Omethoate
(endorsed 2011)
Guideline
Based on human health concerns, omethoate in drinking water should not exceed 0.001 mg/L.
Related chemicals
Omethoate (CAS 1113-02-6) belongs to the organophosphate class of chemicals. There are many other pesticides in this class including dimethoate, fenthion, parathion, profenofos and ethoprophos (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, omethoate would not be a health concern unless the concentration exceeded 0.001 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: Omethoate is an insecticide and acaricide (miticide) for the control of aphids, flies, caterpillars, bugs, moths, mites and fleas. Omethoate is a metabolite of dimethoate.
There are registered products containing omethoate in Australia. The products are intended for both professional and home garden use. Omethoate is formulated as a liquid concentrate, to be diluted and sprayed onto plants and bare earth. It can be applied to a variety of crops including pastures, legumes, oilseeds, faba bean, poppies, ornamentals, citrus, apples, cotton, lupins, onions, pears and potatoes. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main sources of public exposure to omethoate are the use of home garden products, and residues in food. Residue levels in food produced according to good agricultural practice and potential dietary exposure are generally low.
Agricultural use of omethoate 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 reports of omethoate in Australian drinking waters have been identified.
Treatment of drinking water
There is insufficient information on the treatment of omethoate in drinking water, but it is expected that advanced treatment methodologies such as ozonation and advanced oxidation would be effective.
Measurement
Omethoate can be determined in water by hydrophilic interaction liquid chromatography with tandem mass spectrometry, with a limit of quantitation of 0.05 µg/L (Hayama et al. 2008). A capillary electrophoresis method has also been developed for the analysis of omethoate with a method detection limit of 50 µg/L (Tao et al. 2008). A variety of solid phase extraction materials may be used for the effective extraction of omethoate from water (Geiss and Gebert 2006).
History of the health values
The current acceptable daily intake (ADI) for omethoate is 0.0004 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.04 mg/kg bw/day from 2-year dietary study in rats. The NOEL was based on inhibition of acetylcholinesterase. The ADI incorporates a safety factor of 100 and was established in 2005.
The acute reference dose (ARfD) of 0.003 mg/kg bw/day for omethoate was established in 2005, based on a NOEL of 0.25 mg/kg bw/day from an acute neurotoxicity study in rats. The NOEL was based on inhibition of acetylcholinesterase activity. The ARfD incorporates a safety factor of 100.
A health value has not been previously established by NHMRC.
Health considerations
Metabolism: Omethoate is absorbed rapidly and extensively from the gastrointestinal tract in rats and is rapidly eliminated in the urine, mainly as unchanged omethoate (88% eliminated after 8 hours). It is widely distributed throughout the body with the highest levels found in the thyroid. Identified metabolites include N-methyl-methyl-sulfinyl-acetamide, O-desmethylated omethoate, O,O dimethyl phosphoric acid and O,O-dimethyl phosphorothioic acid.
Acute effects: Omethoate has high acute oral toxicity and moderate acute dermal toxicity. It can cause skin sensitisation in guinea pigs.
Short-term and long-term effects: Short-term dermal exposure studies and long-term dietary exposure studies in rats reported symptoms indicative of nervous system toxicity. A 2-year dietary study in rats reported inhibition of red blood cell and brain cholinesterase activity at dose levels equivalent to 0.12 mg/kg bw/day and above. The NOEL of 0.04 mg/kg bw/day from this study is the basis of the ADI.
Carcinogenicity: There was evidence of benign thyroid tumours at high dose levels in rats. These levels were well in excess of the expected levels of human exposure.
Genotoxicity: Omethoate is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: A multigeneration reproduction study in rats reported no evidence of effects on reproductive parameters. Developmental studies in rats and rabbits reported maternal and foetal toxicity as a result of cholinesterase inhibition at 1 mg/kg bw/day in rats and 0.2 mg/kg bw/day in rabbits. There was no evidence of teratogenicity.
Neurotoxicity: Omethoate did not cause delayed neurotoxicity in tests conducted on hens.
Poisons Schedule: Omethoate is in Schedules 5, 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 0.001 mg/L for omethoate was determined as follows:
where:
0.04 mg/kg bw/day is the NOEL based on a long-term (2-year) 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 amount (maximum) of water consumed by an adult.
100 is the safety factor applied to the NOEL derived from animal studies. The safety factor of 100 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.
Geiss S, Gebert S (2006). Extraction of highly polar organophosphorus pesticides from water. Acta Hydrochimica et Hydrobiologica, 34(5):464-473.
Hayama T, Yoshida H, Todoroki K, Nohta H, Yamaguchi M (2008). Determination of polar organophosphorus pesticides in water samples by hydrophilic interaction liquid chromatography with tandem mass spectrometry. Rapid Communications in Mass Spectrometry, 22(14):2203-2210.
Tao YG, Wang YM, Ye LB, Li HF, Wang Q (2008). Simultaneous determination of omethoate and dichlorvos by capillary electrophoresis. Bulletin of Environmnetal Contamination and Toxicology, 81(2):210-215.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
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