Pendimethalin
(endorsed 2011)
Guideline
Based on human health concerns, pendimethalin in drinking water should not exceed 0.4 mg/L.
Related chemicals
Pendimethalin (CAS 40487-42-1) belongs to the dinitroaniline class of chemicals. Other pesticides in this class include trifluralin and oryzalin (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, pendimethalin would not be a health concern unless the concentration exceeded 0.4 mg/L. While the health-based guideline is based on long-term effects, there is evidence for maternotoxicity after short-term exposure at levels near the no-observed-effect level (NOEL) used to derive the health-based guideline. Therefore, excursions above the health-based guideline even for a short period are of concern.
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: Pendimethalin is a selective herbicide for the control of broad-leaf weeds and annual grasses in soybean, cotton, wheat, barley, vegetable and turf crops.
There are registered products that contain pendimethalin in Australia. Pendimethalin products are intended for professional use. They are in emulsifiable concentrate or granular formulations, and are applied by aerial spray or ground spray to crops or soil, or mixed with irrigation water for use on fields. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to pendimethalin and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of pendimethalin 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 pendimethalin in Australian drinking waters have been identified.
Treatment of drinking water
No specific data on the treatment of pendimethalin in drinking water have been identified.
Measurement
Pendimethalin can be determined in drinking waters by gas chromatography-mass spectrometry. The typical limit of detection is 0.01 µg/L (WHO 2004).
History of the health values
The current acceptable daily intake (ADI) for pendimethalin is 0.1 mg per kg of bodyweight (mg/kg bw), based on a NOEL of 12.5 mg/kg bw/day from a long-term (2-year) dietary study in dogs. The NOEL is based on decreased bodyweight gain and evidence of mild effects on the liver. The ADI incorporates a safety factor of 100, and was first established in 1987.
The previous health value was 0.3 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Pendimethalin is poorly absorbed in the gastrointestinal tract, with most excreted unchanged in the faeces and the remainder being extensively metabolised and rapidly excreted in urine. Excretion is complete by 96 hours.
Acute effects: Pendimethalin has low acute oral and dermal toxicity. It is not a skin sensitiser in guinea pigs.
Short-term effects: In 30-day dietary studies in rats and dogs, decreased bodyweight gain, and increased absolute liver weights and blood glucose levels were seen in rats at doses of 320 mg/kg bw/day and above. Toxic effects were not seen in dogs even up to high exposure levels of 1000 mg/kg bw/day.
In a 3-month dietary study with pendimethalin in dogs, the only effect was a small decrease in bodyweight gain at the highest dose tested, 1000 mg/kg bw/day.
Long-term effects: In long-term dietary studies in mice, rats and dogs, decreased bodyweight gain and mild liver effects (increased serum alkaline phosphatase, mild inflammation, biliary hyperplasia, hepatic haemosiderosis) were seen in all three species at various doses. The lowest overall NOEL was 12.5 mg/kg/day from a 2-year dietary study in dogs. This NOEL is the basis for the current ADI.
Carcinogenicity: Based on long-term studies in mice and rats, there is no evidence of carcinogenicity for pendimethalin.
Genotoxicity: Pendimethalin is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: In 2- and 3-generation studies in rats, and in developmental studies in rats and rabbits, there was no evidence of effects on reproductive parameters or on foetal development. However, maternotoxicity in these studies at 30 mg/kg bw/day is close to the NOEL used as a basis for the ADI.
Poisons Schedule: Pendimethalin is included in Schedule 5 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 0.4 mg/L for pendimethalin was determined as follows:
where:
12.5 mg/kg bw/day is the NOEL based on a long-term (2-year) 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.
The World Health Organization has a health-based guideline value of 0.02 mg/L for pendimethalin (WHO 2004). This was based on an ADI of 5 mg/kg bw and an uncertainty factor of 1000 (100 for inter- and intra-species variation and 10 for a combination of the use of a lowest-observed-adverse-effect level instead of a no-observed-adverse-effect level and limitations of the database).
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.
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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