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

Objectives. To determine whether outdoor air pollution exposure is associated with breast cancer incidence. Methods. Residential-level concentrations of nitrogen dioxide (NO2, parts per billion [ppb]), fine particulate matter (PM2.5; ≤ 2.5 μ/m3) and ozone (ppb) in the United States were estimated for participants of the Nurses' Health Studies, Women's Health Initiative Clinical Trials and Observational Study Cohort, and Sister Study using high-resolution spatiotemporal models. Cox proportional hazards regression estimated cohort-specific hazard ratios (HRs) and 95% confidence intervals (CIs), and a random effects model determined summary HRs, overall and by estrogen receptor (ER)/progesterone receptor (PR) subtype and census region. Results. NO2 was positively associated with overall breast cancer incidence (n = 28 811 cases; HR = 1.03; 95% CI = 1.00, 1.05), with little variation by subgroups. PM2.5 was associated with higher incidence of ER-/PR- tumors (n = 2367 cases; HR = 1.14; 95% CI = 1.04, 1.24; P-heterogeneity < .001) and with higher overall incidence in the Midwest (HR = 1.15; 95% CI = 1.01, 1.32; P-heterogeneity = .01). Ozone was not associated with overall incidence, but was associated with ER-/PR- tumors (n = 3406 cases; HR = 1.10; 95% CI = 1.00, 1.21; P-heterogeneity = .03). Conclusions. In this largest US study to date, we confirmed an association between NO2 and breast cancer, and we present novel associations of PM2.5 and ozone with ER-/PR- tumors. (Am J Public Health. Published online ahead of print September 25, 2025:e1-e14. https://doi.org/10.2105/AJPH.2025.308247).