BACKGROUND: Air pollution has been classified as a human carcinogen based largely on findings for respiratory cancers. Emerging, but limited, evidence suggests that it increases the risk of breast cancer, particularly among younger women. We characterized associations between residential exposure to ambient fine particulate matter (PM(2.5)) and nitrogen dioxide (NO(2)) and breast cancer. Analyses were performed using data collected in the Ontario Environmental Health Study (OEHS). METHODS: The OEHS, a population-based case-control study, identified incident cases of breast cancer in Ontario, Canada among women aged 18-45 between 2013 and 2015. A total of 465 pathologically confirmed primary breast cancer cases were identified from the Ontario Cancer Registry, while 242 population-based controls were recruited using random-digit dialing. Self-reported questionnaires were used to collect risk factor data and residential histories. Land-use regression and remote-sensing estimates of NO(2) and PM(2.5,) respectively, were assigned to the residential addresses at interview, five years earlier, and at menarche. Logistic regression was used to estimate odds ratios (OR) and their 95 % confidence intervals (CI) in relation to an interquartile range (IQR) increase in air pollution, adjusting for possible confounders. RESULTS: PM(2.5) and NO(2) were positively correlated with each other (r = 0.57). An IQR increase of PM(2.5) (1.9 µg/m(3)) and NO(2) (6.6 ppb) at interview residence were associated with higher odds of breast cancer and the adjusted ORs and 95 % CIs were 1.37 (95 % CI = 0.98-1.91) and 2.33 (95 % CI = 1.53-3.53), respectively. An increased odds of breast cancer was observed with an IQR increase in NO(2) at residence five years earlier (OR = 2.16, 95 % CI: 1.41-3.31), while no association was observed with PM(2.5) (OR = 0.96, 95 % CI 0.64-1.42). CONCLUSIONS: Our findings support the hypothesis that exposure to ambient air pollution, especially those from traffic sources (i.e., NO(2)), increases the risk of breast cancer in young women.