Chlorinated drinking water linked to colorectal cancer risk in major study

<p><strong>BACKGROUND:</strong> Colorectal cancer is the third most common malignancy worldwide, and it is strongly linked to lifestyle and environmental risk factors. While several drinking water disinfection by-products are confirmed rodent carcinogens, there is still inconclusive evidence for human carcinogenicity, including colorectal cancer.</p><p><strong>METHODS:</strong> 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 two 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 (&lt;15µg/L) and high exposure (≥15µg/L). Incident cases of colorectal cancer were ascertained using the Swedish National Cancer Register.</p><p><strong>RESULTS:</strong> During an average follow-up of 16.8 years (988,144 person-years), 1,913 cases of colorectal cancer were ascertained (1,176 and 746 men and women, respectively). High drinking water THM concentrations (≥15 µg/L) was associated with increased risk of colorectal cancer in men (hazard ratio, HR: 1.26, 95% confidence interval, CI: 1.05 to 1.51) compared to no exposure. When assessing subsites, the association was significant for proximal colon cancer (HR: 1.59, 95% CI: 1.11 to 2.27) but not distal colon cancer or rectal cancer. In women, we observed overall no association of THMs with colorectal cancer.</p><p><strong>CONCLUSION:</strong> These results add further support to 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 in most previous studies.</p>