Case-control study of colon and rectal cancers and chlorination by-products in treated water

This population-based case-control study was conducted in southern Ontario, Canada from 1992 to 1994 to assess the relationship between chlorination by-products in public water supplies and cancers of the colon and rectum, Interviews providing residence and water source histories were completed by 76% of eligible cancer cases and 72% of eligible controls. Supplemental data from municipal water supplies were used to estimate individual exposure to water source, chlorination status, and byproduct levels as represented by trihalomethanes (THMs) during the 40-year period before the interview. The analyses included 767 colon cases, 661 rectal cases, and 1545 controls with exposure information for at least 30 of these years (75% of subjects with completed interviews). Among males, colon cancer risk was associated with cumulative exposure to THMs, duration of exposure to chlorinated surface water, and duration of exposure to a THM level greater than or equal to 50 mu g/liter and 75 mu g/liter. Males exposed to chlorinated surface water for 35-40 years had an increased risk of colon cancer compared with those exposed for <10 years (odds ratio, 1.53; 95% confidence interval, 1.13-2.09), Males exposed to an estimated THM level of 75 mu g/liter for greater than or equal to 35 years had double the risk of those exposed for <10 years (odds ratio, 2.10; 95% confidence interval, 1.21-3.66), In contrast, these relationships were not observed among females. No relationship was observed between rectal cancer risk and any of the measures of exposure to chlorination byproducts. The results of this study should be interpreted with caution because they are only partially congruent with the limited amount of literature addressing this issue.