Diclofop-methyl
(endorsed 2011)
Guideline
Based on human health concerns, diclofop-methyl in drinking water should not exceed 0.005 mg/L.
Related chemicals
Diclofop-methyl (CAS 51338-27-3) belongs to the aryloxyphenoxy propionate and chlorophenoxy class of chemicals. Other pesticides in these classes include dichlorprop, fenoprop, 2,4-D, 2,4,5-T and MCPA (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, diclofop-methyl would not be a health concern unless the concentration exceeded 0.005 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: Diclofop-methyl is used as a post-emergence herbicide for the control of grass weeds in turf and agricultural crops.
There are currently products registered in Australia that contain diclofop-methyl. All of these are intended for professional use and not for use in the home garden. Diclofop-methyl is available in concentrated solutions to be applied in diluted form using aircraft, ground sprayers or knapsack 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 diclofop-methyl and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of diclofop-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 reports of diclofop-methyl in Australian drinking waters have been identified.
Treatment of drinking water
No specific data on the treatment of diclofop in drinking water have been identified.
Measurement
Diclofop-methyl can be analysed in water by solid phase extraction followed by high performance liquid chromatography with diode array detection (Ozhan et al. 2005). The limit of detection for this method is 0.04 µg/L. Diclofop-methyl can also be analysed in environmental samples by gas chromatography–mass spectrometry (Tadeo et al. 1996).
History of the health values
The current acceptable daily intake (ADI) for diclofop-methyl is 0.002 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.25 mg/kg bw/day from a long- term study. The NOEL is based on liver toxicity observed in a 2-year study in mice. The ADI incorporates a safety factor of 100 and was established in 1986.
The previous health value was 0.005 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Diclofop-methyl is readily absorbed via the gastrointestinal tract. It is extensively metabolised and is excreted in the urine and faeces, within 96 hours.
Acute effects: Diclofop-methyl has low oral acute toxicity and moderate acute dermal toxicity. It is a skin sensitiser.
Short-term effects: Medium-term dietary studies in rats and dogs reported increases in liver weight and enlargement of hepatocytes, in addition to changes in blood chemistry at dose levels of 1.5 mg/kg bw/day in rats and at 4 mg/kg bw/day in dogs. Fatty infiltration of the adrenal cortex was reported at high doses in dogs.
Long-term effects: Long-term dietary studies in mice, rats and dogs demonstrated the most sensitive toxic effects to be increased liver, heart and kidney weights in addition to increased serum alkaline phosphatase levels in the mouse, at dose levels of 0.8 mg/kg bw/day. Decreased bodyweight and liver pathology and changes in blood chemistry were observed at high doses in all species. The NOEL of 0.25 mg/kg bw in mice is the basis for the current ADI.
Carcinogenicity: Two-year studies in mice reported an increase in hepatocellular adenomas at 2.5 mg/kg bw/day, considered to be related to liver toxicity. In the absence of positive genotoxicity, these tumours were considered to be a threshold effect.
Genotoxicity: Diclofop-methyl 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 developmental studies in rabbits showed no adverse effects on reproduction or development
Poisons Schedule: Diclofop-methyl is included in Schedule 6 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 health-based guideline
The health-based guideline of 0.005 mg/L for diclofop-methyl was determined as follows:
where:
0.25 mg/kg bw/day is the NOEL based on a long-term (2-year) dietary study in mice.
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 rom the consumption of drinking water.
2 L/day is the estimated maximum amount of water consumed by an adult.
200 is the safety factor applied to the NOEL derived from animal studies. This safety factor incorporates a factor of 10 for interspecies extrapolation, 10 for intraspecies variation and 2 for uncertainty due to potential for tumour promotion.
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.
Ozhan G, Ozden S, Alpertunga B (2005). Determination of commonly used herbicides in surface water using solid-phase extraction and dual-column HPLC-DAD. Journal of Environmental Science and Health Part B – Pesticides Food Contaminants and Agricultural Wastes, 40(6):827-840.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
Tadeo JL, SanchezBrunete C, GarciaValcarcel AI, Martinez L, Perez RA (1996). Determination of cereal herbicide residues in environmental samples by gas chromatography. Journal of Chromatography A, 754(1-2):347-365.
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