Sunlight Exposure and the Risk of Mortality and Breast Cancer Incidence

Background: Prolonged sun exposure increases the risk of skin cancer. However, multiple sunlight-related health benefits have been identified. The overall impact of sun exposure on mortality is not clear. Sunlight could be associated with breast cancer risk via stimulating vitamin D photosynthesis; however, evidence has been mixed. Objectives: To investigate the association of time spent outdoors in daylight (in warmer and cooler months, and the total [annual]) and UV irradiance using the residential locations of participants with the risks of mortality (all-cause and cause-specific: cancer, cardiovascular [CVD], and non-cancer non-CVD) and primary invasive breast cancer. Methods: This research included participants of the Adventist Health Study 2 cohort of North America. Multivariable Cox regression was our statistical method to investigate these associations. Results: Among 83,205 participants included in the mortality studies, we observed nonlinear (reverse J-shaped) associations between time outdoors in warmer months and the risks of all-cause, CVD, and non-cancer non-CVD mortalities ([HR: 0.90; 95% CI: 0.86–0.93], [0.89; 0.83–0.95], and [0.83; 0.78–0.89]), respectively, comparing 2 hours/day to 30 minutes/day, but no clear evidence of an association between time outdoors in daylight and cancer mortality. The time-dependent UV irradiance was associated with higher risks of all-cause, cancer, CVD, and non-cancer non-CVD mortalities ([1.14, 1.10–1.18), [1.08; 1.02–1.14], [1.14; 1.08–1.20], and [1.19; 1.13–1.26], respectively, comparing the 75th percentile of 164 mW/m2 to the 25th percentile of 116 mW/m2). Among 36,625 women included in the breast cancer study, we found no association between our sun exposure metrics (time spent outdoors in daylight and UV irradiance) and the risk of breast cancer. These associations did not vary by women’s menopausal status. There was no significant interaction between UV irradiance and time spent outdoors. Conclusion: Time spent outdoors in daylight during warmer months could be associated with a lower risk of all-cause and non-cancer mortalities. The UV irradiance was associated with a higher risk of mortality outcomes. Both sun exposure metrics were not related to breast cancer risk. Future studies should measure sun exposure using biomarkers or wearable devices and investigate how to balance sunlight-related health risks and benefit.