Maldison (Malathion)
(endorsed 2011)
Guideline
Based on human health concerns, maldison in drinking water should not exceed 0.07 mg/L.
Related chemicals
Maldison (malathion)(CAS 121-75-5) belongs to the organophosphate class of chemicals. There are many other pesticides in this class, including fenthion, parathion, profenofos and ethoprophos (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, maldison would not be a health concern unless the concentration exceeded 0.07 mg/L. Minor 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: Maldison is an insecticide and parasiticide for the control of various insect pests such as fruit fly and locusts on various crops, and in grain storage facilities. It is also used on dogs and cats and in aviaries for treatment of lice, brown dog tick and mange.
There are registered products containing maldison in Australia. The products are intended for both professional and home garden/veterinary use. Products are formulated as liquid concentrates (to be diluted and sprayed), dusts, lures/traps and insecticidal washes. Aerial ultra-low volume (ULV) application is permitted, as well as ground-based ULV application. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main sources of public exposure to maldison and its metabolites are the use of home garden and home veterinary products, and residues in food. Residue levels in food produced according to good agricultural practice are anticipated to be generally low.
Agricultural use of maldison may potentially lead to contamination of source waters through processes such as run-off, spray drift (especially from aerial and ULV application) or entry into groundwater.
Reported values in Australian waters
No data were found on maldison in Australian waters. Maldison was not detected in surveys of municipal and private drinking-water supplies conducted in Canada between 1971 and 1986 (Health Canada 1989). It was detected in 4 of 949 stream samples in southern Ontario agricultural watersheds at concentrations of 0.24 to 1.8 μg/L (Health Canada 1989). In the USA, maldison has been reported in surface water at levels up to 0.18 μg/L and in drinking-water at 0.1 μg/L (ATSDR 2000, WHO 2004a).
Treatment of drinking water
There is insufficient information on the treatment of maldison in drinking water, but it is expected that advanced treatment methodologies such as ozonation and advanced oxidation would be effective. Maldison has been shown to have relatively high removal rates when water undergoes advanced oxidation with iron-catalysed ultraviolet irradiation and peroxide (Fenton reaction) (Huston and Pignatello 1999).
Measurement
Maldison can be measured by routine gas chromatography–mass spectrometry analysis, with a limit of reporting of 0.1 µg/L (Queensland Health 2007).
History of the health values
The current acceptable daily intake (ADI) for maldison is 0.02 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 2 mg/kg bw/day from a 2-year dietary study in rats. This NOEL is based on inhibition of red blood cell cholinesterase. The ADI incorporates a safety factor of 100 and it was established in 2005.
The previous ADI was 0.02 mg/kg bw/day, based on a NOEL of 0.26 mg/kg bw/day for inhibition of red blood cell and plasma cholinesterase in a human study and a safety factor of 10. The NOEL in this study was not maintained due to the absence of information regarding the purity of the material tested.
The acute reference dose (ARfD) of 1.5 mg/kg bw/day for maldison was established in 2005, based on a NOEL of 15 mg/kg bw/day from an acute dietary study in humans. The ARfD incorporates a safety factor of 10.
The previous health value was 0.05 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Maldison is rapidly and extensively absorbed from the gastrointestinal tract. Metabolism is extensive, with oxidation to malaoxon, and further hydroxylation to another six to eight metabolites. Maldison has a low potential for accumulation, with <1% of the dose present in blood and tissues after 72 hours.
Acute effects: Maldison has low acute oral and dermal toxicity. It is not a skin sensitiser in guinea pigs, although there is some evidence of skin sensitisation in humans.
Short-term and long-term effects: Short-term and long-term dietary studies in rats report cholinesterase inhibition as the main toxicological effect. In a 3-month study, cholinesterase inhibition occurred at 34 mg/kg bw/day and above. In a 2-year study, inhibition of red blood cell cholinesterase occurred at 29 mg/kg bw/day and above. The NOEL of 2 mg/kg bw/day in this rat study is the basis for the current ADI.
A 56-day human study reported inhibition of red blood cell cholinesterase at a dose of 0.4 mg/kg bw/day.
Carcinogenicity: Adenomas were reported in the liver in mice and rats and in the thyroid in rats at high dose levels, but these were considered rodent-specific. There was no other evidence of carcinogenicity for maldison.
Genotoxicity: Maldison 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 reported no evidence of effect on reproductive parameters or foetal development.
Neurotoxicity: Maldison did not cause delayed neurotoxicity in hens.
Poisons Schedule: Maldison (Malathion) is included in Schedule 3, 5 and 6 of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010), depending on its 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 0.07 mg/L for maldison was determined as follows:
where:
2 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 from animal studies. The safety factor of 100 incorporates a factor of 10 for interspecies extrapolation and 10 for intraspecies variation.
The World Health Organization has not established a health-based guideline value for malathion and it is excluded from the list of agricultural chemicals guideline value derivation because it “occurs in drinking-water at concentrations well below those at which toxic effects may occur” (WHO 2004b).
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.
ATSDR (Agency for Toxic Substances and Disease Registry) (2000). Toxicological profile information sheet – Malathion. Draft for public comment. Atlanta, GA, US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry.
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.
Health Canada (1989). Guidelines for Canadian Drinking Water Quality – Technical Documents: Malathion. Available at https://www.canada.ca/en/health-canada/services/publications/healthy-living/guidelines-canadian-drinking-water-quality-supporting-documents-malathion.html
Huston PL, Pignatello JJ (1999). Degradation of selected pesticide active ingredients and commercial formulations in water by the photo-assisted Fenton reaction. Water Research 33(5):1238-1246.
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.
Queensland Health (2007). Organochlorine, organophosphorous and synthetic pyrethroid pesticide, urea and triazine herbicides and PCBs in water. QHFSS SOP 16315.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
WHO (World Health Organization) (2004a). Malathion in Drinking-water. Background document for development of WHO Guidelines for Drinking-water Quality. WHO.
WHO (World Health Organization) (2004b). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
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