Breast Cancer Risk in Relation to Ambient Air Pollution Exposure at Residences in the Sister Study Cohort.

Background: Some but not all past studies reported associations between components of air pollution and breast cancer, namely fine particulate matter ≤2.5 μm (PM2.5) and nitrogen dioxide (NO2). It is yet unclear whether risks differ according to estrogen receptor (ER) and progesterone receptor (PR) status.

Methods: This analysis includes 47,591 women from the Sister Study cohort enrolled from August 2003 to July 2009, in whom 1,749 invasive breast cancer cases arose from enrollment to January 2013. Using Cox proportional hazards and polytomous logistic regression, we estimated breast cancer risk associated with residential exposure to NO2, PM2.5, and PM10.

Results: Although breast cancer risk overall was not associated with PM2.5 [HR = 1.03; 95% confidence intervals (CI), 0.96-1.11], PM10 (HR = 0.99; 95% CI, 0.98-1.00), or NO2 (HR = 1.02; 95% CI, 0.97-1.07), the association with NO2 differed according to ER/PR subtype (P = 0.04). For an interquartile range (IQR) difference of 5.8 parts per billion (ppb) in NO2, the relative risk (RR) of ER(+)/PR(+) breast cancer was 1.10 (95% CI, 1.02-1.19), while there was no evidence of association with ER(-)/PR(-) (RR = 0.92; 95% CI, 0.77-1.09; Pinteraction = 0.04).

Conclusions: Within the Sister Study cohort, we found no significant associations between air pollution and breast cancer risk overall. But we observed an increased risk of ER(+)/PR(+) breast cancer associated with NO2.

Impact: Though these results suggest there is no substantial increased risk for breast cancer overall in relation to air pollution, NO2, a marker of traffic-related air pollution, may differentially affect ER(+)/PR(+) breast cancer.