An association between mammographic breast density and fine particulate matter among postmenopausal women.

Increasing breast density is a risk factor for breast cancer. Geographic variations in breast density may be due to differences in lifestyle and diet, as well as environmental factors such as air pollution exposure. However, these environmental contributors have not been established. In this study, we evaluated an association between air pollution and mammographic breast density. The study population for this study was postmenopausal women who had undergone screening mammography at the Center for Preventive Medicine, St. Luke's International Hospital, from April 2004 to September 2018. Individual mammography results were obtained from electronic charts. The ambient air pollution (PM2.5) density of the locations of interest, namely, the patients' residential areas during the study period, was obtained. The mean PM2.5 exposure levels for 1, 3, 5, and 7 years were determined. A generalized estimating equations model was used to examine the association between air pollution density and dense breast. A total of 44,280 mammography results were included in this study, and 29,135 were classified in the non-dense breast group and 15,145 in the dense breast group. There was a 3% increase in the odds of having dense breasts after 1 year (OR = 1.027, 95% confidence interval (CI) 1.019-1.034) and 3 years of PM2.5 exposure (OR = 1.029, 95% CI 1.022-1.036). This further increased to 4% at 5-year exposure (OR = 1.044, 95% CI 1.037-1.052) and 5% at 7-year exposure (OR = 1.053, 95% CI 1.044-1.063). The risk for dense breasts increased if the factors of smoking, family history of breast and/or ovarian cancer, and history of childbirth were present.