Methidathion
(endorsed 2011)
Guideline
Based on human health concerns, methidathion in drinking water should not exceed 0.006 mg/L.
Related chemicals
Methidathion (CAS 950-37-8) belongs to the organophosphate class of chemicals. There are many other pesticides in this class, which includes terbufos, ethion, fenamiphos, and acephate (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, methidathion would not be a health concern unless the concentration exceeded 0.006 mg/L. Excursions above this level even for a relatively short period are of concern as the health-based guideline is based on medium-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: Methidathion is an insecticide for the control of insect pests in orchards, sub-tropical, vegetable, cereal, pasture, cotton, and sunflower agricultural crops.
There are registered products containing methidathion in Australia. The products are intended for professional use, and are available in concentrated solutions, which are diluted and applied to infested areas by ground or aerial spray. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to methidathion is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of methidathion 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
Methidathion was not detected in the Mt Lofty Ranges, the main catchment area for Adelaide’s drinking water supply (Oliver et al. 2005). No other published reports on methidation occurrence in Australian drinking water supplies were found. Methidathion exposures from water are expected to be non-significant, based on pesticide tolerances (USEPA 2002) and drinking water risk assessment reports (Lewis 2001).
Treatment of drinking water
Water treatment processes such as granular activated carbon and membranes are capable of removing methidation (USEPA 2000). During chlorination in water treatment plants, methidathion can be oxidised forming oxons and/or other oxidation analogues (Kamel et al. 2009).
Measurement
Methidathion can be extracted from water by liquid/liquid and analysed by gas chromatography coupled with a nitrogen phosphorus detector and flame photometric detector. The method can achieve a limit quantitation (LOQ) of 0.05 μg/L. Methidathion can also be extracted by polypropylene hollow fiber liquid phase microextraction and analysed by gas chromatography with flame thermionic detection. The method can achieve a LOQ of 3 ng/L (Lambropoulou and Albanis 2005). Methidathion can also be accurately quantified by solid-phase extraction (SPE) and liquid chromatography (LOQ 0.01 μg/L) (Wang et al. 2007). SPE with multiwalled carbon nanotubes, without the need for chromatographic separation, can achieve a LOQ of 3 μg/L for methidathion (Al-Degs et al. 2009). SPE with liquid chromatography and tandem mass spectrometry reported a LOQ of 2.3 μg/L for methidathion (Rodrigues et al. 2007).
History of the health values
The current acceptable daily intake (ADI) for methidathion is 0.002 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.16 mg/kg bw/day from a 90-day dietary study in dogs. The NOEL is based on increased liver cholesterol and decreased red blood cell cholinesterase activity at doses of 1.96 mg/kg bw/day and above. The ADI incorporates a safety factor of 100 and was established in 2004.
The acute reference dose (ARfD) of 0.01 mg/kg bw/day for methidathion was established in 2004, based on a NOEL of 1 mg/kg bw/day from an acute neurotoxicity study based on inhibition of red blood cell and brain cholinesterase activity in rats. The ARfD incorporates a safety factor of 100.
The previous health value was 0.03 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Methidathion is readily and extensively absorbed via the gastrointestinal tract in rats. It is extensively metabolised via hydrolysis and oxidation to sulfoxides, sulfones, and oxons. Excretion is via the urine and is almost complete within 24 hours.
Acute effects: Methidathion has a moderate to high acute oral toxicity in dogs, rats and mice, and a moderate acute dermal toxicity in rats. It is not a skin sensitiser.
Short-term effects: In a 21-day dermal toxicity study in rabbits, there was decreased red blood cell and brain cholinesterase activity at 10 mg/kg bw/day. In a 4-week oral toxicity study in rats, there was decreased red blood cell cholinesterase activity at 0.83 mg/kg bw/day. In 90-day oral toxicity studies in rats and dogs, there was decreased red blood cell cholinesterase activity and increased levels of cholesterol in the liver at 2 mg/kg bw/day. The lowest overall NOEL was 0.16 mg/kg bw/day in dogs. This is the basis for the current ADI.
In a 6-week oral toxicity study in humans, there was no effect on cholinesterase activity and no clinical signs of toxicity at 0.11 mg/kg bw/day.
Long-term effects: In a 2-year oral toxicity study in mice, there was no evidence of toxicity apart from clinical signs of cholinesterase inhibition at 7 mg/kg bw/day. In 2-year oral toxicity studies in rats and monkeys, there was decreased plasma and red blood cell cholinesterase activity at 1.7 mg/kg bw/day in rats and at 1.0 mg/kg bw/day in monkeys. In a 1-year oral toxicity study in dogs, there were decreases in red blood cell cholinesterase activity and evidence of liver toxicity at 1.3 mg/kg bw/day.
Carcinogenicity: Based on 2-year studies in mice and rats, there is no evidence of carcinogenicity for methidathion.
Genotoxicity: Methidathion is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: Two- and 3-generation reproduction studies in rats, and developmental studies in rats and rabbits did not produce any evidence of effects on reproductive parameters or foetal development.
Neurotoxicity: In a 21-day neurotoxicity study in hens, there was no evidence of delayed neurotoxicity.
Poisons Schedule: Methidathion is included in Schedule 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 0.006 mg/L for methidathion was determined as follows:
where:
0.16 mg/kg bw/day is the NOEL based on a medium-term (90-day) dietary study in dogs.
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.
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.
Al-Degs YS, Al-Ghouti MA, El-Sheikh AH (2009). Simultaneous determination of pesticides at trace levels in water using multiwalled carbon nanotubes as solid-phase extractant and multivariate calibration. Journal of Hazardous Materials, 169(1-3):128-35.
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.
Kamel A, Byrne C, Vigo C, Ferrario J, Stafford C, Verdin G, Siegelman F, Knizner S, Hetrick J (2009). Oxidation of selected organophosphate pesticides during chlorination of simulated drinking water. Water Research, 43(2):522-34.
Lambropoulou DA, Albanis TA (2005). Application of hollow fiber liquid phase microextraction for the determination of insecticides in water. Journal of Chromatography A, 1072(1):55-61.
Lewis CM (2001). Methidathion: Dietary and drinking water risk assessment. Medical Toxicology Branch, Department of Pesticide Regulation, Sacramento, CA,
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.
Oliver D, Kookana R (2005). Pesticide use in the 6th Creek Sub-catchment, Mt. Lofty Ranges, S.A. and Assessment of Risk of Off-site Movement using Pesticide Impact Rating Index (PIRI). CSIRO Land and Water Technical Report, Adelaide.
Rodrigues, A. M., V. Ferreira, V. V. Cardoso, E. Ferreira and M. J. Benoliel. 2007. Determination of several pesticides in water by solid-phase extraction, liquid chromatography and electrospray tandem mass spectrometry. J Chromatogr A 1150(1-2), 267-78
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
USEPA (United States Environmental Protection Agency) (2000). Components and Methodologies of the Calendex™, Lifeline™, and REx Models as Tools for Dietary and Residential Pesticide Exposure and Risk Assessments; and Assessment of Pesticide Concentrations in Drinking Water and Water Treatment Effects on Pesticide Removal and Transformation. Session VI – Progress Report on Estimating Pesticide Concentrations in Drinking Water and Assessing Water Treatment Effects on Pesticide Removal and Transformation.
USEPA (United States Environmental Protection Agency) (2002). Interim Reregistration Eligibility Decision for Methidathion. List A, Case 0034. Washington, DC, Office of Prevention, Pesticides and Toxic Substances, USEPA.
Wang, S., P. Zhao, G. Min and G. Fang. 2007. Multi-residue determination of pesticides in water using multi-walled carbon nanotubes solid-phase extraction and gas chromatography-mass spectrometry. J Chromatogr A 1165(1-2), 166-71.
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