Terbufos
(endorsed 2011)
Guideline
Based on human health concerns, terbufos in drinking water should not exceed 0.0009 mg/L.
Related chemicals
Terbufos (CAS 13071-79-9) belongs to the organophosphate group of chemicals. There are many other pesticides in this class, which includes chlorpyrifos, dimethoate, ethoprophos and ethion (Tomlin 2006).
Human risk statement
With good water quality management practices, the exposure of the general population to terbufos is expected to be well below levels that may cause health concerns.
Terbufos is an acutely poisonous organophosphate pesticide. If it is detected in water as a result of spillage or misuse at levels above 0.0009 mg/L, remedial action should be taken. Concentrations of terbufos greatly exceeding this guideline present an acute human health risk.
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: Terbufos is used as an insecticide and nematocide for the control of various above-ground insects, soil arthropods, and nematodes in agriculture, including food crops.
There are registered products containing terbufos in Australia. The products are granular formulations to be applied to the soil in agricultural settings. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to terbufos and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of terbufos may potentially lead to contamination of source waters through processes such as run-off or entry into groundwater.
Typical values in Australian drinking water
No reports of terbufos in Australian drinking waters have been identified.
Treatment of drinking water
No specific data on the treatment of terbufos in drinking water have been identified.
Measurement
Terbufos can be measured in drinking water by solid phase extraction and capillary column gas chromatography–mass spectrometry (USEPA 2000). The practical limit of detection is 0.05 mg/L.
History of the health values
The current acceptable daily intake (ADI) for terbufos is 0.0002 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.0025 mg/kg bw/day from a short-term (6-month dietary) study. The NOEL is based on decreased serum cholinesterase activity in dogs. The ADI incorporates a safety factor of 10, and was established in 1992.
The previous ADI of 0.00003 mg/kg bw/day set in 1980 was based on the same study and endpoint. The use of a safety factor of 100 was reconsidered in 1992 following an evaluation by the WHO. The 1992 review affirmed the validity of cholinesterase inhibition as an endpoint of toxicity, but decreased the safety factor to 10.
The previous health value was 0.0005 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Terbufos is readily absorbed via the gastrointestinal tract and is widely distributed in tissues and blood. In rats, terbufos was slowly excreted as metabolites (within 7 days), mainly in the urine. The major metabolites were S-methylated metabolites, which are of similar or lower toxicity to terbufos.
Acute effects: Terbufos has high acute oral toxicity in rats and high acute dermal toxicity in rabbits. The skin sensitisation potential of terbufos is unknown. Clinical signs of acute poisoning were typical of cholinesterase inhibition and included hyperexcitability, salivation, bronchoconstriction, headache, vomiting and other behavioural changes.
Short-term effects: Short-term dietary studies in rats and dogs reported symptoms indicative of nervous system toxicity. In 28-day dietary studies, decreased cholinesterase activity was reported at doses above 0.0125 mg/kg bw/day in rats and above 0.01 mg/kg bw/day in dogs. In a 3-month dietary study in rats and a 6-month dietary study in dogs, cholinesterase inhibition occurred at doses above 0.05 mg/kg bw/day in rats and 0.0025 mg/kg bw/day in dogs. The NOEL of 0.0025 mg/kg bw/day in dogs is the basis for the current ADI.
Long-term effects: Long-term dietary studies in rats and dogs reported symptoms indicative of nervous system toxicity. A 1-year dietary study in dogs reported inhibition of cholinesterase at the lowest dose tested, 0.015 mg/kg bw/day.
Carcinogenicity: Based on long-term dietary studies in mice or rats, there is no evidence of carcinogenicity for terbufos.
Genotoxicity: Terbufos is not considered to be genotoxic, based on in vitro or in vivo short-term studies.
Reproductive and developmental effects: Reproduction studies and developmental studies in rats reported no effects on reproductive parameters or foetal development other than that resulting from maternal toxicity.
Neurotoxicity: There was no clinical evidence of delayed or residual neurotoxicity or demyelination in 21-day toxicity studies in hens.
Poisons Schedule: Terbufos 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 0.0009 mg/L for terbufos was determined as follows:
where:
0.0025 mg/kg bw/day is the NOEL based on a medium-term (6-month) 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.
10 is the safety factor applied to the NOEL derived from animal studies. The safety factor of 10 was considered to provide an adequate margin of safety, as despite being derived from animal studies, the end-point (inhibition of plasma cholinesterase) is considered to be a very sensitive indicator.
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.
USEPA (United States Environmental Protection Agency (2000). Method 526. Determination of selected semivolatile organic compounds in drinking water by solid phase extraction and capillary column gas chromatography/ mass spectrometry (GC/MS).
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