Dichlorprop/Dichlorprop-P
(endorsed 2011)
Guideline
Based on human health concerns, dichlorprop in drinking water should not exceed 0.1 mg/L.
Related chemicals
Dichlorprop (CAS 7547-66-2/CAS 15165-67-0) is in the phenoxycarboxylic acid group of chemicals. Dichlorprop possesses a single asymmetric carbon and is a chiral molecule. Dichlorprop is a racemic mixture of stereoisomers and dichlorprop-P is the R-isomer of dichlorprop. Other pesticides in this group include 2,4-D, MCPA and mecoprop (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, dichlorprop would not be a health concern unless the concentration exceeded 0.1 mg/L. Excursions above this level even for a relatively short period are of concern as the health-based guideline is based on short- to 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: Dichlorprop is a herbicide used for the control of weeds and as a plant growth regulator for citrus fruits.
There are registered product containing either dichlorprop (as the potassium salt) or dichlorprop-P (as the 2-ethylhexyl ester) in Australia. These are for professional use and applied by spray (backpack, boom spray or helicopter) to citrus fruits or non-crop areas. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to dichlorprop is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use 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
There are few available data for dichlorprop in Australian drinking water. In Australian treated sewage, dichlorprop was below 0.5 µg/L (supporting data, NRMMC/EPHC/NHMRC 2008). In Canada, the maximum concentration in drinking water was 0.10 µg/L (Donald et al. 2007).
Treatment of drinking water
Although no empirical data are currently available, it is likely that activated carbon would provide efficient removal based on its chemical structure.
Measurement
Dichlorprop in water can be analysed by gas chromatography with mass spectrometry, with a detection limit of 0.42 ng/L (Donald et al. 2007).
History of the health values
The acceptable daily intake (ADI) for dichlorprop (R and S isomers) is 0.03 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 3.1 mg/kg bw/day from a 13-week study in dogs. The NOEL was based on changes in clinical chemistry and kidney discolouration. The ADI incorporates a safety factor of 100 and was established in 1998.
The ADI for dichlorprop-P is 0.03 mg/kg bw, based on a NOEL of 6 mg/kg bw/day in an 18-month mouse dietary study. The NOEL is based on chronic necropathy. The ADI incorporates a safety factor of 200 and was established in 2006.
An acute reference dose (ARfD) has not been set for dichlorprop. An acute reference dose (ARfD) of 0.20 mg/kg bw/day for dichlorprop-P was established based on a NOEL of 20 mg/kg bw/day from a developmental study in rats. The ARfD incorporates a safety factor of 100.
A health value has not been previously established by NHMRC for dichlorprop-P.
Health considerations
The toxicology data on dichlorprop-P largely form the basis for the public health standards for both of these chemicals.
Metabolism: Dichlorprop-P is rapidly and extensively absorbed via the gastrointestinal tract in rats. There is limited metabolism of dichlorprop and no evidence of tissue accumulation. It is largely excreted unchanged in the urine within 24-48 hours. Metabolism and toxicokinetic data are similar for the dichlorprop-P acid and 2-ethylhexylester forms.
Acute effects: Dichlorprop-P has low acute oral and dermal toxicity in rats. It is not a skin sensitiser.
Short-term effects: Short-term studies in mice reported effects on the liver and on blood cholesterol at very high dose levels only. Studies in rats and dogs also reported effects on the kidney and liver at high dose levels. Blood lipids were decreased in dogs at 17 mg/kg bw/day.
Long-term effects: Long-term dietary studies in mice and dogs reported the kidney as the main target organ. The lowest NOEL was 6 mg/kg bw/day in mice and this NOEL is the basis of the ADI.
Carcinogenicity: Based on long-term studies in mice, there is no evidence of carcinogenicity for dichlorprop-P.
Genotoxicity: Dichlorprop-P is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Neurotoxicity: In an oral single dose and a dietary repeat dose study in rats, there was no evidence of delayed neurotoxicity.
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.
Poisons Schedule: Dichlorprop and dichlorprop-P are 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 health-based guideline
The health-based guideline of 0.1 mg/L for dichlorprop and dichlorprop-P was determined as follows:
where:
6 mg/kg bw/day is the NOEL for dichlorprop-P based on a long-term (18-month) dietary study in mice.
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.
200 is a safety factor applied to the NOEL derived from a study conducted in animals. The safety factor incorporates a factor of 10 for interspecies extrapolation, a factor of 10 for intraspecies variation, and an additional factor of 2 for uncertainty in the toxicology database.
The World Health Organization has established a health-based guideline value of 0.1 mg/L for dichlorprop, in 1993 (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.
Donald DB, Cessna AJ, Sverko E, Glozier NE (2007). Pesticides in surface drinking-water supplies of the Northern Great Plains. Environmental Health Perspectives, 115:1183-1191.
NRMMC (Natural Resource Management Ministerial Council), EPHC (Environment Protection and Heritage Council), NHMRC (National Health and Medical Research Council) (2008). Australian Guidelines for Water Recycling: Managing health and environmental risks (Phase 2) - Augmentation of drinking water supplies. NRMMC, EPHC, NHMRC.
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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