40-60 Hermann (1974b) 1.6 g (28.1% of a.i.) Scardinius erythrophthalmus 6 cm 18 30-40 Hermann (1974c) 1.3 g Cirrhana mrigala (larvae) 51 ± 3 mg 20 1.45 Verma et al. (1984) Demeton-S-methyl was applied on fields of winter wheat by fixed-wing aircraft using conventional boom-and-nozzle equipment. The applied amount was 245 g a.i. per hectare at a volume rate of 20 litres/hectare. Samples of the soil surface and crop foliage fauna were collected 1-2 times before and 4-5 times after application from a treated field and from a control untreated field. The number of crop foliage but not of soil surface entomophagus invertebrates was reduced soon after application of demeton-S-methyl. Empididai (dance flies) was the only group to be significantly reduced in numbers by demeton-S-methyl. Predatory Coleoptera (beatles) (Carabidae and Staphylinidae), Araneae (spiders) and predatory Diptera (flies) (except Empididae) were not affected by demeton-S-methyl. Among the ephytophagus fauna, cereal aphids markedly declined in number soon after application, but a rapid increase was observed 2 months later, when numbers of Diptera and Thripidae (thrips) were also increased. Numbers of entomobryid and sminthurid springtails decreased soon after the application but were higher than in the control field 2 months later (Shires, 1985). Demeton-S-methyl was slightly toxic to the predatory mite Phytoseiulus persimilis since a 70% population reduction was obtained with a concentration of 0.025% a.i. (Kniehase, 1984). The contact LD50 for bees has been reported to be 0.60 µg/bee (Westlake et al. 1985). A field study was conducted on bee colonies following application of a 0.2% formulation of Metasystox (at 600 litres/hectare) to field beans (Vicia faba) after bee foraging had ceased in the evening. Toxicity to bees was assessed by collecting dead insects on canvas sheets in front of the hive entrance. Before spraying, the mortality in exposed and control hives was comparable. Following spraying, there was a mortality rate of 123 bees per hive in the exposed and 90 bees in the controls (prior to exposure mortality was 98 and 95 respectively). The mortality increase persisted for 3 days after the application (Bayer, 1970). 9.2.3 Birds Some oral LD50 values are given in Table 7. Table 7. Acute oral LD50 values for birds Species Vehicle Purity LD50 Reference (mg/kg b.w.) Japanese quail Cremephor EL 99.7% 44-50 Flucke (1984) (Coturnix japonica) Canary ? 99.7% 10-20 Grau (1984) (Serinus canarius) Groups of 10 starlings (Sturnus vulgaris) were given (by gavage) 0, 0.2, 1.5 or 2 mg/kg body weight of demeton-S-methyl (technical grade) dissolved in corn oil. Serum butyrylcholinesterase and carboxylesterase and brain cholinesterase activities were measured at 3 (high-dose only), 6 and 24 h after dosing (n=2 for brain, n=4 for serum). The highest dose inhibited brain cholinesterase by about 20%, serum carboxylesterase by about 50% and serum butyrylcholinesterase by about 80%. The peak effect was at 3 h with some recovery after 24 h. In vitro treatment of the enzyme preparation with pralidoxine chloride (2-PAM) caused almost complete reactivation of brain cholinesterase but had no effect on the other enzymes (Thompson et al., 1991). 9.2.4 Effects in field The activities of brain cholinesterase, serum butyrylcholinesterase and glutamate oxaloacetate transaminase (GOT) were measured in house sparrows (Passer domesticus) captured 1 (n=7), 2 (n=6) or 3 (n=6) days after spraying wheat fields with demeton-S-methyl (0.485 litres a.i./hectare from an emulsifiable concentrate formulation in a xylene solvent base). Liver weight was also recorded and histological examination of the liver was performed. Serum butyrylcholinesterase was reduced to 64% of that of controls (n=5) (statistically not significant) and brain cholinesterase was 82% of that of controls (statistically significant) for birds trapped on day 2, but serum GOT was not significantly affected in the exposed birds. The authors claimed that there was evidence of liver damage in exposed birds because of a significant increase of binucleation (3.3-5 vs. 1.9 binucleation/10 fields) and inflammatory cell foci (more evident after 3 days of exposure) (Tarrant et al., 1992). 10. EVALUATION OF HUMAN HEALTH RISKS AND EFFECTS ON THE ENVIRONMENT The major metabolites of demeton-S-methyl in plant and mammals are oxydemeton-methyl and demeton-S-methylsulfone. These two compounds are also commercial insecticides. Their mechanism of action is the same as that of demeton-S-methyl (i.e. inhibition of acetylcholinesterase) and their toxicological profiles have been reported to be similar to that of demeton-S-methyl (FAO/WHO, 1990). 10.1 Evaluation of human health risks Sources of exposure of humans to demeton-S-methyl are dietary and through dermal absorption and inhalation during the manufacture or use of the compound. There is very limited information on actual dietary exposure to demeton-S-methyl. In an early study, during application of a mixture of demeton-S- methyl (30%) and its isomer, demeton-O-methyl (70%), an ambient air concentration of 8.8-27 mg/m3 of the combined isomers was found. This is much higher than the conservative TLV/TWA for methyl-demeton proposed by ACGIH (0.5 mg/m3). However, no inhibition of plasma and erythrocyte cholinesterase activities was found after exposure. Cases of acute intoxication, which required pharmacological treatment, have been reported where workers were filling bottles with a 50% xylene solution of demeton-S-methyl. Skin absorption was considered the cause of poisoning because of inadequate personal protection. Agricultural workers exposed to demeton-S-methyl while mixing or spraying during three consecutive days did not show significant inhibition of either plasma or erythrocyte cholinesterase. Agricultural workers exposed to demeton-S-methyl while spraying cotton for 1-18 days (average of 3 days) showed signs and symptoms of poisoning. Improper working conditions could not be excluded. Cases of severe demeton-S-methyl poisoning (suicide attempts) have been reported. No delayed toxicity developed in the patients who survived. No data are available on the effect of repeated human exposure to demeton-S-methyl. No reproductive, developmental or carcinogenic effects have been found in laboratory animals exposed to demeton-S-methyl. NOAEL values have been based on inhibition of brain acetyl cholinesterase or, in a rat reproduction study, on reduced pup viability and body weight in the F1b generation. The available information is insufficient to permit an adequate assessment of the genotoxic potential of demeton-S-methyl. 10.2 Evaluation of effects on the environment The information on utilization and application rates that has been employed for this risk assessment is derived from the agricultural use of demeton-S-methyl within the European Union. It should be possible to extrapolate this assessment to other agricultural uses at similar application rates elsewhere in the world. The application rates for demeton-S-methyl can be summarised as follows: arable (tractor-mounted/drawn hydraulic spray boom applications), 0.32 kg/ha; top fruit (broadcast air-assisted applications), 0.49 kg/ha. The following risk assessment is based on the principle of calculating toxicity-exposure ratios (TERs) (Fig. 3), which follows the European and Mediterranean Plant Protection Organisation and Council of Europe (EPPO/CoE) Environmental Risk Assessment Scheme model and associated trigger values (EPPO/CoE, 1993a,b). 10.2.1 Aquatic organisms The main risk to aquatic organisms from the use of demeton-S- methyl is from spray drift during either arable applications (0.32 kg/ha) or top fruit air-assisted (0.49 kg/ha). For each of these risk scenarios, the predicted environmental concentration (PEC) in a 30-cm-deep static surface water body, arising from either arable-based spray drift at 1 m from the edge of the spray boom or from top fruit air-assisted spray drift at 3 m from the point of application (both based on Ganzelmeier et al., 1995), was calculated as follows: PEC (mg demeton-S-methyl/litre) = max application rate (kg/ha) × A (% spray drift) 300 where A = 5 for ground-based hydraulic spray applications 1 m from edge of boom or 30 for air-assisted application 3 m from point of application 10.2.1.1 Acute risk The acute LC/EC50 values for the most sensitive fish was 0.59 mg/litre, for the most sensitive aquatic invertebrate (mollusc) was 0.0042 mg/litre (0.022 mg/litre for Daphnia), and for the most sensitive algal species was 8 mg/litre.
(a) Spray drift from ground-based applications The acute PEC for spray drift (1 m from the edge of the spray boom into a 30-cm-deep static water body at the maximum application rate (see PEC assumptions above) is 0.005 mg/litre. Therefore, the TERs based on this PEC and the above LC/EC50 toxicity values are: fish, 110; aquatic invertebrates, 0.8 (4.1 for Daphnia); and algae, 1500. Based on the EPPO/CoE risk assessment scheme for aquatic organisms, these TERs indicate a low acute risk to fish and algae. However, for aquatic invertebrates the TER is less than 10, indicating a potential risk to aquatic invertebrates (both molluscs and arthropods). In such risk situations the use of a "no-spray" restriction zone next to surface waters may reduce the risk to such aquatic invertebrates. For example, arable spray drift at 5 m from the edge of boom is 0.6% (Ganzelmeier et. al., 1995). Based on this 5 m drift data, the PEC is 0.0006 mg/litre and results in a 5 m TER of 7 for molluscs and 37 for Daphnia. The TER for the most sensitive invertebrate (mollusc) is still below the EPPO trigger of 10, indicating a borderline risk. However, the Daphnia 5 m TER, which is above 10, indicates that the use of a 5 m "no-spray" restriction zone next to surface waters would help reduce the acute risk to aquatic invertebrates. (b) Spray drift from broadcast air-assisted top fruit applications The acute PEC for spray drift (3 m from the point of application into a 30-cm-deep static water body at the maximum application rate (see PEC assumptions above) is 0.049 mg/litre. Therefore, the TERs based on this PEC and the above LC/EC50 toxicity values are: fish, 12; aquatic invertebrates, 0.09 (0.4 for Daphnia); and algae, 163. Based on the EPPO/CoE risk assessment scheme for aquatic organisms, these TERs indicate a low acute risk to fish and algae. However, for aquatic invertebrates the TER is less than 1 indicating a high risk to aquatic invertebrates (both molluscs and arthropods). In such risk situations the use of a "no-spray" restriction zone next to surface waters may reduce the risk to such aquatic invertebrates. For example, spray drift from broadcast air-assisted applications to top fruit at 15 m from point of application is 6.0% (Ganzelmeier et al., 1995). Based on this 15 m drift data, the PEC is 0.006 mg/litre and results in a 15 m TER of 0.7 for molluscs and 3.4 for Daphnia. These TERs at 15 m for the most sensitive invertebrates are still below the EPPO trigger of 10, indicating a potential risk. Therefore, even with a 15 "no-spray" restriction zone next to surface waters, there is still a potential risk to aquatic invertebrates from the broadcast air-assisted use of demeton-S-methyl. Table 8 summarizes the acute TERs for demeton-S-methyl to aquatic organisms at 1 m for arable and 3 m for broadcast air-assisted applications. Table 8. Acute toxicity-exposure ratios (TER) for aquatic organisms at 1 m for arable and 3 m for broadcast air-assisted applications Species LC/EC50 PEC (mg/litre) at 1 m PEC (mg/litre) at 3 m TER TER (mg/litre) (arable) (air assisted) (arable) (air assisted) Fish 0.59 0.005 0.049 110 12 (Oncorhynchus mykiss) Aquatic invertebrate 0.0042 0.005 0.049 0.8 0.09 (Paphia laterisulca) Aquatic invertebrate 0.022 0.005 0.049 4.1 0.4 (Daphnia magna) Alga 8.0 0.005 0.049 1500 163 (Scenedesmus subspicatus) 10.2.1.2 Chronic risk No chronic toxicity data are available for fish. However, a chronic NOEC of 0.0056 mg/litre has been reported for Daphnia magna. There are no data reporting the persistence or degradation of demeton-S-methyl in water, and so a chronic PEC could not be derived. Therefore, owing to the lack of data on chronic fish toxicity and environmental fate in water, it is not possible to assess the chronic risk to either fish or aquatic invertebrates. There are also no data available on either the toxicity of demeton-S-methyl to sediment- dwelling invertebrates or fate of demeton-S-methyl in aquatic sediments. Therefore, the risk to sediment-dwelling invertebrates could also not be assessed. No fish bioaccumulation data are available, but, as the log Pow is 1.3 (i.e. < 3), the risk of bioaccumulation in fish should be low. 10.2.2 Terrestrial organisms Vertebrates are likely to be exposed to demeton-S-methyl from either grazing on treated vegetation or consuming contaminated insects. For this risk assessment, typical application rates of 0.32 kg/ha are used for ground spray application on arable crops and 0.49 kg/ha for application by air-assisted spraying for fruit. 10.2.2.1 Birds The lowest reported acute oral LD50 for birds is 10 mg/kg body weight for the canary. Dietary data are not available. Indicator birds for use in the risk assessment are: * Greylag goose (Anser anser), as a grazing species, with a body weight of 3 kg and total daily food consumption of 900 g vegetation (dry weight) (Owen, 1975) * Blue tit (Parus caeruleus), as an insectivorous species, with a body weight of 11 g and total daily food consumption of 8.23 g (dry weight) (Kenaga, 1973) a) Grazing birds Initial residues on short grass or cereal shoots are estimated to be 35.8 mg/kg dry weight (based on 112 × application rate) arising from application at 0.32 kg/ha to arable crops (EPPO/CoE, 1993a,b) and 55 mg/kg from application to fruit at a rate of 0.49 kg/ha. This gives an estimated total oral intake for the goose of 32.2 mg and 49.5 mg for the two application rates assuming that the goose ate exclusively food contaminated at this level. This is equivalent to a daily intake of 10.7 and 16.5 mg/kg body weight, respectively. TERs can be calculated as follows: End-point LD50/LC50 Application rate Predicted TER (mg/kg b.w.) (kg/ha) concentration in food (mg/kg) Bird acute 10 0.32 (arable) 35.8 0.93 oral (Canary) 0.49 (fruit) 55 0.61 The calculated TER values fall well below the EPPO/CoE trigger values for concern (TER <10) and indicate a high risk to grazing birds. b) Insectivorous birds Initial residues on small insects are estimated to be 9.3 mg/kg dry weight (based on 29 × application rate) arising from application at 0.32 kg/ha to arable crops (EPPO/CoE, 1993a,b) and 14.2 mg/kg from application to fruit at a rate of 0.49 kg/ha. This gives an estimated total oral intake for the blue tit of 0.08 mg and 0.12 mg for the two application rates assuming that the blue tit ate exclusively food contaminated at this level. This is equivalent to a daily intake of 6.94 and 10.6 mg/kg body weight, respectively. TERs can be calculated as follows: End-point LD50/LC50 Application rate Predicted TER (mg/kg b.w.) (kg/ha) concentration in food (mg/kg) Bird acute 10 0.32 (arable) 9.3 1.44 oral (Canary) 0.49 (fruit) 14.2 0.94 The TERs for acute toxicity to insectivorous birds are substantially less than the trigger value of <10, indicating high acute risk to these birds. 10.2.2.2 Mammals The lowest reported acute oral LD50 for laboratory mammals is 63 mg/kg body weight for the rat. Indicator mammals for use in the risk assessment will be: * Rabbit (Oryctolagus cuniculus), as a grazing mammal, with a body weight of 1200 g and a total daily food consumption of 500 g vegetation (dry weight) (Ross, personal communication to the IPCS). * Shrew (Sorex araneus), as an insectivorous mammal, with a body weight of 18 g and a total daily food consumption of 18 g (Churchfield, 1986) a) Grazing mammals Initial residues on short grass or cereal shoots are estimated to be 35.8 mg/kg dry weight (based on 112 × application rate) arising from application at 0.32 kg/ha to arable crops (EPPO/CoE, 1993a,b) and at 55 mg/kg from application to fruit at a rate of 0.49 kg/ha. This gives an estimated total oral intake for the rabbit of 17.9 mg and 27.4 mg for the two application rates, assuming that the rabbit ate exclusively food contaminated at this level. This is equivalent to a daily intake of 15 and 23 mg/kg body weight, respectively. TERs can be calculated as follows: End-point LD50 Application rate Predicted TER (test (mg/kg b.w.) (kg/ha) concentration species) in food (mg/kg) Acute oral 63 0.32 (arable) 17.9 4.20 (Rat) 0.49 (fruit) 27.5 2.76 Since the TERs are < 10, this indicates a high risk to grazing mammals. b) Insectivorous mammals Initial residues on large insects are estimated to be 0.86 mg/kg dry weight (based on 2.7 × application rate) arising from application at 0.32 kg/ha to arable crops (EPPO/CoE, 1993a,b) and 1.32 mg/kg from application to fruit at a rate of 0.49 kg/ha. This gives an estimated total oral intake for the shrew of 0.016 mg and 0.024 mg for the two application rates, assuming that the shrew ate exclusively food contaminated at this level. This is equivalent to a daily intake of 0.86 and 1.32 mg/kg body weight, respectively. TERs can be calculated as follows: End-point LD50 Application rate Predicted TER (mg/kg b.w.) (kg/ha) concentration in food (mg/kg) Mammal 63 (Rat) 0.32 (arable) 0.86 72.9 acute oral 0.49 (fruit) 1.32 47.6 These TERs fall outside the trigger for high risk for insectivorous mammals but within the range for medium risk. 10.2.2.3 Bees The reported contact toxicity to bees gives an LD50 of 0.26 µg/bee. Using application rates at 0.32 and 0.49 kg/ha for cereals and fruit respectively, hazard quotients are calculated to be 1231 and 1885 (application (g/ha)/LD50 (µg/bee)). The trigger for concern is >50 (EPPO/CoE, 1993a,b) and, therefore, there is substantial concern for exposed bees. The compound should not be applied to flowering plants, and exposure of flying bees should be avoided. 10.2.2.4 Earthworms Earthworms are likely to be exposed to demeton-S-methyl. Based on a soil depth of 5 cm and a soil density of 1.5 g/cm3, the soil PEC would be 0.43 mg/kg assuming even distribution in the medium. The reported LC50 for earthworms (Eisenia foetida) is 60 mg/kg soil giving a TER of 140.6. As this is above the trigger value of 10 (EPPO/CoE, 1993a,b), the acute risk to earthworms is very low. 11. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT Demeton-S-methyl causes acute cholinergic toxicity in humans and laboratory animals. Carcinogenic, reproductive and developmental effects have not been found in laboratory animals. Effects due to chronic human exposure are unlikely to occur. Exposure of the general population may only occur through food residues. However, the levels of exposure are unlikely to cause adverse effects. Given the high acute toxicity, as determined in a number of test species and the known cases of human poisoning, it can be concluded that the risk of occupational poisoning with demeton-S-methyl exists when adequate protective measures and good practices are not adopted. Therefore, handling and application of demeton-S-methyl should only be performed by supervised and trained workers who must adhere to good application practices and adopt adequate safety measures. Demeton-S-methyl does not persist in the environment and is not accumulated by organisms. It has high acute toxicity to aquatic invertebrates and is toxic to fish and birds, leading to high or moderate risk factors for these organisms. However, significant field kills of organisms have not been reported for the compound. Precautions should be taken to minimize exposure of non-target organisms (e.g., do not spray over water bodies, minimize exposure by spray drift). In view of the negative results obtained in developmental and carcinogenicity studies, it is felt that further clarification of the genotoxic potential of demeton-S-methyl is not necessary. 12. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES Demeton-S-methyl was classified as highly hazardous (class Ib) by WHO (1996). Demeton-S-methyl was evaluated by the Joint FAO/WHO Meeting on Pesticide Residues (JMPR) in 1972, 1973, 1979, 1982, 1984, 1989, 1992. The 1989 Meeting (FAO/WHO, 1990) evaluated demeton-S-methyl, oxydemeton-methyl and demeton-S-methylsulfone and set the following NOAEL levels: demeton-S-methyl: mouse: 1 mg/kg diet (equal to 0.24 mg/kg body weight per day) rat: 1 mg/kg diet (equal to 0.05 mg/kg body weight per day) dog: 1 mg/kg diet (equal to 0.036 mg/kg body weight per day) oxydemeton-methyl: mouse: 30 mg/kg diet (equal to 0.03 mg/kg body weight per day) rat: 0.57 mg/kg diet (equal to 0.03 mg/kg body weight per day) dog: 0.125 mg/kg body weight per day demeton-S-methylsulfone: rat: 1 mg/litre in drinking water (equal to 0.06 mg/kg body weight per day) dog: 10 mg/kg diet (equal to 0.36 mg/kg body weight per day) The estimated ADI for demeton-S-methyl was established at 0-0.0003 mg/kg body weight. This ADI applies to demeton-S-methyl alone or in combination with oxydemeton-methyl and/or demeton-S- methylsulfone because residues are determined after oxidation and are expressed as demeton-S-methyl. The 1992 JMPR reviewed the residue data on these three related compounds. Updated figures were available only for oxydemeton-methyl, which is replacing demeton-S-methyl in most registrations. The Meeting decided therefore to change the definition of the residues to "the sum of demeton-S-methyl, oxydemeton-methyl and demeton-S-methylsulfone expressed as oxydemeton-methyl". The MRLs varied from 0.05 to 1 mg/kg commodity and have been obtained from supervised trials on oxydemeton-methyl rather than on demeton-S-methyl. Demeton-S-methyl has not been evaluated by the International Agency for Research on Cancer (IARC). REFERENCES ACGIH (1993) 1993-1994 Threshold limit values for chemical substances and physical agents. 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Wilmes R (1984) Orientating light stability: demeton-S-methyl. Leverkusen, Germany, Bayer AG (Unpublished report No. IM1408). Winkler KO & Arent H (1970) Medical observation of personnel occupied with spraying in hop cultivation during the application of the organic phosphoric acid esters mevinphos and demeton-S-methyl (Metasystox i). Leverkusen, Germany, Bayer AG (Unpublished report No. 24618). Xue SZ, Ding XJ, & Ding Y (1985) Clinical observation and comparison of the effectiveness of several oxime cholinesterase reactivators. Scand J Work Environ Health, 11(Suppl 4): 46-48. Ziegler W, Engelhardt G, Wallnöfer PR, Oehlmann L, & Wagner K (1980) Degradation of demeton-S-methyl sulfoxide (Metasystox R) by soil microorganisms. J Agric Food Chem, 28: 1102-1106. RÉSUMÉ ET ÉVALUATION, CONCLUSIONS ET RECOMMANDATIONS 1. Résumé et évaluation 1.1 Identité, propriétés physiques et chimiques Le déméton-S-méthyl se présente sous la forme d'un liquide huileux jaune pâle à l'odeur pénétrante. C'est un organophosphoré agissant par la voie générale et par contact qui est utilisé comme insecticide et acaricide sur les fruits, les légumes, les céréales et les plantes ornementales pour lutter contre Acarina, Thysanoptera, Hymenoptera et Homoptera. Sa tension de vapeur est de 63,8 mPa à 20°C et il est facilement soluble dans la plupart des solvants organiques. Il est très soluble dans l'eau (3,3 g/litre à la température ambiante) et son coefficient de partage entre l'octanol et l'eau (log Pow) est de 1,32. Le déméton-S-méthyl est stable dans les solvants non aqueux. Pour la recherche et le dosage des résidus et l'analyse des échantillons prélevés dans l'environnement, on procède à une extraction au moyen d'un solvant organique, suivie d'une oxydation de la sulfone correspondante. Le dosage s'effectue ensuite par chromatographie en phase gazeuse avec un détecteur spécifique du phosphore. 1.2 Sources d'exposition humaine et environnementale Jusqu'en 1957, le déméton-méthyl a été commercialisé sous la forme d'un mélange de deux isomères, le déméton-S-méthyl et le déméton-O-méthyl. La formulation commerciale est un concentré émulsionnable que l'on utilise en pulvérisations sur les céréales, les fruits, les légumes et les plantes ornementales. On est en train de le remplacer par l'oxydéméton-méthyl qui est un métabolite produit par les plantes et les mammifères ou qui résulte du séjour du déméton-S-méthyl dans le sol. 1.3 Transport, distribution et transformation dans l'environnement La décomposition par hydrolyse du déméton-S-méthyl dépend du pH de la solution. A 22°C, sa demi-vie est de 56 jours à pH 7 et de 8 jours à pH 9. Dans le sol, sa voie de décomposition principale est la biodégradation, avec une demi-vie d'environ 4 h. Au bout de 24 h, il y a encore dans le sol une proportion d'oxydéméton-méthyl qui représente 20 à 30% de la dose initiale de déméton-S-méthyl. Le coefficient de sorption dans le sol (Kd) du déméton-S-méthyl est de 0,68 à 2,66, selon la composition du sol. Dans l'environnement, l'une des principales voies de dégradation du composé est la photolyse. La métabolisation par le blé de printemps est rapide et elle analogue à celle qui se produit dans le sol et chez les mammifères. 1.4 Niveaux d'exposition environnementale et humaine La population générale est principalement exposée au déméton-S-méthyl par l'intermédiaire des résidus qui subsistent sur les cultures vivrières. La réunion conjointe FAO/OMS sur les résidus de pesticides (JMPR) a recommandé une dose journalière admissible (DJA) de 0,0003 mg/kg de poids corporel. Il s'agit d'une DJA collective qui englobe le déméton-S-méthyl, l'oxydéméton-méthyl et la déméton-S-méthylsulfone, étant donné que les méthodes habituelles d'analyse ne distinguent pas ces différents composés. Il est arrivé qu'une exposition excessive au déméton-S-méthyl par la voie transcutanée entraîne une intoxication de type cholinergique chez des ouvriers mal protégés qui procédaient au conditionnement du concentré. En revanche, des volontaires qui s'étaient prêtés à l'épandage simulé d'un mélange de déméton-S-méthyl et de déméton-O-méthyl (30 et 70% respectivement) et avaient été exposés à des concentrations de 8,8 à 27 mg/m3 des deux matières actives, n'ont présenté aucune réduction de leur activité cholinestérasique plasmatique ou érythrocytaire. 1.5 Cinétique et métabolisme Chez le rat, le déméton-S-méthyl est rapidement et presque complètement résorbé au niveau de l'intestin et il se répartit uniformément dans les tissus (exception faite des hématies dans lesquelles il se concentre plus fortement). Il est rapidement métabolisé et excrété dans les urines. Sa concentration sanguine diminue avec une demi-vie qui est initialement de 2 h environ. Au bout de 24 h, il reste environ 1% de la dose dans l'organisme. La principale voie métabolique du déméton-S-méthyl consiste en une oxydation de la chaîne latérale qui conduit à la formation du sulfoxyde correspondant (oxydéméton-méthyl) et, dans une moindre proportion, après une oxydation plus poussée, à la sulfone. L'O-déméthylation constitue une autre voie métabolique importante. 1.6 Effets sur les animaux de laboratoire et les systèmes d'épreuve in vitro 1.6.1 Exposition unique Le déméton-S-méthyl provoque des intoxications de type cholinergique. Chez les mammifères, la DL50 varie de 7 à 100 mg/kg p.c. selon la voie d'administration et l'espèce. 1.6.2 Exposition de courte durée Une des premières études, comportant une exposition par la voie alimentaire, a montré que des rats qui recevaient le composé à raison de 50 mg/kg de nourriture, avaient une activité cholonestérasique cérébrale sensiblement réduite au bout de 26 semaines de ce régime. Des signes d'intoxication de type cholinergique ont été observés au bout de 5 semaines de traitement chez des rats recevant 200 mg de composé par kg de nourriture. Lors d'une étude d'alimentation d'un an effectuée sur des chiens, on a obtenu une dose sans effet nocif observable (NOAEL), basée sur les effets cholinergiques, de 1 mg/kg de nourriture, soit l'équivalent quotidien de 0,036 mg/kg de poids corporel. 1.6.3 Exposition de longue durée Des souris ont reçu pendant 21 mois des rations contenant respectivement 0, 1, 15 ou 75 mg de déméton-S-méthyl par kg de nourriture. La NOAEL, a été trouvée égale à 1 mg/kg de nourriture, soit l'équivalent quotidien de 0,24 mg/kg de poids corporel en prenant comme base l'inhibition de la cholinesterase cérébrale. Chez des rats ayant reçu une alimentation contenant respectivement 0, 1, 7 ou 50 mg/kg de déméton-S-méthyl, la NOAEL, basée sur l'inhibition de la cholinestérase cérébrale, s'est révélée égale à 1 mg/kg de nourriture, soit l'équivalent quotidien de 0,05 mg/kg p.c. Chez aucune espèce il n'a été constaté d'augmentation de l'incidence des tumeurs. 1.6.4 Irritation et sensibilisation cutanée ou oculaire Le déméton-S-méthyl est légèrement irritant pour l'oeil et la peau. Le test de Magnusson et Klingman a donné des résultats positifs chez des cobayes. En revanche, le test de sensibilisation de Buehler (application d'un timbre cutané) n'a pas fourni d'indice d'une sensibilisation cutanée, ce qui donne à penser que, dans la pratique, l'usage du déméton-S-méthyl ne devrait pas poser de problème de sensibilisation. 1.6.5 Effets sur la reproduction, embryotoxicité et tératogénicité Lors d'une étude sur deux générations de rats, au cours de laquelle les animaux ont été exposés au composé par la voie alimentaire, on a constaté une réduction de la viabilité et du poids corporel des ratons (uniquement la génération F1b) à la dose de 5 mg/kg de nourriture. La NOAEL était de 1 mg/kg de nourriture, soit l'équivalent quotidien de 0,07 mg/kg de poids corporel. Le déméton-S-méthyl ne s'est révélé ni embryotoxique, ni tératogène chez le rat et le lapin. 1.6.6 Mutagénicité et points apparentés d'aboutissement des effets toxiques Le déméton-S-méthyl produit des mutations ponctuelles in vitro. On n' a mis en évidence des effets chromosomiques in vivo qu'avec des formulations du commerce. Les données disponibles sont insuffisantes pour que l'on puisse procéder à une évaluation satisfaisante du pouvoir génotoxique de ce composé. 1.6.7 Neurotoxicité retardée Le déméton-S-méthyl n'a produit ni polyneuropathie, ni inhibition de l'estérase cible correspondante (NTE) lorsqu'on l'a administré à des poules en dose égale à la DL50 par voie orale. 1.6.8 Toxicité des métabolites Deux métabolites du déméton-S-méthyl produits par des plantes et des mammifères, à savoir l'oxydéméton-méthyl et la déméton-S-méthylsulfone, sont également des pesticides du commerce et ont été très largement étudiés. Il ressort de ces travaux que le profil toxicologique de ces composés ne diffère pas sensiblement, quantitativement ou qualitativement, de celui du déméton-S-méthyl. 1.7 Mécanisme de la toxicité - Mode d'action Le déméton-S-méthyl est un inhibiteur direct de la cholinestérase et sa toxicité est liée au fait qu'il inhibe l'acétylcholinestérase (AChE) au niveau des terminaisons nerveuses. Une fois inhibée par le déméton-S-méthyl, l'acétylcholinestérase se réactive spontanément avec une demi-vie in vitro de 1,3 h environ, comme on peut s'y attendre pour une AChE diméthylphosphorylée. 1.8 Effets sur l'homme On a signalé quelques cas d'intoxication aiguë avec syndrome cholinergique, consécutifs à des tentatives de suicide. Aucun effet retardé n'a été observé chez les survivants, dont une femme enceinte. A la suite d'une exposition subie par négligence lors du conditionnement de formulations du commerce, quelques employés ont présenté des troubles d'origine cholinergique qui ont nécessité un traitement pharmacologique. Le composé avait probablement pénétré par la voie transcutanée. De même, il est possible que des conditions de travail laissant à désirer aient entraîné une absorption excessive de déméton-S-méthyl au cours de l'épandage de ce composé sur des champs de coton. 1.9 Effets sur les autres êtres vivants au laboratoire et dans leur milieu naturel Pour les algues vertes, la CE50 à 96 h va de 8 à 37 mg/litre. On a mesuré une CL50 allant de 0,004 à 1,3 mg/litre chez une série d'invertébrés aquatiques. Chez les poisson, la toxicité est variable, avec une CL50 à 96 h qui peut aller de 0,59 mg/litre pour la truite arcen-ciel à environ 40 mg/litre pour l'orfe, le cyprin doré et la carpe. Chez la caille japonaise et le canari, on a obtenu une DL50 (effet aigu, voie orale) de 10-50 mg/kg de poids corporel. Chez des étourneaux, une dose unique par voie orale de 2 mg/kg p.c. a provoqué une inhibition de l'acétylcholinestérase cérébrale 3 h après le traitement. Pour les lombrics terricoles, la CL50 du déméton-S-méthyl est de 66 mg/kg sur 14 jours. Pour les abeilles, on a obtenu des valeurs de la DL50 de contact et par voie orale (effet aigu) respectivement égales à 0,21 et 0,6 par insecte. En traitant du blé d'hiver à la dose recommandée, on a constaté une réduction sensible du nombre d'invertébrés présents sur les feuilles (principalement des diptères du genre Empididae) sans effet sur les invertébrés entomophages présents à la surface du sol. 2. Conclusions Le déméton-S-méthyl est un insecticide organophosphoré fortement toxique (classe Ib de la classification de l'OMS) (OMS, 1966). Son action toxique s'explique par l'inhibition de l'acétylcholinestérase au niveau des terminaisons nerveuses. L'exposition de la population générale résulte principalement des résidus présents dans les denrées alimentaires provenant des cultures traitées. Moyennant de bonnes pratiques agricoles et le respect des règles d'hygiène et de sécurité, la manipulation du composé lors de sa fabrication ou de son épandage ne devrait entraîner aucun effet indésirable. Des effets résultant d'une exposition chronique sont improbables. Le déméton-S-méthyl ne persiste pas dans l'environnement et il ne s'accumule pas chez les êtres vivants. Il est très toxique pour les invertébrés aquatiques et présente une toxicité notable pour les poissons et les oiseaux, aussi doit on considérer que pour ces organismes, le facteur de risque est élevé ou modéré. Néanmoins, on n'a pas signalé de mortalité importante dans la nature du fait de ce composé. Des précautions sont à prendre pour réduire au minimum l'exposition des organismes non visés (par ex. ne pas le pulvériser sur des étendues d'eau et veiller à ce que les gouttelettes d'insecticide ne soient pas entraînées vers les zones à respecter). 3. Recommandations Afin de protéger la santé et le bien-être des travailleurs et de la population dans son ensemble, il ne faut confier la manipulation et l'épandage du déméton-S-méthyl qu'à des opérateurs dûment formés et encadrés, qui auront le souci de respecter les mesures de sécurité et d'épandre le produit selon les règles. RESUMEN Y EVALUACION, CONCLUSIONES Y RECOMENDACIONES 1. Resumen y evaluación 1.1 Identidad, propiedades físicas y químicas, y métodos analíticos El demeton-S-metilo, un líquido oleoso de color amarillo pálido y olor penetrante, es un insecticida y acaricida organofosfatado sistémico y de contacto que se utiliza para combatir Acarina, Thysanoptera y Homóptera en frutas, cereales, plantas ornamentales y hortalizas. Tiene una presión de vapor de 63,8 mPa a 20°C, es fácilmente soluble en la mayoría de los disolventes orgánicos, tiene una gran solubilidad en agua, de 3,3 g/litro a temperatura ambiente, y un coeficiente de reparto octanol/agua (log Pow) de 1,32. El demeton-S-metilo es estable en disolventes no acuosos. Los análisis de residuos y ambientales se realizan por extracción con un disolvente orgánico y oxidación a la sulfona correspondiente. Las mediciones se efectúan por cromatografía de gases utilizando un detector específico del fósforo. 1.2 Fuentes de exposición humana y ambiental Antes de 1957, el metildemeton se comercializaba como mezcla de los isómeros demeton-S-metilo y demeton-O-metilo. El demeton-S-metilo se utiliza desde 1957. Su presentación es la de un concentrado emulsionable y se aplica por rociamiento a los cereales, frutas, plantas ornamentales y hortalizas. Se está sustituyendo por el oxidemetonmetilo, que es un metabolito del demeton-S-metilo producido en las plantas, el suelo y los mamíferos. 1.3 Transporte, distribución y transformación en el medio ambiente La degradación hidrolítica del demeton-S-metilo depende del pH de la solución; a 22°C la semivida es de 63 días con un pH de 4, de 56 días con un pH de 7 y de 8 días con un pH de 9. La principal vía de degradación en el suelo es la biodegradación. La semivida del demeton-S-metilo en el suelo es de unas 4 horas. Sin embargo, al cabo de 24 horas, el oxidemetonmetilo representa aún el 20-30 % de la dosis aplicada de demeton-S-metilo. El coeficiente de sorción (Kd) del demeton-S-metilo en el suelo oscila entre 0,68 y 2,66, dependiendo de la composición del suelo. La fotolisis no es uno de los principales mecanismos de degradación del demeton-S-metilo en el medio ambiente. Su metabolismo en el trigo de primavera es rápido y similar al que se produce en el suelo y en los mamíferos. 1.4 Niveles ambientales y exposición humana La exposición primaria de la población humana en general se produce a través de los residuos de demeton-S-metilo en los cultivos alimentarios. En la reunión conjunta FAO/OMS sobre residuos de plaguicidas (JMPR) se recomendó una ingesta diaria admisible (IDA) de 0,0003 mg/kg de peso corporal. Se trata de una IDA colectiva para el demeton-S-metilo, el oxidemetonmetilo y el demeton-S-metilsulfona, ya que los métodos habituales de análisis no diferencian entre estos tres compuestos. La exposición excesiva al demeton-S-metilo y su absorción cutánea produjeron toxicidad colinérgica en trabajadores insuficientemente protegidos durante el envasado de la formulación concentrada. Tras haber expuesto a voluntarios participantes en una simulación de rociamiento con una mezcla de demeton-S-metilo y demeton-O-metilo (30 y 70% respectivamente) a 8,8-27 mg/m3 de ambos componentes activos combinados, no se observaron efectos adversos en la actividad de la colinesterasa en el plasma ni en los eritrocitos. 1.5 Cinética y metabolismo El demeton-S-metilo se absorbe rápida y casi completamente por el tracto intestinal en las ratas y se distribuye uniformemente a los tejidos (excepto una concentración elevada en los eritrocitos). Se metaboliza rápidamente y se excreta por orina. La concentración en la sangre disminuye, con una semivida inicial de unas 2 horas. Aproximadamente el 1% de la dosis oral sigue presente en el organismo 24 horas después del tratamiento. La principal ruta metabólica del demeton-S-metilo en las ratas es la oxidación de la cadena lateral, que da lugar a la formación del sulfóxido correspondiente (oxidemetonmetilo) y, en menor medida, a la sulfona, tras una oxidación mayor. Otra ruta metabólica importante es la O-demetilación. 1.6 Efectos en mamíferos de laboratorio y en sistemas de ensayo in vitro 1.6.1 Exposición única El demeton-S-metilo produce toxicidad colinérgica. Los valores de la DL50 para los mamíferos oscilan entre 7 y 100 mg/kg de peso corporal, dependiendo de la vía de administración y de la especie. 1.6.2 Exposición breve Un estudio alimentario inicial reveló que las ratas alimentadas con 50 mg de demeton-S-metilo por kg de la dieta habían sufrido una reducción notable de la actividad de la colinesterasa cerebral y eritrocitaria tras una exposición de 26 semanas. Se observaron síntomas colinérgicos en ratas alimentadas con 200 mg/kg de la dieta durante las 5 primeras semanas de exposición. En un estudio alimentario de un año de duración efectuado en perros se estableció un nivel sin efectos adversos observados (NOAEL) de 1 mg/kg de la dieta (equivalente a 0,036 mg/kg de peso corporal por día) sobre la base de los efectos de la colinesterasa cerebral. 1.6.3 Exposición prolongada Se administró durante 21 meses a ratones una dieta que contenía 0, 1, 15 ó 75 mg/kg de demeton-S-metilo. El NOAEL, determinado sobre la base de la inhibición de la colinesterasa cerebral, fue de 1 mg/kg de la dieta (equivalente a 0,24 mg/kg de peso corporal por día). En ratas cuya dieta contenía 0, 1, 7 ó 50 mg/kg de demeton-S-metilo, el NOAEL, determinado sobre la bases de la inhibición de la colinesterasa cerebral, fue de 1 mg/kg de la dieta (equivalente a 0,05 mg/kg de peso corporal por día). La incidencia de tumores no aumentó en ninguna de esas especies. 1.6.4 Irritación y sensibilización de la piel y los ojos El demeton-S-metilo es un irritante cutáneo y ocular leve. La prueba de maximización de Magnusson y Klingman dio resultados positivos en cobayos. Sin embargo, la prueba del parche cutáneo de Buehler no mostró indicios de sensibilización cutánea, lo que parece indicar que el uso práctico del demeton-S-metilo no debería acarrear problemas de sensibilización. 1.6.5 Reproducción, embriotoxicidad y teratogenicidad En un estudio alimentario llevado a cabo en dos generaciones de ratas, se observó una reducción de la viabilidad y del peso corporal de los cachorros (generación F1b solamente) con una dosis de 5 mg de demeton-S-metilo por kg de la dieta. El NOAEL fue de 1 mg/kg de la dieta, equivalente a 0,07 mg/kg de peso corporal por día. El demeton-S-metilo no resultó embriotóxico ni teratogénico en ratas ni en conejos. 1.6.6 Mutagenicidad y parámetros conexos El demeton-S-metilo produce mutaciones puntuales in vitro. Se han demostrado efectos cromosómicos in vivo con formulaciones comerciales solamente. La información disponible es insuficiente para realizar una evaluación adecuada del potencial genotóxico del demeton-S-metilo. 1.6.7 Neurotoxicidad retardada El demeton-S-metilo no causó polineuropatías retardadas ni inhibición de la Esterasa Diana de Neuropatía en pruebas con gallinas a un nivel igual a la DL50 oral. 1.6.8 Toxicidad de los metabolitos Las plantas y los mamíferos producen dos metabolitos del demeton-S-metilo (el oxidemetonmetilo y el demeton-S-metilsulfona) que también son plaguicidas comerciales y han sido ampliamente estudiados. Se ha señalado de que el perfil toxicológico de estos dos compuestos no difiere de manera significativa, cuantitativa ni cualitativamente, del demeton-S-metilo. 1.7 Mecanismo de toxicidad: modalidad de acción El demeton-S-metilo es un inhibidor indirecto de la colinesterasa y su toxicidad que está relacionada con la inhibición de la acetilcolinesterasa en las terminales nerviosas. La acetilcolinesterasa inhibida por el demeton-S-metilo se reactiva espontáneamente, con una semivida in vitro de aproximadamente 1,3 horas, como cabe esperar en la acetilcolinesterasa dimetil fosforilada. 1.8 Efectos en el ser humano Se han notificado varios casos de intoxicación aguda con síndrome colinérgico tras intentos de suicidio. Los pacientes que sobrevivieron, inclusive una mujer embarazada, no presentaron efectos retardados. Tras una exposición laboral por inatención durante el envasado de la formulación comercial, algunos trabajadores desarrollaron una toxicidad colinérgica que precisó tratamiento farmacológico. El demeton-S-metilo se absorbió probablemente a través de la piel. Análogamente, condiciones de trabajo inapropiadas podrían haber sido la causa de una absorción cutánea excesiva durante la aplicación del demeton-S-metilo en algodonales. 1.9 Efectos en otros organismos en el laboratorio y sobre el terreno Las CE50 96-h para las algas verdes se sitúan entre 8 y 37 mg/litro. Las CL50 para diversos invertebrados acuáticos se encuentran entre 0,004 y 1,3 mg/litro. La toxicidad para los peces varía; la CL50 96-h oscila entre 0,59 mg/litro para la trucha arcoiris y unos 40 mg/litro para Leuciscus idus, Carassius auratus y la carpa. La DL50 oral aguda para la codorniz japonesa y el canario es de 10-50 mg/kg de peso corporal. En el estornino, una dosis oral única de 2 mg/kg de peso corporal produjo inhibición del 20% de la AChE cerebral 3 horas después del tratamiento. La CL50 del demeton-S-metilo en el suelo para las lombrices de tierra es de 66 mg/kg para un período de 14 días. La DL50 aguda oral y de contacto del demeton-S-metilo fue de 0,21 y 0,6 g/abeja respectivamente. Después de haber sido aplicado al trigo de invierno según la proporción propuesta, el demeton-S-metilo redujo significativamente el número de invertebrados en el follaje de los cultivos (principalmente Empididae), pero no el número de invertebrados entomófagos de la superficie del suelo. 2. Conclusiones El demeton-S-metilo es un éster organofósforado sumamente tóxico (pertenece a la clase Ib de la clasificación de la OMS) (OMS, 1996) utilizado como insecticida. La acción tóxica consiste en la inhibición de la acetilcolinesterasa en las terminales nerviosas. La exposición de la población en general se produce principalmente a causa de los residuos presentes en los productos agrícolas. Con buenas prácticas laborales y medidas de higiene y de seguridad, el demeton-S-metilo no debería producir efectos adversos durante la fabricación o la aplicación. Es poco probable que aparezcan efectos debidos a la exposición crónica. El demeton-S-metilo no es persistente en el medio ambiente y no se acumula en los organismos. Tiene una toxicidad aguda elevada para los invertebrados acuáticos y es tóxico para los peces y las aves, llevando aparejados factores de riesgo agudo o moderado elevados para esos organismos. Sin embargo, no se han comunicado muertes masivas significativas de organismos en el campo a causa de este compuesto. Habría que tomar precauciones para reducir al mínimo la exposición de organismos no combatidos (por ejemplo, no rociar sobre masas de agua y reducir al mínimo la exposición por desviación del rociado). 3. Recomendaciones Para la salud y el bienestar de los trabajadores y del público en general, el manejo y la aplicación del demeton-S-metilo deberían encomendarse exclusivamente a personal bien adiestrado y supervisado que aplique las medidas de seguridad necesarias y buenas prácticas de aplicación.
See Also: Toxicological Abbreviations Demeton-s-methyl (ICSC) Demeton-s-methyl (Pesticide residues in food: 1979 evaluations) Demeton-S-methyl (Pesticide residues in food: 1984 evaluations)