Thiometon
(endorsed 2011)
Guideline
Based on human health concerns, thiometon in drinking water should not exceed 0.004 mg/L.
Related chemicals
Thiometon (CAS 640-15-3) belongs to the organophosphate class of chemicals. There are many other pesticides in this class, which includes chlorpyrifos, terbufos, azinphos methyl (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, chemical would not be a health concern unless the concentration exceeded 0.004 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: Thiometon is an insecticide and acaricide used to control lice, mites and sawflies on a wide variety of agricultural crops.
There are currently no products containing thiometon registered for use in Australia, but de-registered compounds may still be detected in water. After a review, the toxicological database was considered inadequate to support continued registration of products containing thiometon.
Exposure sources: If used in the future, the main source of public exposure to thiometon and its metabolites would be residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of any thiometon products in the future 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 thiometon in Australian drinking waters have been identified.
Treatment of drinking water
No specific data on the treatment of thiometon in drinking water have been identified.
Measurement
The National Measurement Institute reports that gas chromatography with mass spectrometry can achieve a limit of reporting of 0.0001 mg/L for thiometon.
History of the health values
The current acceptable daily intake (ADI) for thiometon is 0.001 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.12 and 0.15 mg/kg bw/day from long-term (2-year) dietary studies in rats and dogs, respectively. The NOEL is based on decreased erythrocyte cholinesterase activity. The ADI incorporates a safety factor of 100, and was first established in 1989.
The previous health value was 0.003 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Thiometon is readily and extensively absorbed via the gastrointestinal tract in rats. It is extensively metabolised, primarily through sulfoxidation pathways to form sulfoxides and sulfones. These metabolites are rapidly excreted in urine as sulfates, glucoronides, and glutathione conjugates, almost completely within 24 hours.
Acute effects: Thiometon has moderate acute oral and dermal toxicity. Skin sensitisation studies are unavailable, but it has not been associated with sensitisation during occupational exposure. Clinical signs of acute poisoning were typical of cholinesterase inhibition and included hyperexcitability, salivation, bronchoconstriction, headache, vomiting and other behavioural changes.
Short-term effects: In 90-day dietary studies in rats and dogs, cholinesterase inhibition was seen at doses of 1.5 mg/kg bw/day (rats) and 0.6 mg/kg bw/day (dogs). No other effects were seen up to the highest doses tested, 4.5 mg/kg bw/day in rats and 1.2 mg/kg bw/day in dogs.
Long-term effects: In 1- and 2-year dietary studies in dogs and rats, respectively, erythrocyte cholinesterase inhibition was seen at doses of 0.3 mg/kg bw/day (dogs) and 1.5 mg/kg bw/day (rats) in 1- and 2-year dietary studies. Plasma cholinesterase inhibition was the only other effect seen up to the highest dose tested, 1.2 mg/kg bw/day, in dogs. In rats, increased serum alkaline phosphatase activity, and decreased serum cholesterol, protein, and glucose, and brain cholinesterase activity was seen at the highest dose tested, 14.4 mg/kg bw/day. The NOELs were 0.12 mg/kg bw/day (rats) and 0.15 mg/kg bw/day (dogs), and these are the basis for the current ADI of 0.001 mg/kg bw/day.
Carcinogenicity: Based on a 2-year study in rats, there is no evidence of carcinogenicity for thiometon.
Genotoxicity: Thiometon is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: Three-generation reproduction studies in rats, and developmental studies in rats and rabbits did not produce any evidence of effects on reproductive parameters or foetal development.
Neurotoxicity: No delayed developmental toxicity was seen in a 21-day dietary study in chickens up to the highest dose tested, 4 mg/kg bw/day.
Poisons Schedule: Thiometon is included in Schedule 6 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 0.004 mg/L for thiometon was determined as follows:
where:
0.12 mg/kg bw/day is the NOEL based on a long-term (2-year) dietary study in rats. This NOEL is closely supported by a NOEL of 0.15 mg/kg bw/day from a long-term (2-year) dietary 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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