Association between ambient air pollution and breast cancer risk: The multiethnic cohort study

Previous studies using different exposure methods to assess air pollution and breast cancer risk among primarily whites have been inconclusive. Air pollutant exposures of particulate matter and oxides of nitrogen were estimated by kriging (NO x , NO 2 , PM 10 , PM 2.5 ), land use regression (LUR, NO x , NO 2 ) and California Line Source Dispersion model (CALINE4, NO x , PM 2.5 ) for 57,589 females from the Multiethnic Cohort, residing largely in Los Angeles County from recruitment (1993–1996) through 2010. Cox proportional hazards models were used to examine the associations between time‐varying air pollution and breast cancer incidence adjusting for confounding factors. Stratified analyses were conducted by race/ethnicity and distance to major roads. Among all women, breast cancer risk was positively but not significantly associated with NO x (per 50 parts per billion [ppb]) and NO 2 (per 20 ppb) determined by kriging and LUR and with PM 2.5 and PM 10 (per 10 μg/m 3 ) determined by kriging. However, among women who lived within 500 m of major roads, significantly increased risks were observed with NO x (hazard ratio [HR] = 1.35, 95% confidence interval [95% CI]: 1.02–1.79), NO 2 (HR = 1.44, 95% CI: 1.04–1.99), PM 10 (HR = 1.29, 95% CI: 1.07–1.55) and PM 2.5 (HR = 1.85, 95% CI: 1.15–2.99) determined by kriging and NO x (HR = 1.21, 95% CI:1.01–1.45) and NO 2 (HR = 1.26, 95% CI: 1.00–1.59) determined by LUR. No overall associations were observed with exposures assessed by CALINE4. Subgroup analyses suggested stronger associations of NO x and NO 2 among African Americans and Japanese Americans. Further studies of multiethnic populations to confirm the effects of air pollution, particularly near‐roadway exposures, on the risk of breast cancer is warranted.