Diquat (ion), Diquat dibromide
(endorsed 2011)
Guideline
Based on human health concerns, diquat in drinking water should not exceed 0.007 mg/L.
Related chemicals
Diquat (CAS 2764-72-9; 85-00-7) is a quaternary ammonium compound and belongs to the bipyridilium class of chemicals. The other pesticide in this class is paraquat (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, diquat would not be a health concern unless the concentration exceeded 0.007 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: Diquat is a contact herbicide for the control of aquatic weeds and weeds in agricultural crops. It is also used as a desiccant in seed crops and to clear weeds from bodies of water and sewerage systems.
There are currently products registered in Australia that contain diquat, mostly as diquat dibromide. Diquat products are intended for professional use and are available as concentrated solutions to be applied in diluted form using ground, aerial or hand-held 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 diquat and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
The agricultural use of diquat involves direct application into water ways and sewerage systems, which may then enter source water for drinking water.
Typical values in Australian drinking water
No reports of diquat in Australian drinking waters have been identified.
Treatment of drinking water
No specific data on the treatment of diquat in drinking water have been identified.
Measurement
Diquat can be measured in drinking water by solid phase extraction followed by high performance liquid chromatography with mass spectrometry, with a method detection limit of 0.1 µg/L (Grey et al. 2002, Nunez et al. 2004, Rial-Otero et al. 2006).
History of the health values
The current acceptable daily intake (ADI) for diquat is 0.002 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.2 mg/kg bw/day from a long-term study. The NOEL is based on cataract formation in a 2-year rat dietary study. The ADI incorporates a safety factor of 100. It was established in 1985 and reaffirmed in 2002.
The acute reference dose (ARfD) of 0.05 mg/kg bw/day for diquat was established in 2002, based on a NOEL of 26.5 mg/kg bw/day from an acute dietary study in dogs. The ARfD incorporates a safety factor of 500.
The previous health value was 0.005 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Diquat is poorly absorbed via the gastrointestinal tract. The absorbed fraction undergoes minimal metabolism before being rapidly excreted in the urine, almost completely within 48 hours.
Acute effects: Diquat has a high acute oral and dermal toxicity. It is a skin sensitiser in guinea pigs. Human poisoning incidents indicate the lowest lethal dose to be 6 g. Doses greater than 12 g are usually fatal.
Short-term effects: A 4-week dietary study in rats reported decreased bodyweight gain and food consumption as well as clinical biochemistry changes at doses of 17 mg/kg bw/day.
In a 3-week dermal study, rats were treated with diquat up to 80 mg/kg bw/day. The reported effects were irritation and tissue destruction at the application site at dose levels of 5 mg/kg bw/day. Systemic effects included decreased bodyweight gain, hypothermia and emaciation at the highest dose. Treatment-related increases in mortality were evident at 40 mg/kg bw/day.
Medium-term dietary studies were conducted in rats and dogs. In rats, there was increased inflammation of the tongue and palate and effects on the kidney. Cataract formation was reported at doses above 8.9 mg/kg bw/day. In dogs, the reported dose-related effects were tongue lesions, decreased appetite and decreased bodyweight gain.
Long-term effects: Two-year dietary studies were conducted in mice, rats and dogs. In mice, there was decreased bodyweight gain at doses of 4.2 mg/kg bw/day. In rats, there were effects in the kidney at doses of 10.7 mg/kg bw/day. Cataract formation was increased in both in rats and dogs at doses of 1 mg/kg bw/day and 1.7 mg/kg bw/day, respectively. The NOEL of 0.2 mg/kg bw forms the basis for the current ADI.
Carcinogenicity: Based on long-term studies in mice and rats, there is no evidence of carcinogenicity for diquat.
Genotoxicity studies: Diquat gave positive results in some in vitro short-term assays, but overall, it is not considered to be genotoxic.
Reproduction and developmental effects: A 3-generation reproduction study in rats and developmental studies in rats and rabbits did not produce any evidence of reproductive effects or teratogenicity. There were effects on maternotoxicity and foetal development only at dose levels well in excess of the likely level of human exposure.
Poisons Schedule: Diquat 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 its concentration and use. Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline value
The health-based guideline value of 0.007 mg/L for diquat was determined as follows:
where:
0.2 mg/kg bw/day is the NOEL based on a long-term (2-year) dietary study 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 diquat based on the available evidence that the chemical is rarely found in drinking water, even though it may be used as an aquatic herbicide. A provisional guideline value of 0.01 mg/L was established based on the practical limit of detection in water (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.
Grey L, Nguyen B, Yang P (2002). Liquid chromatography-electrospray ionization isotope dilution mass spectrometry analysis of paraquat and diquat using conventional and multilayer solid-phase extraction cartridges. Journal of Chromatography A, 958(1-2):25-33.
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.
Nunez O, Moyano E, Galceran MT (2004). Time-of-flight high resolution versus triple quadrupole tandem mass spectrometry for the analysis of quaternary ammonium herbicides in drinking water. Analytica Chimica Acta, 525(2):183-190.
Rial-Otero R, Cancho-Grande B, Perez-Lamela C, Simal-Gandara J, Arias-Estevez M (2006). Simultaneous determination of the herbicides diquat and paraquat in water. Journal of Chromatographic Science, 44(9):539-542.
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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