Imazapyr
(endorsed 2011)
Guideline
Based on human health concerns, imazapyr in drinking water should not exceed 9 mg/L.
Related chemicals
Imazapyr (CAS 81334-34-1) belongs to the imidazolinone class of chemicals. Other pesticides in this class include imazamox and imazethapyr (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, imazapyr would not be a health concern unless the concentration exceeded 9 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: Imazapyr is a broad spectrum herbicide for the pre- and post-emergence control of certain annual grass and broad-leaf weeds in agricultural crops
There are registered products that contain imazapyr, or its isopropylamine or ammonium salt, in Australia. The products are intended for professional use and are available as soluble concentrates or water-dispersible granule formulations which are diluted and applied using ground boom sprayers. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to imazapyr is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of imazapyr 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 occurrence data for imazapyr in Australian waters were found. In the USA, the modelled estimated surface and groundwater concentration were 0.34–79 µg/L and 39 µg/L, respectively (USEPA 2006).
Treatment of drinking water
No data on drinking water treatment removal efficiency were found for imazapyr.
Measurement
Imazapyr can be measured in water by liquid chromatography–mass spectrometry with an electrospray interface, with a limit of detection of 0.004 µg/L (D’Ascenzo et al. 1998).
History of the health values
The current acceptable daily intake (ADI) for imazapyr is 2.5 mg per kg body weight (mg/kg bw), based on a no-observed-effect level (NOEL) of 250 mg/kg bw/day from a 1-year dietary study in dogs. The NOEL is based on the absence of signs of toxicity at the highest dose tested (250 mg/kg bw/day). The ADI incorporates a safety factor of 100 and was established in 1998.
A health value has not been previously established by NHMRC.
Health considerations
Metabolism: Imazapyr is readily and extensively absorbed via the gastrointestinal tract in rats. It was rapidly excreted in the urine and faeces (within 2 days).
Acute effects: Imazapyr and its isopropylamine salt have low acute oral toxicity and low dermal toxicity. Neither imazapyr nor its isopropylamine salt is a skin sensitiser.
Short-term effects: In a 13-week dietary study in rats, no treatment-related effects were observed at the highest dose tested (879 mg/kg bw/day).
Long-term effects: In an 18-month dietary study in mice, there was no evidence of treatment-related toxicity at the highest dose tested (1639 mg/kg bw/day). In 1-year dietary studies in rats and dogs, there was no evidence of treatment-related toxicity at the highest dose tested (639 and 250 mg/kg bw/day in rats and dogs respectively). The NOEL of 250 mg/kg bw/day in dogs is the basis of the current ADI.
Carcinogenicity: There was no evidence of carcinogenicity, based on an 18-month dietary study in mice and a 2-year dietary study in rats.
Genotoxicity: Imazapyr 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 did not produce any evidence of reproductive or developmental effects.
Poisons Schedule: Imazapyr 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 9 mg/L for imazapyr was determined as follows:
where:
250 mg/kg bw/day is the NOEL based on a long-term (1-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.
D’Ascenzo G, Gentili A, Marchese S, Perret D (1998). Development of a method based on liquid chromatography-electrospray mass spectrometry for analyzing imidazolinone herbicides in environmental water at part-per-trillion levels. Journal of Chromatrography A, 800(1):109-119.
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.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
USEPA (United States Environmental Protection Agency) (2006). Reregistration Eligibility Decision for Imazapyr. EPA 738-R-06-007, OPP-2005-0495.
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