Azinphos-methyl
(endorsed 2011)
Guideline
Based on human health concerns, azinphos-methyl in drinking water should not exceed 0.03 mg/L.
Related chemicals
Azinphos-methyl (CAS 86-50-0) belongs to the organophosphate class of chemicals and is structurally related to azinphos-ethyl. There are many pesticides in this chemical class, including acephate, chlorfenvinphos, diazinon, ethion, phorate, and terbufos (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, azinphos-methyl would not be a health concern unless the concentration exceeded 0.03 mg/L. Excursions above this level even for a limited 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: Azinphos-methyl is an insecticide, acaricide (miticide), and molluscicide for the control of pests such as moths, grasshoppers, beetles, aphids, and slugs in agricultural crops.
There are registered products that contain azinphos-methyl in Australia. These products are intended for professional use on pome fruit, citrus fruit, macadamia nuts, grape, lychee, and blueberry crops. They are available as concentrated solutions to be applied in both concentrated and diluted forms by ground and aerial sprays. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to azinphos-methyl and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of azinphos-methyl 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 data are available on occurrence of azinphos-methyl in Australian waters. In the USA, the estimated environmental concentration in surface waters was 0.016 mg/L (USEPA 2006). Azinphos-methyl was not found in a survey of Canadian drinking water (Health Canada 1989).
Treatment of drinking water
No specific data on the treatment of azinphos-methyl in drinking water have been identified.
Measurement
Azinphos-methyl can be measured by routine gas chromatrography–mass spectrometry analysis, with a limit of reporting of 0.1 µg/L (Queensland Health 2007).
History of the health values
The current acceptable daily intake (ADI) for azinphos-methyl is 0.025 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.25 mg/kg bw/day from a short-term (28-day) oral study in humans. The NOEL is based on the absence of cholinesterase inhibition at this dose, which was the only dose tested. The ADI incorporates a safety factor of 10, and was established in 2002.
The previous ADI was 0.001 mg/kg bw, based on a NOEL of 0.125 mg/kg bw/day for cholinesterase inhibition in a 2-year dog study, with a 100-fold safety factor. The ADI was amended following a review in 2002 of all available data on azinphos-methyl.
The acute reference dose (ARfD) of 0.075 mg/kg bw for azinphos-methyl was also established in 2002, based on the absence of cholinesterase inhibition and clinical signs of toxicity at the highest dose tested of 0.75 mg/kg bw/day in a single oral dosing study in humans. The ARfD incorporates a safety factor of 10.
The previous health value was 0.003 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Azinphos-methyl is readily absorbed via the gastrointestinal tract in rats. It is extensively metabolised, and is rapidly excreted in the urine and faeces, almost completely within 48 hours. The major metabolite was benzamide.
Acute effects: Azinphos-methyl has high acute oral toxicity in rats and mice, and high dermal toxicity in rats. It is a skin sensitiser in guinea-pig. Clinical signs of acute poisoning were typical of cholinesterase inhibition and included hyperexcitability, salivation, bronchoconstriction, headache, vomiting and other behavioural changes.
Short-term effects: A 28-day dietary studies in rats reported dose-dependent red blood cell and plasma cholinesterase inhibition at 0.8 mg/kg bw/day and above, and brain cholinesterase inhibition at 3.2 mg/kg bw/day. A 28-day oral dosing study with humans reported no toxic effects at 0.25 mg/kg bw/day. This NOEL is the basis for the current ADI.
In 12-16 week dietary studies in rats and dogs, cholinesterase in brain, erythrocytes, and plasma was inhibited at 1 mg/kg bw/day in rats. No changes were noted histologically. Deaths were reported at doses of 2.5 mg/kg bw/day and above. In dogs, red blood cell cholinesterase was inhibited at 0.25 mg/kg bw/day.
Long-term effects: In 2-year dietary studies in mice, rats, and dogs, erythrocyte and plasma cholinesterase were inhibited at doses of 0.5 mg/kg bw/day in dogs and brain cholinesterase was inhibited at 3 mg/kg bw/day and above. Clinical signs of toxicity were noted at 6.25 mg/kg bw/day and above.
Carcinogenicity: Based on a 2-year study in mice and rats, there is no evidence of carcinogenicity for azinphos-methyl.
Genotoxicity: Azinphos-methyl was positive in some in vitro short-term studies, but not in in vivo studies. Overall, it was not considered to be genotoxic.
Reproductive and developmental effects: Three-generation reproduction studies in rats and mice, and developmental studies in rats, mice and rabbits did not produce any evidence of effects on reproductive parameters or foetal development.
Neurotoxicity: Special neurotoxicity studies in rats by dietary administration found no evidence of delayed neurotoxicity.
Poisons Schedule: Azinphos-methyl is included in Schedule 7 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010). Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.03 mg/L for azinphos-methyl was determined as follows:
where:
0.25 mg/kg bw/day is the NOEL based on a short-term (28-day) oral dosing study in humans.
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.
30 is the safety factor applied to the NOEL derived from human studies. This safety factor incorporates a factor of 10 for intraspecies variation, with an additional safety factor of 3 to account for the use of short-term data.
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.
Health Canada (1989). Guidelines for Canadian Drinking Water Quality – Technical documents: Azinphos-methyl.
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.
Queensland Health (2007). Organochlorine, organophosphorous and synthetic pyrethroid pesticide, urea and triazine herbicides and PCBs in water. QHFSS SOP 16315.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
USEPA (United States Environmental Protection Agency) (2006). Reregistration Eligibility Decision for Azinphos-Methyl. USEPA, Office of Pesticide Programs.
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