Metaldehyde
(endorsed 2011)
Guideline
Based on human health concerns, metaldehyde in drinking water should not exceed 0.02 mg/L.
Related chemicals
Metaldehyde (CAS 108-62-3) belongs to aldehyde class of chemicals. There are no other pesticides in this class (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, metaldehyde would not be a health concern unless the concentration exceeded 0.02 mg/L. Minor excursions above this level would need to occur over a significant period, as the health-based guideline is based on moderate- to 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
Users: Metaldehyde is used to control snails and slugs in food crops.
There are registered products containing metaldehyde in Australia. The products are for professional and home garden use and are available in bait or pellet form, as powders, and in granular formations. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to metaldehyde is the use of home garden products.
Agricultural and home garden use of metaldehyde may potentially lead to contamination of source waters through processes such as run-off or entry into groundwater.
Typical values in Australian drinking water
No reports of metaldehyde in Australian drinking waters have been identified.
Treatment of drinking water
There is insufficient information on the treatment of metaldehyde in drinking water, but it is expected that advanced treatment methodologies such as ozonation and advanced oxidation would be effective.
Measurement
No suitable analytical techniques have been identified for analysis of metaldehyde in drinking water. However, if a need to monitor for metaldehyde is identified, it is expected that gas chromatography–mass spectroscopy should be suitable, since this technique has been used for monitoring metaldehyde in human and animal blood serum.
History of the health values
The current acceptable daily intake (ADI) for metaldehyde is 0.005 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 5 mg/kg bw/day in a rat study and applying a safety factor of 1000. The NOEL was based on neurological effects in rats and was established in 1986.
A health value has not been previously established by NHMRC.
Health considerations
Metabolism: Metaldehyde is readily absorbed from the gastrointestinal tract. Only a small percentage of the parent compound is excreted in the urine.
Acute effects: Metaldehyde has a moderate acute oral toxicity and low dermal toxicity. Clinical symptoms of toxicity reported in human poisoning cases included salivation, restlessness, muscle cramps and increased heart rate, indicative of mild neurotoxicity. Similar symptoms were observed in animal studies.
Short-term effects: No short-term studies have been evaluated.
Long-term effects: A 2-year dietary study in rats reported liver enzyme changes and increased liver weight at 12 mg/kg bw/day; however, the study was poorly reported.
Carcinogenicity: Based on a long-term study in rats, there is no evidence of carcinogenicity for metaldehyde; however, the study was poorly reported.
Genotoxicity: Only short-term in vitro studies are available; these report no evidence that metaldehyde is mutagenic.
Reproductive and developmental effects: A 3-generation reproduction study in rats produced effects on reproductive parameters at extremely high dose levels only. No developmental toxicity studies have been evaluated.
Poisons Schedule: Metaldehyde is included in Schedule 5 or 6 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010), depending on the concentration and use. Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.02 mg/L for metaldehyde was determined as follows:
where:
5 mg/kg bw/day is the NOEL based on a a long-term (1-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.
1000 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 an additional factor of 10 because of the poor quality of the data.
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.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
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