Pyrazophos
(endorsed 2011)
Guideline
Based on human health concerns, pyrazophos in drinking water should not exceed 0.02 mg/L.
Related chemicals
Pyrazophos (CAS 13457-18-6) belongs to the phosphorothiolate class of chemicals. There are no other pesticides in this class (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, pyrazophos 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: Pyrazophos is a fungicide for the control of powdery mildew in vegetable crops.
There are no registered products containing pyrazophos in Australia, but de-registered compounds may still be detected in water. When used previously, products containing pyrazophos were available as concentrated solutions and applied in diluted form using ground and aerial sprays.
Exposure sources: The main source of public exposure to pyrazophos, if used in the future, would be residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Typical values in Australian drinking water
No published reports on pyrazophos occurrence in Australian drinking water supplies were found.
Treatment of drinking water
Activated carbon can be added in a powder form to the coagulation processes or in a granular form as part of the filtration process for the removal of pyrazophos. Effective removal of organophosphorous pesticides has also been observed with softening, disinfection, membrane treatments, and in some cases air-stripping (USEPA 2001).
Measurement
Pyrazophos can be extracted from the water by liquid/liquid extraction with dichloromethane. The extract is dried with sodium sulfate, concentrated, and analysed by gas chromatography coupled with a nitrogen phosphorus detector and flame photometric detector. The method can achieve a limit of quantitation (LOQ) of 0.05 μg/L. A fully automated at-line solid-phase extraction–gas chromatography procedure can achieve a LOQ of 0.05 μg/L (Hankemeier et al. 1996). Solid-phase microextraction with high resolution gas chromatography and mass spectrometry can achieve a LOQ of 10 ng/mL (Souza et al. 2003).
History of the health values
The current acceptable daily intake (ADI) for pyrazophos is 0.007 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.07 mg/kg bw/day from a 10-day oral study in humans. The NOEL is based on headaches and decreased plasma cholinesterase activity at 0.15 mg/kg bw/day. The ADI incorporates a safety factor of 10, and was first established in 1991.
The previous health value was 0.03 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Pyrazophos is readily and extensively absorbed via the gastrointestinal tract in rats. It is extensively metabolised, primarily through sulfoxidation to form organophosphate compounds. Excretion is rapid, being almost complete by 72 hours and proceeding through urine.
Acute effects: Pyrazophos has moderate acute oral toxicity and low acute dermal toxicity. It is not a skin sensitiser.
Short-term effects: In a 28-day dietary study in rats, there was serum cholinesterase inhibition at 0.25 mg/kg bw/day, and erythrocyte cholinesterase inhibition at 0.75 mg/kg bw/day. In a 10-day oral study in humans, plasma cholinesterase activity was decreased at 0.15 mg/kg bw/day. The NOEL was 0.07 mg/kg bw/day and this is the basis of the ADI.
Three-month dietary studies were conducted in rats and dogs. In rats, there was decreased erythrocyte cholinesterase activity at 0.48 mg/kg bw/day and an increase in absolute adrenal and spleen weights at 8 mg/kg bw/day. In dogs, there was decreased erythrocyte and plasma cholinesterase activity at 0.25 mg/kg bw/day and decreased physical activity and anaemia at 16 mg/kg bw/day.
Long-term effects: Long-term dietary studies in mice, rats and dogs showed decreased plasma and erythrocyte cholinesterase activity at 0.7 mg/kg bw/day in mice, 1 mg/kg bw/day in rats, and 0.25 mg/kg bw/day in dogs.
Carcinogenicity: Based on a 2-year study in rats and an 18-month study in mice, there is no evidence of carcinogenicity for pyrazophos.
Genotoxicity: Pyrazophos 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 and development studies in rats and rabbits did not produce any evidence for effects on reproductive parameters or foetal development.
Neurotoxicity: A delayed neurotoxicity study in hens using doses up to 150 mg/kg bw found no evidence for delayed neurotoxicity from pyrazophos.
Poisons Schedule: Pyrazophos is included in Schedule 6 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 pyrazophos was determined as follows:
where:
0.07 mg/kg bw/day is the NOEL based on a short-term (10-day) oral dosing study in human volunteers.
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 a study in humans, to allow for variation of response within the human population.
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.
Hankemeier T, Steketee E, Vreuls JJ, Brinkman UAT (1996). Automated at-line solid-phase extraction-gas chromatographic analysis of micropollutants in water using the PrepStation. Journal of Chromatography A, 750:161-174.
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.
Souza DA, Lancas FM (2003). Solventless sample preparation for pesticides analysis in environmental water samples using solid-phase microextraction-high resolution gas chromatography/mass spectrometry (SPME-HRGC/MS). Journal of Environmental Science and Health B, 38(4):417-28,
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
USEPA (United States Environmental Protection Agency) (2001). FIFRA Scientific Advisory Panel Meeting. Session VI- A Set of Scientific Issues Being Considered by the Environmental Protection Agency Regarding: Progress Report on Estimating Pesticide Concentrations in Drinking Water and Assessing Water Treatment Effects on Pesticide Removal and Transformation: A Consultation. Arlington, Virginia.
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