Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Dimethoate
Table of Contents
The maximum acceptable concentration (MAC) for dimethoate in drinking water is 0.02 mg/L (20 µg/L).
Identity, Use and Sources in the Environment
Dimethoate released to the environment does not adsorb onto the soil and is subject to considerable leaching. It is also lost from the soil through evaporation and biodegradation. The half-life of dimethoate in soil ranges from four to 16 days.Footnote 4 It is relatively stable in aqueous media at pH 2 to 7.Footnote 5 Reported half-lives for dimethoate in raw river water range from 18 hours to eight weeks.,Footnote 4 Dimethoate is degraded in the environment to another more toxic pesticide, omethoate; the proportion of omethoate in the total residue reaches about 50% after five weeks.Footnote 6
Dimethoate was not detected in 98 samples from municipal and private drinking water supplies in Nova Scotia, Quebec, Metropolitan Toronto and Manitoba surveyed from 1971 to 1986 (reported detection limits 0.2 and 0.6 µg/L).Footnote 7 It was detected at trace levels in a private well in Nova Scotia (detection limit 0.01 µg/L).Footnote 8 It was not detected in a survey of surface water at 38 sites in the Prairies from 1973 to 1974 (detection limit not reported),,Footnote 7 nor was it found in 11 agricultural watersheds in southern Ontario from 1975 to 1977 (detection limit 0.5 µg/L)Footnote 9 or in three Ontario river basins from 1981 to 1985 (detection limit 0.5 µg/L).Footnote 10
Analytical Methods and Treatment Technology
No information on the effectiveness of current treatment technologies in removing dimethoate from drinking water was identified.
Dimethoate is readily absorbed from the gastrointestinal tract; 76 to 100% of an orally administered dose (quantity unspecified) was excreted in the urine of human volunteers within 24 hours.Footnote 14 In rats, about 60% of an administered dose (quantity unspecified) was excreted in the urine and expired air in 24 hours.Footnote 15 The principal metabolite of dimethoate is the thiocarboxy derivative.Footnote 16 Based on comparisons of the rate of degradation in liver specimens from humans and other species and acute toxicity data in animals, the oral LD50 for humans is predicted to be about 30 mg/kg.,Footnote 16
Dimethoate is a cholinesterase inhibitor. Thirty-six male and female human volunteers were administered oral doses of dimethoate of 5, 15, 30, 45 and 60 mg/d for periods of 14 to 57 days. There were no significant effects on cholinesterase levels in 12 persons ingesting 5 mg/d for 28 days or in nine persons ingesting 15 mg/d for 39 days. There was a decrease in whole blood cholinesterase activity by day 20 in the eight volunteers receiving 30 mg/d, which persisted until the end of the study. This depression occurred earlier and to a somewhat greater degree in the higher dose groups. The no-observed-adverse-effect level (NOAEL) was considered to be 15 mg/d, or 0.2 mg/kg bw per day.Footnote 17 In other studies in humans, no inhibition of cholinesterase activity was observed in two adults ingesting 9 and 18 mg/d for 21 days or in 20 adults ingesting 2.5 mg/d for four weeks.,Footnote 14
The NOAEL for erythrocyte and plasma cholinesterase inhibition of dimethoate administered daily in the diet of dogs was 10 and 50 ppm, respectively, or 0.2 and 1.0 mg/kg bw per day.Footnote 18 In rats, the reported NOAEL for cholinesterase inhibition has been found to range from 1 ppm (about 0.05 mg/kg bw per day) to 32 ppm dimethoate in the diet.,Footnote 2
Dimethoate was mutagenic in a number of in vivo and in vitro short-term tests.,Footnote 6 In mice drinking water containing 60 ppm (9.5 to 10.5 mg/kg bw per day) dimethoate, there were adverse reproductive effects on mating success, survival of pups and growth of surviving pups; however, no teratogenic effects were observed.Footnote 22 Effects on foetal development were observed in cats and rats exposed to 12 mg/kg bw per day but not at 3 or 6 mg/kg bw per day in either species.Footnote 23,Footnote 24
In 1984, the Food and Agriculture Organization (FAO) and the World Health Organization (WHO),Footnote 6 withdrew their estimate of an acceptable daily intake (ADI) for dimethoate because of the incomplete data base. Until further experimental studies are completed, a temporary ADI has been established by WHO,Footnote 6 as follows:
- 0.2 mg/kg bw per day is considered to be the NOAEL from studies in human volunteers,Footnote 17
- 100 is the uncertainty factor.
Based on the above ADI, the maximum acceptable concentration (MAC) for dimethoate in drinking water is derived as follows:
- 0.002 mg/kg bw per day is the temporary ADI established by the FAO/WHO
- 70 kg bw is the average body weight of an adult
- 0.20 is the proportion of daily intake of dimethoate allocated to drinking water
- 1.5 L/d is the average daily consumption of drinking water by an adult.
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