Trichlorobenzenes
1,2,3-trichlorobenzene (1,2,3-TCB); 1,2,4-trichlorobenzene (1,2,4-TCB); 1,3,5-trichlorobenzene (1,3,5-TCB) (endorsed 1996)
Guideline
Based on aesthetic considerations (taste and odour), the concentration of trichlorobenzenes in drinking water, either individually or in total, should not exceed 0.005 mg/L.
Trichlorobenzenes would not be a health concern unless the concentration exceeded 0.03 mg/L.
General description
Trichlorobenzenes are present in the environment mainly as a result of a variety of industrial processes. They have only occasionally been found in drinking water supplies overseas, and rarely above 0.001 mg/L. Food and air are the primary routes of exposure.
Taste and odour thresholds vary from 0.005 mg/L to 0.03 mg/L, depending on individual sensitivities and water temperature.
Industrial-grade TCB is more than 90% 1,2,4-TCB with the remainder 1,2,3-TCB. The compound has a wide variety of uses. It is used as a solvent for high-melting products, an electrical coolant, a lubricant and an insecticide, and in polyester dyeing and termiticide preparations.
Typical values in Australian drinking water
TCBs have not been found in Australian drinking waters. They are included here to provide guidance in the unlikely event of contamination, and because they have been detected occasionally in drinking water supplies overseas.
Treatment of drinking water
The concentration of TCBs in drinking water can be reduced by adsorption onto granular activated carbon.
Measurement
TCBs can be analysed using a solvent extraction procedure (USEPA Method 612 1984). The TCBs are extracted using dichloromethane and analysed using gas chromatography with electron capture detection. The limit of determination for 1,2,4-TCB is 0.00005 mg/L (50 ng/L). The purge and trap method can also be used (USEPA Draft Method 502.1 1986).
Health considerations
The TCBs are readily absorbed from the gastrointestinal tract and distributed in fat, skin and the liver. In rats and rabbits the TCBs are metabolised into trichlorophenols and mercapturic acids.
An extensive review and summary of the human and animal toxicity data for chlorobenzenes is available (IPCS 1991).
There are very few studies on the effects of human exposure. TCBs have caused marked irritation of the mucous membranes following inhalation over short periods of exposure. No data are available on the effects of long-term exposure.
Animal studies are of short-term duration. A 13-week study using rats reported that toxic effects of the three isomers were similar: low doses produced no adverse effects, but higher doses (77 mg/kg body weight per day) caused changes to the liver and thyroid.
No increase in the incidence of tumours was observed in longer-term animal studies. TCBs did not exhibit mutagenic activity in tests with bacteria.
Derivation of guideline
As the three TCBs have similar toxic effects, the guideline value can be based on the total concentration of all the TCBs rather than on the individual compounds. The health-based guideline value for total TCBs in drinking water was determined as follows:
where:
7.7 mg/kg body weight per day is the no-effect level from a 13-week dietary study using rats (Côté et al. 1988).
70 kg is the average weight of an adult.
0.1 is the proportion of total daily intake attributable to the consumption of water.
2 L/day is the average amount of water consumed by an adult.
1000 is the safety factor in using the results of an animal study as a basis for human exposure (10 for interspecies variations, 10 for intraspecies variations and 10 for a less than lifetime study).
This health-based guideline value exceeds the taste and odour threshold of 0.005 mg/L.
The World Health Organization guideline value of 0.02 mg/L was based on an adult body weight of 60 kg. The difference in guideline values is not significant.
References
Côté M, Chu I, Villeneuve DC, Secours VE, Valli VE (1988). Trichlorobenzenes: results of a 13 week feeding study in the rat. Drug and Chemical Toxicology, 11:11–28.
IPCS (International Programme on Chemical Safety) (1991). Chlorobenzenes other than hexachlorobenzene. Environmental Health Criteria, 128. World Health Organization, IPCS.
USEPA Method 612 (1984). Guidelines establishing test procedures for the analysis of pollutants under the Clean Water Act. Federal Register, 40, CFR Part 136, 43234–43442.
USEPA Draft Method 502.1 (1986). Volatile halogenated organic compounds in water by purge and trap gas chromatography. United States Environmental Protection Agency, Environmental Monitoring and Support laboratory (EMSL), Cincinnati, Ohio.
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