Methomyl
(endorsed 2011)
Guideline
Based on human health concerns, methomyl in drinking water should not exceed 0.02 mg/L.
Related chemicals
Methomyl (CAS 16752-77-5) belongs to the carbamate class of insecticides. There are many other pesticides in this class, including carbaryl, methiocarb, and oxamyl (Tomlin 2006).
Human risk statement
With good water quality management practices, the exposure of the general population is expected to be well below levels that may cause health concerns.
If present in drinking water as a result of a spillage or through misuse, methomyl would not be a health concern unless the concentration exceeded 0.02 mg/L. Excursions above this level even for a relatively short period are of concern as the health-based guideline is based on short-term effects.
With good water quality management practices, pesticides should not be detected in source waters used for drinking water supplies. Persistent detection of pesticides may indicate inappropriate use or accidental spillage, and investigation is required in line with established procedures in the risk management plan for the particular water source.
General description
Uses: Methomyl is an insecticide for the control of fruit flies in agricultural settings, and domestic flies in home garden, public and industrial settings.
There are registered products that contain methomyl in Australia. The products are intended for professional and home garden use. They are applied by aerial and ground boom spray on agricultural crops and as baits in domestic and commercial buildings. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main sources of public exposure to methomyl are the use of home garden products and residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of methomyl may potentially lead to contamination of source waters through processes such as run-off, spray drift or entry into groundwater.
Typical values in Australian drinking water
No occurrence data for methomyl in Australian waters could be found. In the USA, the maximum predicted concentrations for methomyl are 20 and 30 µg/L in groundwater and surface water, respectively (USEPA 1998).
Treatment of drinking water
Oxidation has been reported to be an effective method of removing methomyl. Both ozone and chlorination (using chlorine gas) were successful treatment methods for methomyl removal; however, harmful by-products are formed during oxidation processes (Mason et al. 1990).
Activated carbon technologies are limited in their success in the removal of methomyl, and very large doses of powdered activated carbon are required to achieve any significant removal (Hu et al. 1998).
Measurement
The practical limit of quantification for methomyl in water is 1 µg/L by liquid chromatography with tandem mass spectrometry (Alder et al. 2006).
History of the health values
The current acceptable daily intake (ADI) for methomyl is 0.01 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 1.25 mg/kg bw/day from a medium-term (3-month) dietary study in rats. The NOEL is based on decreased cholinesterase activity. The ADI incorporates a safety factor of 100, and was first established in 1991.
The previous health value was 0.03 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Methomyl is readily absorbed via the gastrointestinal tract in rats, and is extensively metabolised. The major metabolic pathway is via isomerisation to the E-isomer, then hydrolysis followed by rearrangement to form acetonitrile. The metabolites are eliminated in the urine almost completely within 24 hours as sulfate conjugates.
Acute effects: Methomyl has high acute oral toxicity and moderate acute dermal toxicity. It is not a skin sensitiser.
Short-term effects: In 28-day dietary studies in mice and rats, there was decreased cholinesterase activity at 10 mg/kg bw/day and above. No other effects were observed. Medium-term (3-month) dietary studies reported decreased cholinesterase activity at 12 mg/kg bw/day in mice and decreased bodyweight gain, decreased kidney weight and adverse effects in the spleen in rats. A medium-term study in dogs was not considered acceptable for regulatory purposes. The lowest overall NOEL was 1.25 mg/kg bw/day in the medium-term study in rats. This NOEL is the basis for the current ADI.
Long-term effects: Long-term dietary studies were conducted in mice, rats and dogs. In mice, decreased cholinesterase activity was reported at 13 mg/kg bw/day. In rats, decreased bodyweight gain and adverse effects on the spleen were reported at 10 mg/kg bw/day. In dogs, splenic haematopoiesis, and kidney swelling and pigmentation were reported at 10 mg/kg bw/day.
Carcinogenicity: Based on 2-year studies in mice and rats, there is no evidence of carcinogenicity for methomyl.
Genotoxicity: Methomyl is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: Reproduction studies in rodents reported decreased litter size and increased still births in first generation pups at the lowest dose, 3.75 mg/kg bw/day. Developmental studies in rodents reported increased embryo deaths at 1 mg/kg bw/day. The potential effects of methomyl on reproduction and development are the subject of a current review.
Poisons Schedule: Methomyl is included in Schedule 6 and 7 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010), depending on its concentration and use. Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.02 mg/L for methomyl was determined as follows:
where:
1.25 mg/kg bw/day is the NOEL based on a medium-term (3-month) dietary study in rats.
70 kg is taken as the average weight of an adult.
0.1 is a proportionality factor based on the assumption that 10% of the ADI will arise from the consumption of drinking water.
2 L/day is the estimated maximum amount of water consumed by an adult.
200 is the safety factor applied to the NOEL derived from animal studies. This safety factor incorporates a factor of 10 for interspecies extrapolation and 10 for intraspecies variation., with an additional safety factor of 2 to account for uncertainty in the ADI (which may change as a result of the current review).
The World Health Organization has not established a health-based guideline value for methomyl and it is excluded from the list of agricultural chemicals guideline value derivation because it is “unlikely to occur in drinking water” (WHO 2004).
References
NOTE: The toxicological information used in developing this fact sheet is from reports and data held by the Department of Health, Office of Chemical Safety.
Alder L, Greulich K, Kempe G, Vieth B (2006). Residue analysis of 500 high priority pesticides: better by GC-MS or LC-MS/MS? Mass Spectrometry Reviews, 25(6):838-65.
DoHA (2010) The Poisons Standard; Schedule 1-Standard for the Uniform Scheduling of Medicines and Poisons, Department of Health and Ageing, Commonwealth of Australia, Canberra.
Hu J-Y, Aizawa T, Asami M, Magara Y (1998). Analysis of thermolabile and polar pesticides in environmental water with capillary chromatography-frit-fast atom bombardment mass spectrometry. Water Science and Technology, 38(7):169-177.
Mason Y, Choshen E, Rav-Acha C (1990). Carbamate insecticides: Removal from water by chlorination and ozonation. Water Research, 24(1):11-21.
NHMRC (National Health and Medical Research Council), NRMMC (Natural Resources Management Ministerial Council) (2004). Australian Drinking Water Guidelines. National Water Quality Management Strategy, Paper 6. NHMRC and NRMMC.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
USEPA (1998) Reregistration Eligibility decision (RED) Methomyl Prevention, Pescticides and Toxic Substances (7508C), United States Environment Protection Agency, EPA 738-R-98-021, December 1998
WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
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