Air Pollutants and Breast Cancer Risk: A Parallel Analysis of Five Large US Prospective Cohorts.

This article examines the association between outdoor air pollution exposure—specifically nitrogen dioxide (NO₂), fine particulate matter (PM₂.5; particles ≤ 2.5 μm), and ozone—and breast cancer incidence in the United States. Using high-resolution spatiotemporal exposure models and data from five large prospective cohorts (Nurses' Health Studies I and II, Sister Study, Women's Health Initiative Clinical Trials and Observational Study), the study found a modest positive association between NO₂ exposure and overall breast cancer risk (hazard ratio [HR] = 1.03 per 10 ppb increase). PM₂.5 and ozone were not associated with overall breast cancer incidence but showed novel positive associations with estrogen receptor–negative/progesterone receptor–negative (ER-/PR-) breast cancer subtypes, with PM₂.5 also linked to higher breast cancer incidence in the U.S. Midwest. These findings suggest that certain air pollutants may contribute to breast cancer risk, particularly for hormone receptor–negative tumors, which have fewer established risk factors and poorer prognoses.