Abstract
BACKGROUND: Accumulating evidence suggests that domestic water hardness is linked to health outcomes, but its association to all-cause and cause-specific cancers warrants investigation. OBJECTIVE: The aim of this study was to investigate the association of domestic hard water with all-cause and cause-specific cancers. METHODS: In the prospective cohort study, a total of 447,996 participants from UK Biobank who were free of cancer at baseline were included and followed up for 16 y. All-cause and 22 common cause-specific cancer diagnoses were ascertained using hospital inpatient records and self-reported data until 30 November 2022. Domestic water hardness, measured by CaCO3 concentrations, was obtained from the local water supply companies across England, Scotland, and Wales in 2005. Data were analyzed using Cox proportional hazard models, with adjustments for known measured confounders, including demographic, socioeconomic, clinical, biochemical, lifestyle, and environmental factors. RESULTS: Over a median follow-up of 13.6 y (range: 12.7-14.4 y), 58,028 all-cause cancer events were documented. A U-shaped relationship between domestic water hardness and all-cause cancers was observed (p for nonlinearity < 0.001). In comparison with individuals exposed to soft water (0 - 60 mg/L), the hazard ratios (HRs) and 95% confidence intervals (CIs) of all-cause cancer were 1.00 (95% CI: 0.98, 1.02) for those exposed to moderate hard water ( > 60 - 120 mg/L), 0.88 (95% CI: 0.84, 0.91) for those exposed to hard water ( > 120 - 180 mg/L) and 1.06 (95% CI: 1.04, 1.08) for those exposed to very hard water ( > 180 mg/L). Additionally, domestic water hardness was associated with 11 of 22 cause-specific cancers, including cancers of the esophagus, stomach, colorectal tract, lung, breast, prostate, and bladder, as well as non-Hodgkin lymphoma, multiple myeloma, malignant melanoma, and hematological malignancies. Moreover, we observed a positive linear relationship between water hardness and bladder cancer. DISCUSSION: Our findings suggest that domestic water hardness was associated with all-cause and multiple cause-specific cancers. Findings from the UK Biobank support a potentially beneficial association between hard water and the incidence of all-cause cancer. However, very hard water may increase the risk of all-cause cancer. https://doi.org/10.1289/EHP13606.