Background Occupational exposures have been hypothesized as contributors to breast cancer. Nevertheless, studies exploring the risk of breast cancer by occupation or industry generally show somewhat inconsistent results. In addition to small study size and lack of lifetime work history, potential confounders, such as reproductive history, have often not been taking into account and mortality has often been used as outcome. We conducted a nationwide register based nested case-control study in order to explore associations between occupation and incidence of breast cancer, including reproductive history.

Methods A total of 38,347 employed women born in Denmark during the period 1946 to 1995, and diagnosed with breast cancer 1970–2015 were identified in the nationwide Danish Cancer Registry. Five control subjects, work active and born in Denmark, and free of breast cancer at the date of diagnosis of the corresponding case, were chosen randomly from the Danish Civil Register. Using the unique personal identification number assigned to all residents in Denmark, we obtain lifetime employment history from the nationwide Pension Fund Register, which on an individual level has kept detailed information on all employments in Denmark since 1964. Lastly, information on reproductive factors, and socioeconomic status was retrieved from the Danish Civil Register. Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated by conditional logistic regression analysis.

Preliminary results: Risk elevations were e.g. seen in the military (2.1; 1.49–2.90), printing (1.6; 1.22–2.06), airline transportation (2.4; 1.33–4.47), automobile service (1.3; 95% CI 1.12–1.49), laundries (2.4; 1.48–3.79), hairdressing (1.9; 1.11–3.23) and paint factories (1.6; 95% CI 1.22–2.06). Risk reductions were seen in farming (0.4; 0.21–0.59), window cleaning (0.6; 0.44–0.84) and building and carpentry (0.8; 0.70–0.98).

Conclusions The study suggests that breast cancer risk varies significantly by industry and that occupational exposures, including night-shift work, solvents and outdoor work, may play a possible role.