Trihalomethanes (THMs)
trichloromethane (chloroform); bromodichloromethane; dibromochloromethane; tribromomethane (bromoform) (endorsed 1996)
Guideline
Based on health considerations, the concentration of trihalomethanes, either individually or in total, in drinking water should not exceed 0.25 mg/L.
Trihalomethane concentrations fluctuating occasionally (for a day or two annually) up to 1 mg/L are unlikely to pose a significant health risk.
Action to reduce THMs is encouraged, but must not compromise disinfection, as nondisinfected water poses significantly greater risk than THMs.
General description
In Australia, trihalomethanes are present in drinking water principally as the result of disinfection using chlorination or, to a much lesser extent, chloramination. Chlorine, which produces hypochlorous acid when added to water, can react with naturally occurring organic material, such as humic and fulvic acids, to produce trihalomethanes. The brominated trihalomethanes are produced by the oxidation of bromide present in water to form hypobromous acid, which can then react with organic matter in a similar way.
High trihalomethane concentrations may indicate the presence of other chlorination by-products.
Chloroform is produced commercially and is an important solvent. It is used in the manufacture of refrigerants, and as an ingredient in pharmaceutical and cosmetic preparations. Brominated trihalomethanes are also produced industrially, but less commonly than chloroform.
Typical values in Australian drinking water
In major Australian reticulated supplies concentrations of total trihalomethanes range up to 0.6 mg/L.
Limiting presence in drinking water
The concentration of trihalomethanes can be minimised by removing precursors, by removing trihalomethanes after formation, or by using alternative disinfectants. Precursors can be removed by activated carbon, by coagulation followed with filtration, or by oxidation with ozone or potassium permanganate. Once produced, trihalomethanes can be removed with air stripping or adsorption onto granular activated carbon. Alternative disinfection agents to chlorine, such as chloramines, ozone and chlorine dioxide, can substantially reduce trihalomethane concentrations, but may produce other by-products.
Measurement
There are a number of methods available for the analysis of trihalomethanes, including head-space analysis, solvent extraction, purge and trap, and direct collection on resins. The solvent extraction procedure is relatively simple to use (USEPA Draft Method 551 1990). Sodium chloride is added to the sample and the trihalomethanes extracted using methyl tert-butyl ether. The extracts are then analysed using gas chromatography with electron capture detection. Limits of determination are 0.00002 mg/L (20 ng/L) or less.
Health considerations
The trihalomethanes are rapidly and efficiently absorbed following ingestion. They are metabolised primarily to carbon dioxide and/or carbon monoxide, and rapidly exhaled. They are fat soluble, and accumulate in tissues with the highest lipid content (such as adipose tissue, brain, kidney and blood).
In animals, the trihalomethanes are central nervous system depressants and liver and kidney toxicants. Chloroform and bromoform are also known to cause central nervous system depression in humans.
Some epidemiological studies have reported associations between the ingestion of chlorinated drinking water (which typically contains THMs) and increased cancer mortality rates. The International Agency for Research on Cancer has concluded that the available data for chlorinated water provide inadequate evidence of carcinogenicity in humans (Group 3, inadequate evidence in humans and limited evidence in animals) (IARC 1991).
Long-term carcinogenicity bioassays with animals have shown that trihalomethanes can produce tumours in rats and mice, but only at doses that are toxic to the animals. Chloroform increased the incidence of liver tumours in mice when administered in food at doses from 25 mg/kg body weight per day, but not in drinking water at the same doses, and has induced kidney tumours in male rats at doses from 263 mg/kg body weight per day. Dibromochloromethane, given by gavage 5 days per week, clearly induced liver tumours in female mice at 100 mg/kg body weight per day, and possibly in male mice, but not in rats. Bromodichloromethane, given by gavage 5 days per week, induced kidney tumours in rats at 100 mg/kg body weight, and in male mice at 50 mg/kg body weight; a rare tumour of the large intestine in male rats at doses of 50 and 100 mg/kg body weight; and liver tumours in female mice at 75 and 150 mg/kg body weight. Bromoform induced a small increase in relatively rare tumours of the large intestine in rats at a gavage dose of 200 mg/kg body weight per day, but not in mice.
Results of studies on the genotoxicity of trihalomethanes in bacteria have been inconsistent, with most reporting negative results. Trihalomethanes have, however, induced chromosomal aberrations in human lymphocyte cells in vitro, and in mouse bone-marrow cells in vivo.
Available studies indicate that THMs can produce maternal and foetal toxicity at high doses, but not teratogenicity.
The International Agency for Research on Cancer has concluded that chloroform and bromodichloromethane are possibly carcinogenic to humans (Group 2B, inadequate evidence in humans but sufficient evidence in animals); and that bromoform and dibromochloromethane are not classifiable as to their carcinogenicity to humans (Group 3, inadequate evidence in humans and limited evidence in animals)(IARC 1991).
Derivation of guideline
The guideline value for trihalomethanes in drinking water was determined as follows:
where:
7 mg/kg body weight per day is the no-effect level based on a 90-day study using rats (Chu et al. 1982). The use of this value was recommended by the NHMRC Standing Committee on Toxicity following a review of the available toxicity data for THMs.
70 kg is the average weight of an adult.
0.1 is the proportion of total daily intake attributable to the consumption of water. A higher value was not used because exposure to chloroform from other sources may be significant.
2 L/day is the average amount of water consumed by an adult.
100 is the safety factor in using the results of an animal study as a basis for human exposure (10 for interspecies variations and 10 for intraspecies variations). An additional safety factor for the less than lifetime study was not applied as long-term studies have reported the same effects, and no other forms of toxicity were observed. In addition, the changes observed to the liver at higher doses were mild in nature and disappeared when exposure stopped. The use of this safety factor was recommended by the NHMRC Standing Committee on Toxicity.
Separate guideline values were not derived for each compound as the no-effect levels were similar (ranging from 6.5 to 7.8 mg/kg body weight per day), and the compounds are metabolised in the body in similar ways. The guideline value should therefore apply to the concentration of each compound, or the sum of any combination of individual THM concentrations.
The World Health Organization (WHO) has derived separate guideline values for each compound, but in doing so recognises that the compounds have similar toxicological action.
The WHO guideline values for chloroform (0.2 mg/L) and bromodichloromethane (0.06 mg/L) were based on calculations that estimated additional lifetime risks of one fatal cancer per 100,000 people. The use of this approach is questionable because there is evidence that tumours do not occur at low concentrations.
The WHO guideline values for bromoform (0.1 mg/L) and dibromochloromethane (0.1 mg/L) were based on different studies and safety factors from those recommended by the NHMRC Standing Committee on Toxicity, although toxicological effects were similar.
It is recommended that future reviews of the Guidelines consider the various THMs individually, as data are emerging that suggest the different THMs have different toxic effects. Data were not sufficient at the time of this review to justify individual assessments
In view of the safety factors used in the derivation of the guideline value, it is unlikely that short-term consumption of water containing significantly higher concentrations of trihalomethanes would pose a health risk.
References
Chu I, Villeneuve DC, Secours VE, Becking GC, Valli VE (1982). Trihalomethanes: II Reversibility of toxicological changes produced by chloroform, bromodichloro-methane, chlorodibromomethane and bromoform in rats. Journal of Environmental Science and Health, B17:225–240.
IARC (International Agency for Research on Cancer) (1991). IARC monographs on the evaluation of carcinogenic risks to humans: Chlorinated drinking water; chlorination by-products; some other halogenated compounds; cobalt and cobalt compounds. World Health Organization, IARC, 52, Lyons.
USEPA Draft Method 551 (1990). Determination of chlorination disinfection by-products and chlorinated solvents in drinking water by liquid–liquid extraction and gas chromatography with electron capture detection. United States Environmental Protection Agency, Environmental Monitoring and Support Laboratory (EMSL), Cincinnati, Ohio.
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