Page 11: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trichloroethylene

10.0 Rationale

Both cancer and non-cancer endpoints were considered in the derivation of the MAC of 0.005 mg/L (5 µg/L) for TCE in drinking water.

Animal studies have shown links between TCE exposure and various types of tumours in both rats (kidney and testicular) and mice (pulmonary and liver). Evidence for carcinogenicity resulting from the ingestion of TCE through drinking water is further supported by epidemiological studies that suggest a positive correlation between exposure to TCE and cancer in humans, although the presence of other chemicals was a confounding factor in confirming the association. However, further research is needed to better identify the specific agent(s) that confer this health risk and to estimate the magnitude of this risk. TCE has been classified as "probably carcinogenic to humans," since the evidence of carcinogenicity observed in animal and epidemiological studies suggests a positive association between TCE exposure and cancer.

The cancer risk assessment for TCE is based on kidney tumours observed in male and female rats. Such tumours have also been observed in some epidemiological studies in occupationally exposed industrial workers. The LMS method was used to calculate unit risks for the kidney tumour types observed in rats. A MAC for TCE in drinking water of 0.022 mg/L (22 µg/L) can be derived based on the cancer risk assessment. This assessment assumes a "de minimis" cancer risk level of 10-6, which is considered to be "essentially negligible."

Choice of the developmental toxicity study for the non-cancer risk assessment was based on several factors: the appropriateness of the vehicle used (drinking water); the low dose at which the effects were observed (which coincides with the lowest adverse effect level in all animal studies reviewed); the severity of the endpoint (heart malformations) and existing evidence for similar effects (e.g., cardiac anomalies) from epidemiological studies; and the observation of similar malformations resulting from exposure to metabolites of TCE. The BMD approach was used to estimate the NOAEL, which takes into account the LOAEL observed in the key study. A MAC for TCE in drinking water of 0.005 mg/L (5 µg/L) can be derived based on the developmental effect observed.

The lower of the two calculated MACs (0.005 mg/L) is selected as the guideline value, as it is protective for both cancer and non-cancer endpoints. The MAC is measurable by available analytical methods and achievable by both municipal-scale and residential-scale treatment technologies.

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