Methiocarb
(endorsed 2011)
Guideline
Based on human health concerns, methiocarb in drinking water should not exceed 0.007 mg/L.
Related chemicals
Methiocarb (CAS 2032-65-7) is in the carbamate class of chemicals. Other pesticides in this class include aldicarb, carbaryl, methomyl and pirimicarb (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, methiocarb would not be a health concern unless the concentration exceeded 0.007 mg/L. Minor excursions above this level would need to occur over a significant period to be a health concern, as the health-based guideline is based on long-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: Methiocarb is an insecticide, acaricide and molluscicide used for the control of slugs and snails, mites, thrips, aphids, leaf-hoppers, fruit flies and soil insects.
There are registered products containing methiocarb in Australia. The products are for both professional and home garden use and include granular and soluble powder formulations for use in sprays. Use patterns include spreading pellets onto soil by hand or by sod-seeding machines in the home garden, and spraying wettable powder formulations onto soil by boom spray, airblast, or hand sprays in agriculture. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main sources of public exposure to methiocarb are home garden use and residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use may potentially lead to contamination of sources waters through processes such as run-off, spray drift or entry into groundwater.
Typical values in Australian drinking water
Data on levels of methiocarb in Australian drinking waters are not available. Given the environmental properties of methiocarb, which is highly water insoluble, it is likely that levels in drinking water would be very low and associated with particulate matter and sediments.
Treatment of drinking water
Advanced treatment methods such as ozonation, ozone/biologically activated carbon and advanced oxidation are effective against this type of compound.
Measurement
Measurement at residue levels in water are by high performance liquid chromatography (HPLC) with post-column derivitisation and HPLC with tandem mass spectrometry.
History of the health values
The current acceptable daily intake (ADI) for methiocarb is 0.002 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.2 mg/kg bw/day from a 2-year dietary study in dogs. The NOEL is based on reduced plasma cholinesterase activity. The ADI incorporates a safety factor of 100 and was established in 1986.
The ADI for methiocarb was first set in 1981 at 0.06 mg/kg bw/day, based on a NOEL of 6.25 mg/kg bw/day from a long-term dietary study in dogs, before being revised in 1983 to 0.001 based on a reassessment of data on decreased plasma acetylcholinesterase activity. In 1986, the ADI was revised to its current value on the basis that estimates of food consumption were replaced with actual data.
The acute reference dose (ARfD) of 0.03 mg/kg bw/day for methiocarb was established in 2001, based on NOELs of 3 mg/kg bw/day from developmental toxicity studies in rats and rabbits. The ARfD incorporates a safety factor of 100.
The previous health value was 0.005 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Methiocarb is rapidly absorbed from the gastrointestinal tract, and distributed broadly. It is extensively metabolised and eliminated within 72 hours, mainly in the urine. Methiocarb has a low potential for bioaccumulation. The primary metabolites are methiocarb phenol, methiocarb sulfoxide phenol and methiocarb phenol sulfone.
Acute effects: Methiocarb has a high acute oral and low acute dermal toxicity. Symptoms of acute poisoning include salivation, lacrimation, vomiting, diarrhoea, muscular tremors, restlessness, convulsions, and paralysis. Methiocarb is not a skin sensitiser.
Short-term effects: In short-term dietary studies in rats, there are clinical symptoms indicative of central nervous system toxicity. Plasma, red blood cell and brain acetylcholinesterase activity was decreased at a dose of 10 mg/kg bw/day.
Long-term effects: Long-term dietary studies in rat, mouse and dog also showed effects on the central nervous system with decreased acetylcholinesterase activity in brain and other tissues, together with decreases in physical activity and grooming behaviours at dose levels above 0.2 mg/kg bw/day in dogs. This NOEL is the basis for the ADI.
Carcinogenicity: Based on 2-year studies in mice and rats, there is no evidence of carcinogenicity for methiocarb.
Genotoxicity: Methiocarb is not considered to be genotoxic, based on short-term in vitro and in vivo studies.
Reproductive and developmental effects: A 3-generation reproduction study in rats and developmental studies in rats and rabbits showed no evidence of effects on reproductive parameters or foetal development.
Poisons Schedule: Methiocarb is included in Schedule 5, 6 or 7 in 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 0.007 mg/L for methiocarb was determined as follows:
where:
0.2 mg/kg bw/day is the NOEL based on a long-term (2-year) study in dogs.
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.
100 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.
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.
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.
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.
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