Chlorpyrifos
(endorsed 2011)
Guideline
Based on human health concerns, chlorpyrifos in drinking water should not exceed 0.01 mg/L.
Related chemicals
Chlorpyrifos (CAS 39475-55-3) is in the organophosphate class of chemicals. There are many other pesticides in this class, including fenthion, parathion, profenofos and ethoprofos (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, chlorpyrifos would not be a health concern unless the concentration exceeded 0.01 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: Chlorpyrifos is a broad-spectrum insecticide used to control a broad range of insect pests in many crops, the home and commercial sites.
There are approximately 100 products containing chlorpyrifos. These products are intended for professional and home garden use and are available in many different formulations for the large variety of use patterns. The most common application methods include vehicle-mounted equipment, hand-held equipment, and aircraft for spraying and soil injection. Aerial ultra-low volume (ULV) application is permitted.
Exposure sources: The main sources of public exposure to chlorpyrifos and its metabolites are home and garden uses, and residues in food. Residue levels in food produced according to good agricultural practice are anticipated to be generally low.
Agricultural use of chlorpyrifos may potentially lead to contamination of source waters through processes such as run-off, spray drift (especially from aerial application) or entry into groundwater.
Typical values in Australian drinking water
Surface water contamination arising from agricultural uses of chlorpyrifos are generally below 1 μg/L on the rare occasions that chlorpyrifos is detected in Australian surface waters (APVMA 2000). Extensive monitoring has been conducted in the cotton areas of northern New South Wales and the irrigation areas in southern New South Wales. There are a few high outliers, reaching 26 μg/L in northern rivers and 25 μg/L in irrigation drainage adjacent to rice bays in southern New South Wales, but these appear to be isolated occurrences, with such levels seldom detected because of the limited aquatic persistence of chlorpyrifos (APVMA 2000).
Treatment of drinking water
Chlorpyrifos has been shown to be completely removed from water by chlorination when the chlorine dose is adjusted to match chlorine demand. Nanofiltration has also been shown to be highly effective (Kiso et al, 2000).
Ozonation and activated carbon adsorption for chlorpyrifos removal has also been reported to have moderate success (Ormad et al. 2008). Conventional coagulation/flocculation has been shown to provide a relatively low removal rate.
More research is required to investigate the effectiveness of adsorption or oxidation methods. Jar testing to identify the effectiveness of various removal methods in specific waters is recommended if chlorpyrifos is detected.
Measurement
The practical limit of quantification for chlorpyrifos in water is 0.001 mg/L by liquid chromatography with tandem mass spectrometry (Alder et al. 2006).
History of the health values
The acceptable daily intake (ADI) for chlorpyrifos is 0.003 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.03 mg/kg bw/day. The NOEL is based on plasma cholinesterase inhibition in a 28-day human volunteer study. The ADI was established in 1998 and reaffirmed in 2000, and incorporates a safety factor of 10.
The acute reference dose (ARfD) of 0.1 mg/kg bw/day for chlorpyrifos was established in 2000, based on a NOEL of 1 mg/kg bw/day. The NOEL was based on inhibition of red blood cell acetylcholinesterase inhibition from a 3-day human volunteer oral study. The ARfD incorporates a safety factor of 10.
The previous health value was 0.01 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Chlorpyrifos is readily absorbed from the gastrointestinal tract, widely distributed throughout the body, and excreted rapidly in the urine and faeces. Only low tissue residues have been detected. The major urinary metabolite of chlorpyrifos identified was 3,5,6-trichloro-2-pyridinol (TCP).
Acute effects: Chlorpyrifos has moderate to high acute oral toxicity and low dermal toxicity. It is not a skin sensitiser. Clinical signs of acute poisoning were typical of cholinesterase inhibition and included hyperexcitability, salivation, bronchoconstriction, headache, vomiting and other behavioural changes.
Short-term and long-term effects: Short-term and long-term studies in mice, rats, dogs and monkeys resulted in symptoms indicative of central nervous system toxicity. The most sensitive effect observed was inhibition of plasma acetylcholinesterase in rats at an oral dose of 0.03 mg/kg bw/day. Effects at higher doses included reductions in bodyweight, and increased adrenal and liver weights. In a 28-day human study, the NOEL based on cholinesterase inhibition was 0.03 mg/kg bw/day. This NOEL was the basis for the ADI.
Carcinogenicity: Based on long-term studies in mice and rats, there is no evidence of carcinogenicity for chlorpyrifos.
Genotoxicity: There was no evidence of genotoxicity in short-term in vitro or in vivo studies.
Reproductive and developmental effects: A reproduction study in rats and developmental studies in rats and rabbits did not produce any evidence of effects on reproductive parameters or foetal development.
Neurotoxicity: There was no evidence of delayed neurotoxicity in chicken and rat studies, nor was there any evidence of developmental neurotoxicity in a developmental study in rats.
Poisons Schedule: Chlorpyrifos is included in Schedule 5 or 6 in the Standard for the Uniform Scheduling of Medicines and Poisons (SUSMP) No.1 (2010), depending on the concentration, formulation type or use. Current versions of the Poisons Standard should be consulted for further information.
Derivation of health-based guideline
The health-based guideline of 0.01 mg/L for chlorpyrifos was determined as follows:
where:
0.03 mg/kg bw/day is the NOEL based on a short-term (28-day) volunteer 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 a safety factor applied to the NOEL derived from human studies to allow for intraspecies variation.
The World Health Organization has a guideline value of 0.03 mg/L for chlorpyrifos (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.
Alder L, Greulich K, Kempe G, Vieth B (2006). Residue analysis of 500 high priority pesticides: better by GC-MS or LC-MS/MS?. Mass Spectrometry Reviews, 25(6):838-65.
APVMA (2000) Chlorpyrifos Interim Review Report. Available from https://apvma.gov.au/node/14736
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.
Kiso Y, Nishimura Y, Kitao T, Nishimura K (2000). Rejection properties of non-phenylic pesticides with nanofiltration membranes. Journal of Membrane Science, 171:229-237.
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.
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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