Terbacil
(endorsed 2011)
Guideline
Based on human health concerns, terbacil in drinking water should not exceed 0.2 mg/L.
Related chemicals
Terbacil (CAS 5902-51-2) belongs to the uracil class of chemicals. Other pesticides in this class include bromacil and butafenacil (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, terbacil would not be a health concern unless the concentration exceeded 0.2 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: Terbacil is a selective herbicide for the control of annual and perennial weeds in agricultural crops such as sugarcane, apples, peaches, citrus, and almond trees.
There are registered products that contain terbacil in Australia. The products are intended for professional use. Terbacil is available as concentrated solutions to be applied in diluted form using ground, aerial or hand-held 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 terbacil and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of terbacil 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 terbacil in Australian drinking waters have been identified.
Treatment of drinking water
No specific data on the treatment of terbacil by conventional drinking water treatment processes have been identified. However, advanced oxidation using ultraviolet radiation and hydrogen peroxide is effective under optimised conditions (Shemer et al. 2006, Elovitz et al. 2008).
Measurement
No suitable techniques for the analysis of terbacil in drinking water have been identified. However, it is expected that a suitable method using high performance liquid chromatography with tandem mass spectrometry could be developed if required.
History of the health values
The current acceptable daily intake (ADI) for terbacil is 0.06 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 6.25 mg/kg bw/day from a long-term (2-year) dietary study in dogs. The NOEL is based on increased relative liver weight and elevated serum alkaline phosphatase at the highest dose tested of 60 mg/kg bw/day. The ADI incorporates a safety factor of 100, and was first established in 1987.
The previous health value was 0.03 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Terbacil is readily and extensively absorbed via the gastrointestinal tract in mammals. It is not extensively metabolised, but the limited metabolic pathway proceeded by 6-methyluracil hydroxylation, sulfonation, and conjugation. Excretion was complete by 48 hours as metabolised compound in urine and minor amounts in faeces.
Acute effects: Terabacil has low acute oral and dermal toxicity. It is not a skin sensitiser in guinea-pig.
Short-term effects: A 3-month dietary studies in rats reported an increase in absolute and relative liver weights, associated with hepatocellular hypertrophy and vacuolation at doses of 250 mg/kg bw/day.
Long-term effects: Long-term dietary studies were conducted in mice, rats and dogs. In mice, there was decreased pituitary weight, and centrilobular hypertrophy at doses of 180 mg/kg bw/day. In rats, there were increased absolute and relative liver weights, associated with centrilobular hypertrophy and vacuolation, at doses of 125 mg/kg bw/day. In dogs, there was increased relative liver weight and elevated serum alkaline phosphatase, with no histological evidence of adverse effects, at the highest dose tested, 60 mg/kg bw/day. The next lowest dose and lowest overall NOEL was 6.25 mg/kg bw/day in this study, and this is the basis for the current ADI.
Carcinogenicity: Based on an 18-month study in mice, and a 2-year study in rats, there is no evidence of carcinogenicity for terbacil.
Genotoxicity: Terbacil is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: A 3-generation reproduction study in rats and a developmental study in rats did not produce any evidence of effects on reproductive parameters or foetal development.
Poisons Schedule: Terbacil is considered not to require control by scheduling due to its low toxicity and is therefore in Appendix B 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 0.2 mg/L for terbacil was determined as follows:
where:
6.25 mg/kg bw/day is the NOEL based on 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.
Elovitz MS, Shemer H, Peller JR, Vinodgopal K, Sivaganesan M, Linden KG (2008). Hydroxyl radical rate constants: comparing and pulse radiolysis for environmental pollutants. Journal of Water Supply Research and Technology-Aqua, 57(6):391-401.
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.
Shemer H, Sharpless CM, Elovitz MS, Linden KG (2006). Relative rate constants of contaminant candidate list pesticides with hydroxyl radicals. Environmental Science and Technology, 40(14):4460-4466.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
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