Propachlor
(endorsed 2011)
Guideline
Based on human health concerns, propachlor in drinking water should not exceed 0.07 mg/L.
Related chemicals
Propachlor (CAS 1918-16-7) belongs to the chloroacetamide class of chemicals. Other pesticides in this class include metolachlor and S-metolachlor (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, propachlor would not be a health concern unless the concentration exceeded 0.07 mg/L. Minor excursions above this level would need to occur over a significant period to be of 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: Propachlor is a herbicide for the control of annual grasses and certain broad-leaf weeds in sorghum and selected vegetable crops.
There is at least one registered product that contains propachlor in Australia. Propachlor products are intended for professional use, and are available as an aqueous concentrate to be diluted and applied as a surface spray. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to propachlor and its metabolites is residues in food. Residue levels in food produced according to good agricultural practice are generally low.
Agricultural use of propachlor 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 propachlor in Australian drinking waters have been identified.
Treatment of drinking water
Propachlor can be effectively oxidised in drinking water by suitable dosing chemical oxidants such as ozone, or by photocatalytic advanced oxidation processes (Konstantinou et al. 2002, Liu et al. 2008).
Measurement
No suitable analytical methods have been identified for the analysis of propachlor in drinking waters. However, sensitive liquid chromatography–tandem mass spectrometry methods are available for the analysis of the major environmental degradation products of propachlor and other related herbicides in drinking waters (Fuhrman and Allan 2002, Shoemaker and Bassett 2006).
History of the health values
The current acceptable daily intake (ADI) for propachlor is 0.02 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 2.0 mg/kg bw/day established in two long-term studies (an 18-month study in mice and a 2-year study in rats). The NOELs were based on increased relative liver weights in mice and increased absolute and relative weights of the thyroid and parathyroid glands in rats. The ADI incorporates a safety factor of 100, and was established in 1988.
Earlier ADIs established for propachlor were 0.1 and 0.002 mg/kg bw, set in 1972 and in 1987 respectively. The former ADI was based on a 90-day rat study in which the NOEL was 10 mg/kg bw/day; the latter was based on a NOEL of 3 mg/kg bw/day obtained in a 2-generation rat reproduction study. This NOEL was chosen in the absence of a NOEL derived from long-term toxicity studies.
The previous health value was 0.05 mg/L (NHMRC and NRMMC 2004).
Health considerations
Metabolism: Propachlor is absorbed readily and extensively in the gastrointestinal tract, with maximum blood concentration in 1 hour. It is metabolized rapidly and the major metabolites are excreted in the urine (68%) and faeces (19%). Biliary excretion followed by gut microflora metabolism and re-absorption is significant.
Acute effects: Propachlor has low acute oral and dermal toxicity. It is a skin sensitiser.
Short-term effects: Ninety-day dietary studies in mice, rats and dogs reported no clinical or pathological signs of toxicity at 930 mg/kg bw/day in mice, at 310 mg/kg bw/day in rats, or at 155 mg/kg bw/day in dogs.
Long-term effects: An 18-month dietary study in mice reported increases in liver weight at 7.5 mg/kg bw/day. A two-year dietary studies in rats reported increases in thyroid and parathyroid weights at 25 mg/kg bw/day. A 1-year dietary study in dogs reported a significant decrease in bodyweight in male dogs at 25 mg/kg bw/day. No clinical or pathological signs of toxicity were noted in any of these studies. The NOEL of 2 mg/kg bw/day from the long-term mouse and rat studies is the basis for the ADI.
Carcinogenicity: Based on long-term studies in mice, rats and dogs, there is no evidence of carcinogenicity for propachlor.
Genotoxicity: Propachlor is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: In a 2-generation reproduction study in rats and in a developmental study in rats, there was no evidence of reproductive or developmental effects. A developmental study in rabbits revealed post-implantation foetal losses and a subsequent decrease in the number of viable foetuses at 15 mg/kg bw/day, which was below the dose level causing maternal toxicity.
Poisons Schedule: Propachlor 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 0.07 mg/L for propachlor was determined as follows:
where:
2.0 mg/kg bw/day is the NOEL based on long-term dietary studies in mice (18 months) and rats (2 years)
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.
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.
Fuhrman JD, Allan JM (2002) Determination of acetanilide degradates in ground and surface waters by direct aqueous injection LC/MS/MS. In: Ferrer I, Thurman EM (eds), Symposium on Analysis of Emerging Contaminants Using Liquid Chromatography, Mass Spectrometry and MS held at the 225th National Meeting of the American-Chemical-Society, Orlando, Florida, pp. 256-272.
Konstantinou IK, Sakkas VA, Albanis TA (2002). Photocatalytic degradation of propachlor in aqueous TiO2 suspensions. Determination of the reaction pathway and identification of intermediate products by various analytical methods. Water Research, 36(11):2733-2742.
Liu C, Qiang ZM, Tian F, Zhang T (2008). Reactivity of several classes of pesticides with UV, ozone and permanganate. In: Wang X, Chen R (eds), International Conference on Advances in Chemical Technologies for Water and Wastewater Treatment, Xian, Peoples Republic of China, pp. 279-280.
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.
Shoemaker JA, Bassett MV (2006). Development of EPA method 535 for the determination of chloroacetanilide and other acetamide herbicide degradates in drinking water by solid-phase extraction and liquid chromatography/tandem mass spectrometry. Journal of AOAC International, 89(1):201-209.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th edition, British Crop Production Council, UK.
Last updated
