Case-Control Study of Colon Cancer and Drinking Water Trihalomethanes in Wisconsin

A case-control study was conducted to test the hypothesis that chronic ingestion of trihalomethanes (THMs), occurring as chlorination byproducts in drinking water, carries a risk of colon cancer. Lifetime residential and water source histories and information on water-drinking habits, diet, sociodemographics, medical and occupation histories, lifestyle and other factors were obtained by questionnaire from a statewide sample of newly-diagnosed colon cancer cases (N = 347), controls with cancer of other sites (N = 639) and general population controls (N = 611). Since no data on past THM levels exists, it was necessary to devise a scheme to generate THM estimates for all Wisconsin water sources. For this, a statistical model based on quantitative THM measures and routinely-recorded data taken at 81 municipal water facilities was used in conjunction with individual residential histories to estimate lifetime and period-specific THM exposure for each case and control. Logistic regression was used to estimate odds ratios adjusted for age, sex and urban living, for colon cancer and THM exposure. The study results indicate that THM in Wisconsin drinking water does not pose a significant colon cancer risk. Odds ratios for exposure to the middle and highest category of lifetime cumulative THM were 1.05 (95% Cl = 0.66–1.68) and 0.93(95% Cl = 0.55–1.57) respectively, relative to the cancer control group, and 1.10 (95% Cl = 0.68–1.78) and 0.73 (95%Cl = 0.44–1.21) respectively, relative to the general population controls. Odds ratios based on other THM exposure variables, including cumulative THM indices for 10–30 year periods and dichotomous surrogate variables, were close to 1.00 with few exceptions, consistent with the conclusion that THM exposure is not associated with colon cancer.