BACKGROUND: An association between air pollution and breast cancer risk has been suggested but evidence is sparse and inconclusive.

METHODS: We included 22,877 female nurses from the Danish Nurse cohort who were recruited in 1993 or 1999, and followed them for incidence of breast cancer (N=1,145) until 2013 in the Danish Cancer Register. We estimated annual mean concentrations of particulate matter with diameter < 2.5 µg/m3 (PM2.5) and < 10 µg/m3 (PM10), and nitrogen dioxide (NO2) at nurses' residences since 1990 using an atmospheric chemistry transport model. We examined the association between the 3-year running mean of each pollutant and breast cancer incidence using a time-varying Cox regression.

RESULTS: We found no association between breast cancer and PM2.5 (hazard ratio; 95% confidence interval: 0.99; 0.94-1.10 per interquartile range of 3.3 µg/m3), PM10 (1.02; 0.94-1.10 per 2.9 µg/m3) or NO2 (0.99; 0.93-1.05 per 7.4 µg/m3).

CONCLUSIONS: Air pollution is not associated with breast cancer risk.

IMPACT: Exposure to air pollution in adulthood does not increase the risk of breast cancer, but more data on the effects of early exposure, before first birth, are needed.