Disinfection by-products in drinking water and risk of colorectal cancer: a population-based cohort study

Background: Colorectal cancer is the third most common malignancy worldwide and is strongly linked to lifestyle and environmental risk factors. Although several drinking-water disinfection by-products are confirmed rodent carcinogens, the evidence in humans for carcinogenicity associated with these by-products, including colorectal cancer, is still inconclusive. Methods: We assessed the association of long-term exposure to trihalomethanes (THMs), the most prevalent disinfection by-products in chlorinated drinking water, with incidence of colorectal cancer in 58 672 men and women in 2 population-based cohorts. Exposure was assessed by combining long-term information of residential history with drinking water-monitoring data. Participants were categorized according to no exposure, low exposure (<15 mu g/L), and high exposure (>= 15 mu g/L). Incident cases of colorectal cancer were ascertained by use of the Swedish National Cancer Register. Results: During an average follow-up of 16.8 years (988 144 person-years), 1913 cases of colorectal cancer were ascertained (1176 cases in men and 746 in women, respectively). High THM concentrations in drinking water (>= 15 mu g/L) were associated with increased risk of colorectal cancer in men (hazard ratio = 1.26, 95% confidence interval = 1.05-1.51) compared with no exposure. When subsites were assessed, the association was statistically significant for proximal colon cancer (hazard ratio = 1.59, 95% confidence interval = 1.11 to 2.27) but not for distal colon cancer or rectal cancer. In women, we observed overall no association of THMs with colorectal cancer. Conclusion: These results add further evidence that disinfection by-products in drinking water may be a possible risk factor for proximal colon cancer in men. This observation was made at THM concentrations lower than those in most previous studies.