Lifetime Sunlight Exposure and Risk of Breast Cancer: A Population-Based Incident Case–Control Study

Abstract Background: There is conflicting evidence regarding the association between sunlight exposure and the risk of breast cancer. Methods: This population-based incident case-control study included 600 newly diagnosed breast cancer cases and 600 population controls (18–75 years) recruited in Isfahan, Iran, between 2021 and 2023. Sunlight exposure (hours/day) used disaggregated data for exposure during adolescence (10–20 years), early adulthood (20–30 years), after 30 years, and lifetime exposure assessed in summer and winter. Logistic regression models were used to estimate ORs and 95% confidence intervals (CI) after adjusting for confounders. Results: Each hourly increase in sunlight exposure during adolescence significantly decreased breast cancer odds (exposure during winter OR, 0.84; 95% CI, 0.78–0.91 and exposure during summer OR, 0.89; 95% CI, 0.83–0.96). No associations were observed for sunlight exposure after the age of 30. However, lifetime sunlight exposure was associated with lower odds of breast cancer: each additional hour (per day) reduced odds by 16% in winter (OR, 0.84; 95% CI, 0.76–0.93) and 9% in summer (OR, 0.91; 95% CI, 0.82–1.00). A dose–response association was observed for both adolescence and lifetime sunlight exposure and the odds of breast cancer. Conclusions: Sunlight exposure during adolescence and early adulthood is associated with reduced breast cancer risk. Cumulative lifetime exposure also has protective effects in Iranian females. These findings suggest the need for public health strategies that promote moderate sunlight exposure during critical life stages. Impact: Identifying key periods when sunlight exposure as a modifiable protective factor reduces breast cancer risk offers a simple prevention strategy and informs safer public health guidelines.