Air Pollution, Clustering of Particulate Matter Components and Breast Cancer

To evaluate the relationship between air pollution, particulate matter (PM) components and breast cancer risk in a U.S.-wide prospective cohort. Methods. We estimated annual average ambient outdoor residential levels of PM2.5, PM10 and NO2 using a land- use regression model for 49,533 Sister Study participants (breast cancer-free women with a sister with breast cancer) living in the contiguous U.S. Predictive k-means was used to assign participants to clusters defined by PM2.5 components profile to evaluate the impact of heterogeneity in the air pollution mixture. Participants were also separately clustered by geographic region, without regard to air pollution mixture. Cox proportional hazards regression was used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for invasive breast cancer risk associated with an interquartile range (IQR) increase in pollutants (PM2.5: 3.6 μg/m3, PM10: 5.8 μg/m3, NO2: 5.8 parts per billion (ppb)). For PM2.5, the association was stratified by component cluster membership. Results. During follow-up (mea n = 8.4 years), 2,349 invasive cases were identified. There was little to no increase in risk for invasive breast cancer overall in relation to these air pollutants. However, we observed a higher risk of invasive breast cancer associated with an IQR increase in PM2.5 (HR IQR = 1.15, 95% CI 1.03–1.28), PM10 (HRIQR = 1.05, 95% CI 0.99–1.11) and NO2 (HRIQR = 1.10, 95% CI 1.10–1.21) for women in the Western U.S. Associations also varied by clusters derived from PM2.5 component profiles. PM2.5 was associated with a 28% higher risk of invasive breast cancer (95% CI: 1.00–1.64) in a California-based cluster characterized by low sulfur fractions and high fractions of sodium and nitrate. An elevated, although less precise, risk was observed for another Western U.S. cluster (HR = 1.63, 95% CI: 0.92–2.88), characterized by high fractions of Si, Ca, K and Al. Conclusions. Air pollution measures were related to invasive breast cancer for certain subgroups of women, defined by geographic region and PM component profiles. Particulate matter (PM) is a complex mixture and consideration of geographic variability and mixture components may impact observed associations with breast cancer.