Introduction It has been suggested that exposure to UV radiation from the sun (UVR) may increase the blood level of vitamin D and, in turn, decrease the risk of breast cancer.

Objectives We explored the associations between outdoor occupational solar radiation and subtypes of female breast cancer.

Methods Totally 38,375 female employees under the age 70 years were identified with first primary breast cancer from the Danish Cancer Registry. Five age matched employed controls, were, based on the incidence density principle, randomly chosen. The Danish Supplementary Pension Fund Register was used to retrieve full employment history. A job exposure matrix was used to assess individual occupational UVR exposure. Conditional logistic regression with adjustment for reproductive factors and SES was used to estimate odds ratios (ORs).

Results Longer duration of UVR exposure (≥20 years: OR=0.8, 95% CI: 0.75–0.92) and highest cumulative exposure (OR=0.9, 95% CI: 0.83–0.95) were inversely associated with the risk of breast cancer after age 50 years. Further, the reduced risk pattern for duration of exposure was most pronounced for estrogen receptor negative tumors (≥20 years: OR=0.8, 95% CI: 0.57–0.98).

Conclusion Our study of outdoor workers shows a modest decreased breast cancer risk.