Metsulfuron-methyl
(endorsed 2011)
Guideline
Based on human health concerns, metsulfuron-methyl in drinking water should not exceed 0.04 mg/L.
Related chemicals
Metsulfuron-methyl (CAS 74223-64-6) belongs to the sulfonylurea class of chemicals. There are many other pesticides in this class including azimsulfuron, chlorsulfuron, ethoxysulfuron, halosulfuron-methyl, iodosulfuron methyl-sodium salt, sulfometuron-methyl, sulfosulfuron, triasulfuron, tribenuron-methyl and trifloxysulfuron (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, metsulfuron-methyl would not be a health concern unless the concentration exceeded 0.04 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: Metsulfuron-methyl is used as a post-emergent herbicide for the control of weeds in native pastures, rights of way, commercial and industrial areas, domestic and public service areas and agricultural crops.
There are registered products that contain metsulfuron-methyl in Australia. The products are intended for professional use and are available in wettable powder and granular formulations. Product labels indicate products are to be diluted and applied by boom, hand-held and aerial spray methods. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main sources of public exposure to metsulfuron-methyl and its metabolites are residues in food and contact with treated weeds. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of metsulfuron-methyl 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 published reports on metsulfuron-methyl occurrence in Australian drinking water supplies were found.
Treatment of drinking water
No specific data on the treatment of metsulfuron-methyl in drinking water have been identified.
Measurement
Metsulfuron-methyl can be measured in water after filtration by direct injection on a triple quadrapole liquid chromatography–mass spectrometry instrument in multiple reaction monitoring mode, with a limit of reporting of 10 µg/L. Metsulfuron can also be analysed by solid phase extraction (SPE) followed by high performance liquid chromatography with diode array detection, achieving a limit of detection of 10 µg/L (Ruberu et al. 2000). SPE followed by liquid chromatography with electrospray mass spectrometric detection can achieve method detection limits of 3 ng/L for drinking water samples (Corcia et al. 1999).
History of the health values
The current acceptable daily intake (ADI) for metsulfuron is 0.01 mg per kg of bodyweight (0.01 mg/kg bw), based on a no-observed-effect level (NOEL) of 1 mg/kg bw/day from a long-term (2-year) dietary rat study. The NOEL is based on decreased bodyweight gain. The ADI incorporates a safety factor of 100 and was established in 1985.
The previous health value was 0.03 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Metsulfuron-methyl is readily absorbed via the gastrointestinal tract in rats. It is not extensively metabolised and is rapidly excreted mostly unchanged in the urine (90%) and faeces. Less than 2% was retained in the tissue in rats.
Acute effects: Metsulfuron-methyl has low acute oral and dermal toxicity. Metsulfuron-methyl is not a skin sensitiser in guinea pigs.
Short-term effects: Medium-term dietary studies in rats and dogs reported decreased bodyweight gain and clinical evidence of mild toxicity in both species at doses of 12 mg/kg bw/day and above. Medium-term studies in mice did not report any toxic effects at doses up to 750 mg/kg bw/day.
Long-term effects: Long-term dietary studies conducted in mice, rats and dogs reported decreased bodyweight gain and decreased food consumption in all species at doses of 12 mg/kg bw/day and above. The most sensitive NOEL was 0.93 mg/kg bw/day based on decreased bodyweight gain in a 2-year dietary study in rats. This NOEL was used to set the current ADI.
Carcinogenicity: Based on long-term studies in mice, rats and dogs, there is no evidence of carcinogenicity for metsulfuron-methyl.
Genotoxicity: Metsulfuron-methyl is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: A two-generation reproduction study in rats and developmental studies in rats and rabbits did not produce any effects on reproductive parameters or on foetal development.
Poisons Schedule: Metsulfuron-methyl is considered not to require control by scheduling due to its low toxicity and is therefore included 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 0.04 mg/L for metsulfuron-methyl was determined as follows:
where:
1 mg/kg bw/day is the NOEL based on a long-term (2-year) 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.
Corcia AD, Costantino A, Crescenzi C, Samperi R (1999). Quantification of phenylurea herbicides and their free and humic acid-associated metabolites in natural waters. Journal of Chromatography A, 852(2):465-474.
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.
Ruberu SR, WM Draper, Perera SK (2000). Multiresidue HPLC methods for phenyl urea herbicides in water. Journal of Agricultural and Food Chemistry, 48(9):4109-4115.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
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