Radon Exposure Is Associated with Breast Cancer Risk: A Long-Term Population-Based Study from Finland

Abstract Background Radon is a common environmental carcinogen. It is not known whether radon exposure and breast cancer risk are associated at the population level. Methods Women continuously living in Finland during the 1987-2016 period were linked to municipality-level indoor radon data annually to estimate their cumulative radon exposure over 30 years. Breast cancer incidence was observed over five years from 2017 onwards. Association between radon exposure and breast cancer incidence was assessed using conditional Cox models adjusted for family history of breast cancer, reproductive history, and socio-demographic covariates. Population attributable fractions (PAF) of breast cancer due to radon exposure were estimated. Robustness analyses were conducted in the population who lived only in houses. Results Among 1,335,947 women, 20,067 breast cancer cases were observed during a median follow-up period of 4.9 years. Among women who were exposed to high levels of radon (>100 Bq/m3), overall breast cancer incidence was 10% higher (Hazard ratio 1·10 [95% CI 1·05-1·16]), duct carcinoma incidence was 6% higher (1·06 [1·00-1·13]), lobular carcinoma incidence was 8% higher (1·08 [0·94-1·23]), and the incidence of other morphological sub-types was 34% higher (1·34 [1·17-1·53]) than among other women. Robust associations with overall breast cancer incidence (1·12 [1·05-1·20]) and the incidence of morphological sub-types in the house sample. Correspondingly, suggestive dose-response associations were observed. Conclusions High radon exposure is associated with elevated breast cancer risk at the population level, and this relationship is independent of well-established risk factors. Key messages • This study investigates the association between long-term radon exposure and breast cancer incidence based on national register data. • High radon exposure is robustly associated with breast cancer risk in general Finnish population.