Parathion
(endorsed 2011)
Guideline
Based on human health concerns, parathion in drinking water should not exceed 0.02 mg/L.
Related chemicals
Parathion (ethyl parathion)(CAS 298-00-0) belongs to the organophosphate class of chemicals. Other pesticides in this class include methyl parathion, disulfoton, ethion, and chlorpyrifos (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, parathion would not be a health concern unless the concentration exceeded 0.02 mg/L. Excursions above this level even for a short period are of concern as the health-based guideline is based on short-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: Parathion is an insecticide and acaricide (miticide) for the control of aphids, mites and caterpillars in commercial vegetable, fruit, tobacco and cotton crops.
There are currently no registered products containing parathion in Australia, but de-registered compounds could still be detected in water. Previously registered products were intended for professional use; these products were soluble concentrates intended to be diluted and applied by aerial and ground spray.
Exposure sources: If used in the future, the main source of public exposure to parathion and its metabolites, based on the previous use pattern, would be through residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Typical values in Australian drinking water
Parathion is typically not detected in Australian drinking waters and is not reported in drinking waters internationally under conditions of good agricultural practice. Its presence in drinking waters has not been reported by the World Health Organization (WHO 2004a).
Treatment of drinking water
No specific data on the treatment of parathion in drinking water have been identified.
Measurement
Parathion is effectively extracted from water using liquid-liquid extraction with dichloromethane followed by gas chromatographic analysis employing nitrogen-phosphorus detection. The typical limit of detection using this procedure is 10 ng/L (Culea and Gocan 2000).
History of the health values
The current acceptable daily intake (ADI) for parathion is 0.005 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.05 mg/kg bw/day for a reduction in erythrocyte cholinesterase activity in a 3-week oral toxicity study in humans. The ADI incorporates a safety factor of 10. It was first set in 1967 and reaffirmed in 1997.
The acute reference dose (ARfD) of 0.01 mg/kg bw for parathion was established in 2000, based on a NOEL of 0.125 mg/kg bw/day for erythrocyte cholinesterase inhibition from a 35-day oral toxicity study in humans. The ARfD incorporates a safety factor of 10.
The previous health value was 0.01 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Parathion is readily and extensively absorbed via the gastrointestinal tract. It is extensively metabolised, and is rapidly excreted in the urine and faeces, almost completely within 48 hours. The main metabolite is paraoxon-ethyl, which is of similar acute toxicity to the parent compound.
Acute effects: Parathion has high acute oral and dermal toxicity in animals and humans. The potential for skin sensitisation is unknown, due to its high dermal toxicity. Clinical symptoms of toxicity were typical of cholinesterase inhibition and included tremors, prostration, coma, piloerection, ataxia, and salivation.
Short-term effects: An dietary study in humans for up to 21 days reported reduced erythrocyte cholinesterase activity at the highest dose tested, 0.1 mg/kg bw/day. The NOEL was 0.05 mg/kg bw/day and this NOEL is the basis for the current ADI.
Long-term effects: Long-term dietary studies in rats and dogs reported reduced plasma and erythrocyte cholinesterase activity at doses of 0.03 mg/kg/day and above. In rats, reduced brain cholinesterase activity, retinal atrophy, and myelin degeneration in optical nerves were seen at the highest dose tested, 1.9 mg/kg bw/day.
Carcinogenicity: The evidence from several long-term rodent studies is inadequate to assess carcinogenicity for parathion.
Genotoxicity: Parathion is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: Two-generation reproduction studies in rats up to 2.8 mg/kg bw/day and developmental studies in rats and rabbits up to 1.5 mg/kg bw/day found no evidence for effects on reproduction or foetal development.
Neurotoxicity: In a 10-day study in hens, there was no evidence of delayed neurotoxicity due to parathion.
Poisons Schedule: Parathion is included in Schedule 7 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 0.02 mg/L for parathion was determined as follows:
where:
0.05 mg/kg bw/day is the NOEL based on a short-term (3-week) dietary study in humans.
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.
10 is the safety factor applied to the NOEL derived from human studies. This safety factor covers intraspecies variation.
The World Health Organization has not established a health-based guideline value for parathion 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.
Culea M, Gocan S (2000). Organophosphates. In: Nollet LM, Dekker M (eds). Handbook of Water Analysis, New York, pp571-608.
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.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
WHO (World Health Organization) (2004a). Parathion in Drinking-water; Background Document for Development of WHO Guidelines for Drinking Water.
WHO (World Health Organization) (2004b). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
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