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 b y-products in public water supplies and cancers of the colon and rectum, In terviews providing residence and water source histories were completed by 7 6% 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 c umulative exposure to THMs, duration of exposure to chlorinated surface wat er, 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 3 5-40 years had an increased risk of colon cancer compared with those expose d for <10 years (odds ratio, 1.53; 95% confidence interval, 1.13-2.09), Mal es exposed to an estimated THM level of 75 mu g/liter for greater than or e qual to 35 years had double the risk of those exposed for <10 years (odds r atio, 2.10; 95% confidence interval, 1.21-3.66), In contrast, these relatio nships were not observed among females. No relationship was observed betwee n rectal cancer risk and any of the measures of exposure to chlorination by products. The results of this study should be interpreted with caution beca use they are only partially congruent with the limited amount of literature addressing this issue.