Parathion-methyl
(endorsed 2011)
Guideline
Based on human health concerns, parathion-methyl in drinking water should not exceed 0.0007 mg/L.
Related chemicals
Parathion-methyl (CAS 298-00-0) belongs to the organophosphate class of pesticides, and is structurally related to parathion. This is one of the largest classes of pesticides, and other members include acephate, chlorpyrifos, ethion, phorate, and terbufos (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-methyl would not be a health concern unless the concentration exceeded 0.0007 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: Parathion-methyl is an insecticide for the control of pests in agricultural crops.
There are registered products that contain parathion-methyl in Australia. The products are intended for professional use on vegetables, citrus, pome and stone fruit, grapevines, and cotton crops. Products are capsule suspensions or liquid concentrates that are applied in diluted form to crops by ground boom or aerial sprays. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to parathion-methyl and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of parathion-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 parathion-methyl in Australian drinking waters have been identified.
Treatment of drinking water
Parathion methyl is completely removed from water by chlorination when the chlorine dose is adjusted to match chlorine demand (Ormad et al. 2008).
Ozonation and activated carbon adsorption for parathion-methyl removal has also been reported with moderate success (Ormad et al. 2008). Conventional coagulation/flocculation provides a relatively low removal rate (30%). Jar testing is recommended to optimise dose rates if parathion-methyl is detected.
Measurement
Parathion-methyl can be measured in drinking water by gas chromatography–mass spectrometry. The typical limit of detection is 0.1 µg/L (QFSS 2009).
History of the health values
The current acceptable daily intake (ADI) for parathion-methyl is 0.0002 mg per kg of bodyweight (mg/kg bw), based on a NOEL of 0.02 mg/kg bw/day from long-term (1-year and 2-year) dietary studies in rats. The NOEL is based on time- and dose-dependent neuro-pathological effects in the form of neuronal degeneration and abnormal gait. The ADI incorporates a safety factor of 100, and was established in 1997.
The previous ADI was 0.03 mg/kg bw/day. It was established in 1990 based on a NOEL of 0.3 mg/kg bw/day for erythrocyte cholinesterase inhibition in a 30-day dietary study in humans. The ADI was amended in 1997 to its current level after submission of long-term studies in rats showing effects at levels below this NOEL.
The ARfD of 0.03 mg/kg bw for parathion-methyl was established in 2000, based on the aforementioned NOEL of 0.3 mg/kg bw/day for erythrocyte cholinesterase inhibition in a 30-day dietary study in humans. The ARfD incorporates a safety factor of 10.
The previous health value was 0.1 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Parathion-methyl is readily and extensively absorbed from the gastrointestinal tract. Metabolism was extensive and proceeded by sulfonation, conjugation, and oxidation. The metabolites are of similar or lower toxicity to parathion-methyl. Excretion of parent compound and metabolites is rapid, proceeding through urine and being almost complete by 24 hours.
Acute effects: Parathion-methyl has high acute oral and dermal toxicity in rats. Parathion-methyl is not a skin sensitiser in guinea pigs. Clinical symptoms of toxicity were typical of cholinesterase inhibition and included tremors, prostration, coma, piloerection, ataxia, and salivation.
Short-term effects: A five-day dermal study in rats reported brain cholinesterase inhibition at 1 mg/kg bw/day and above. Clinical symptoms were observed at higher dose levels. A 28-day dermal study in rats reported brain and erythrocyte cholinesterase inhibition at 0.3 mg/kg bw/day and above. A 13-week dietary study in rats reported erythrocyte cholinesterase inhibition at 0.23 mg/kg bw/day and brain cholinesterase inhibition at higher doses. There were no associated histopathological changes. The NOEL in this study was 0.02 mg/kg bw/day.
Long-term effects: Long-term dietary studies were undertaken in rats, dogs, and mice. Effects were typical of organophosphates and in rats included neuronal degeneration and lethargy at doses of 0.1 mg/kg bw/day and above. In mice and rats, effects included behavioural changes and brain and erythrocyte cholinesterase inhibition at doses of 0.5 mg/kg bw/day. Brain demyelination also was seen in rats by 6 months at doses of 2 mg/kg bw/day. In dogs, erythrocyte cholinesterase was inhibited at the lowest dose tested, 0.03 mg/kg bw/day.
The lowest overall NOEL was 0.02 mg/kg bw/day based on neuronal degeneration and decreased physical activity in the rat. This NOEL is the basis for the current ADI.
Carcinogenicity: Based on 2-year studies in mice and rats, there is no evidence of carcinogenicity for parathion-methyl.
Genotoxicity: Parathion-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 reported reduced pup survival during weaning at 1 mg/kg bw/day, without evidence of adverse effects in dams. Developmental studies in rats and rabbits did not identify any effects on foetal development.
Neurotoxicity: Short-term dietary studies in rats, mice and dogs reported symptoms indicative of nervous system toxicity but only at dose levels well in excess of the likely level of human exposure.
Poisons Schedule: Parathion-methyl is included in Schedule 6 and 7 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 0.0007 mg/L for parathion-methyl was determined as follows:
where:
0.02 mg/kg bw/day is the NOEL based on long-term (1-year and 2-year) dietary studies 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.
The World Health Organization has not established a health-based guideline value for parathion-methyl based on the shown evidence that the chemical “occurs in drinking-water at concentrations well below those at which toxic effects may occur” (WHO 2004).
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.
Ormad MP, Miguel N, Claver A, Matesanz JM, Ovelleiro JL (2008). Pesticides removal in the process of drinking water production. Chemosphere, 71: 97–106.
QFSS (Queensland Forensic and Scientific Services) (2009). Personal communication
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
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