Clopyralid
(endorsed 2011)
Guideline
Based on human health concerns, clopyralid in drinking water should not exceed 2 mg/L.
Related chemicals
Clopyralid (CAS 1702-17-6) belongs to the pyridinecarboxylic acid class of chemicals. Other pesticides in this class include aminopyralid, fluroxypyr, picloram, picolinafen and triclopyr (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, clopyralid would not be a health concern unless the concentration exceeded 2 mg/L. 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: Clopyralid is used as a herbicide for the control of broad-leaf weeds in turf, industrial situations and agricultural crops.
There are currently products registered in Australia that contain clopyralid, its potassium salt, its trisopropanolamine salt or its monoethanolamine salt. Clopyralid products are intended for professional use. Products are not intended for use in the home garden. Clopyralid is available as concentrated solutions, powder and granular formulations to be applied in diluted form using boom, aerial or hand-held spray equipment. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to clopyralid and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of clopyralid 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 clopyralid in Australian drinking waters have been identified. However, an estimated mean annual calculated concentration of 24 ng/L and a maximum concentration of 1 µg/L has been reported in drinking water supplies of the northern Great Plains of the USA (Donald et al. 2007).
Treatment of drinking water
No specific data on the treatment of clopyralid in drinking water have been identified.
Measurement
Clopyralid can be measured in drinking waters by solvent extraction followed by gas-chromatography with mass spectrometry detection. The practical limit of detection is 0.6 ng/L (Donald et al. 2007).
History of the health values
The current acceptable daily intake (ADI) for clopyralid is 0.5 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 50 mg/kg bw/day from a long-term (2-year dietary) study. The NOEL is based on decreased bodyweight gain and adverse effects in the stomach epithelium observed in rats. The ADI incorporates a safety factor of 100 and was established in 1982.
The previous health value was 1 mg/L (NHMRC and NRMMC 2004).
Health considerations
The toxicological database for clopyralid consists of toxicity studies on the parent compound.
Metabolism: No metabolism studies in animals are available for clopyralid. A short-term human volunteer study reported elimination of clopyralid to be rapid and occur mainly via the urine. Metabolites were not measured in this study.
Acute effects: Clopyralid has low acute oral and dermal toxicity. It is not a skin sensitiser.
Short-term effects: Two- and four-week dietary studies in rats and dogs indicate the kidney and liver are target organs of clopyralid toxicity. Decreased bodyweight was reported at the highest dose level of 1500 mg/kg bw/day. The most sensitive toxicological effects observed were increased blood urea nitrogen levels and proliferative changes in the stomach epithelium at dose levels above 150 mg/kg bw/day in rats.
Medium-term (90-day) dietary studies were conducted in mice, rats and dogs. In rats, increased kidney and liver weights were observed at doses of 300 mg/kg bw/day. There were no treatment-related effects observed in dogs at the highest dose tested (150 mg/kg bw/day).
Long-term effects: Long-term dietary studies were conducted in mice, rats and dogs. In mice, decreased bodyweight gain was observed at the high dose of 2000 mg/kg bw/day. In dogs, changes in haematological parameters and increased liver and kidney weights were observed above a dose level of 100 mg/kg bw/day. In rats, hyperplasia in stomach epithelium and decreased bodyweight gain were observed at a dose of 100 mg/kg bw/day. The NOEL based on decreased bodyweight gain and effects on the gastric epithelium was 50 mg/kg bw/day, and is the basis for the current ADI.
Carcinogenicity: Based on long-term studies in mice and rats, there is no evidence of carcinogenicity for clopyralid.
Genotoxicity: Clopyralid is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: A 2-generation reproduction study in rats and developmental studies in rats and rabbits produced no evidence of adverse effects on reproduction or the developing foetus.
Poisons Schedule: Clopyralid is included in Schedule 5 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 2 mg/L for clopyralid was determined as follows:
where:
50 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 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.
Donald DB, AJ Cessna, E Sverko, NE Glozier (2007). Pesticides in surface drinking-water supplies of the northern Great Plains (Research). Environmental Health Perspectives, 115(8):1183(9).
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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