Associations between natural radionuclides in borehole water and cancer risks: A comprehensive population-based study from Finland

Background: Carcinogenic effects of radioactive elements in environmental background for all cancer types have not been fully investigated. Methods: This paper assesses the associations between radioactive elements in borehole water and all cancer types based on total Finnish population. National registries of the general population living continuously in Finland during 1987-2016 were linked to annual municipality-level concentrations of uranium (Ur) and radon (Rn) in borehole water. Average radiation exposures over 30 years were calculated individually based on residential location annually. Newly diagnosed cancers were followed from 2017 until 2021 based on national cancer registry. Associations between exposures and cancer incidences were assessed via Cox proportional hazards models, adjusted for pre-existing health conditions and socio-economic factors. Multiplicity was adjusted using Bonferroni correction. Results: Totally 1,013,054 men and 1,101,589 women with non-missing municipality-level exposures were included. During follow-up period, totally 62,085 cancer cases of 70 sub-types were diagnosed in men and 57,627 cancer cases of 73 sub-types were diagnosed in women respectively. Significant associations between environmental radiation exposures and elevated risks of cancer were observed in men (Ur: hazard ratio [HR] 1.05, 95% confidence interval [CI] 1.03-1.07; Rn: 1.04, 1.02-1.05) and women (Ur: 1.07, 1.05-1.09; Rn: 1.05, 1.03-1.07). After correction of multiplicity, significantly elevated risks were observed for cancers of uncertain lymphoid and hematopoietic tissue (1.66, 1.39-1.97), mesothelioma (1.48, 1.18-1.86), myeloma (1.36, 1.18-1.57), melanoma (1.31, 1.22-1.40), leukemia lymphoid (1.28, 1.12-1.48) in men with higher exposure to Ur; and Polycythemia vera (1.79, 1.30-2.45), uncertain lymphoid and hematopoietic tissue (1.61, 1.36-1.91), myeloma (1.52, 1.32-1.75), melanoma (1.22, 1.14-1.31) and bladder (1.20, 1.08-1.32) with higher exposure to Rn. Significantly elevated risks were observed for uncertain lymphoid and hematopoietic tissue (1.78, 1.52-2.08), leukemia lymphoid (1.48, 1.24-1.77), melanoma (1.33, 1.24-1.44), lung (1.26, 1.19-1.34) and breast (1.07, 1.04-1.10) in women with higher exposure to Ur; and uncertain lymphoid and hematopoietic tissue (1.73, 1.48-2.03), Polycythemia vera (1.72, 1.26-2.35), myeloma (1.51, 1.30-1.75), melanoma (1.28, 1.19-1.38) and lung (1.21, 1.14-1.29) with higher exposure to Rn. In sensitivity analysis, similar patterns were observed in samples who only lived in houses during 1987-2016. Dosage-response trends were observed for total cancer risks in men and women respectively. Interpretation: High environmental radiation exposures were significantly associated with elevated total cancer risks and several sub-types in the general population.