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

Abstract

BACKGROUND AND AIM: Exposure to chlorination by-products has been consistently associated with an increased risk of bladder cancer in case-control studies, but confirmation from large-scale cohort studies are lacking. We assessed the association of drinking water trihalomethanes (THM), a surrogate for chlorination by-products, with risk of bladder cancer in 58,672 men and women in two population-based cohorts. METHODS: We used data, including questionnaire information, from the Swedish Infrastructure for Medical Population-Based Life-Course and Environmental Research (SIMPLER). Individual exposure to THM was assessed by combining residential information from register with municipal THM monitoring data. Participants were categorized into no chlorination, low (15 µg/L) or high (15 µg/L) THM exposure dependent on their area of residence. Incident bladder cancer cases were ascertained from 1998 through 2019 via register linkage. Associations were examined using multivariable-adjusted Cox proportional hazards regression models, including information on smoking habits and drinking water arsenic concentrations. RESULTS:The mean THM concentration was 6.6 ± 2.2 µg/L and 17.1 ± 2.1 µg/L in the low and the high exposure category, respectively. During a mean follow-up of 16 years (965,590 person-years), 831 bladder cancer cases were ascertained. We observed no overall association of THM with risk of bladder cancer. The hazard ratio was 0.90 (95% confidence interval: 0.73 – 1.11) when comparing the highest exposed group with those in the non-chlorinated areas. The null association remained after restricting the analysis to long-term residents and across strata of smoking status and cancer stage. CONCLUSIONS:Our results suggest that at THM concentrations below 20 µg/L, which is representative of chlorinated drinking waters in most European countries, chlorination by-product exposure is not associated with risk of bladder cancer. KEYWORDS: Chlorination by-products, Disinfection by-products, Trihalomethanes, Bladder cancer, Water quality, Drinking water