Introduction Both diesel exhaust and specific polycyclic aromatic hydrocarbons (PAHs) have been classified as carcinogenic to humans. However, there is overall limited evidence for an association between these exposures and breast cancer.

Objectives To explore the association between occupational exposure to diesel exhaust and PAHs, respectively, and breast cancer, including subtypes.

Methods The study included 38,375 women with incident breast cancer identified in the Danish Cancer Registry, and five breast cancer-free controls per case matched on year of birth who were randomly selected from the Danish Civil Registration System. Full employment history was obtained for all women from a nationwide pension fund, and exposure to diesel exhaust and PAHs was assessed using a job exposure matrix. Conditional logistic regression was used to estimate of odds ratios (ORs) with adjustment for reproductive factors and socioeconomic status.

Results No noteworthy associations were observed for overall breast cancer in women exposed to diesel exhaust. However, diesel exhaust modestly elevated the risk of estrogen receptor negative breast tumours before age 50 years (OR=1.26, 95% CI: 1.09–1.46). Duration- and dose-response relationships were also observed for this subtype in this age group. No notable risk patterns were generally observed for PAH exposure.

Conclusion Occupational exposure to diesel exhaust may elevate the risk of early-onset estrogen receptor negative breast tumours in women. Future studies exploring this association are warranted.