Chlorination by-products in drinking water and risk of bladder cancer - A population-based cohort study.

Chlorination by-products have been consistently associated with risk of bladder cancer in case-control studies, but confirmation from large-scale cohort studies is lacking. We assessed the association of drinking water trihalomethanes (THM), a proxy for chlorination by-products, with risk of bladder cancer in 58,672 men and women. Data came from two population-based cohorts, parts of the Swedish Infrastructure for Medical Population-Based Life-Course and Environmental Research (SIMPLER). Individual exposure to THM was assessed by combining residential information with tap water monitoring data. Participants were categorized into non-exposed, low (<15 µg/L) or high (≥15 µg/L) THM exposure. Incident cases were ascertained from 1998 through 2019 via register linkage. During 16 years of follow-up (965,590 person-years), 831 bladder cancer cases were ascertained. We observed no overall association of THM with risk of bladder cancer, hazard ratio for the highest exposed compared to the non-exposed 0.90 (95% confidence interval: 0.73 - 1.11). The null association remained after restricting the analysis to long-term residents and across strata of smoking status and cancer stage. Our results indicate that chlorination by-product exposure at THM concentrations representative of chlorinated drinking waters in most European countries, is not associated with an increased risk of bladder cancer.