The association between the incidence of postmenopausal breast cancer and concentrations at street-level of nitrogen dioxide and ultrafine particles

Background: There is scant information as to whether traffic-related air pollution is associated with the incidence of breast cancer. Nitrogen dioxide (NO2) and ultrafine particles (UFPs, < 0.1 mu m), are two pollutants that capture intra-urban variations in traffic-related air pollution and may also be associated with incidence. Methods: We conducted a population-based, case-control study of street-level concentrations of NO2 and UFPs and incident postmenopausal breast cancer in Montreal, Canada. Incident cases were identified between 2008 and 2011 from all but one hospital that treated breast cancer in the Montreal area. Population controls were identified from provincial electoral lists of Montreal residents and frequency-matched to cases using 5-year age groups. Concentrations of NO2 and UFPs were estimated using two separate land-use regression models. Exposures were assigned to residential locations at the time of recruitment, and we identified residential histories of women who had lived in these residences for 10 years or more. Odds ratios (OR) and 95% confidence intervals (CI) were estimated using logistic regression models adjusting for individual level and ecological covariates. We assessed the functional form of NO2 and UFP exposures using natural cubic splines. Results: We found that the functional form of the response functions between incident postmenopausal breast cancer and concentrations of NO2 and UFPs were consistent with linearity. For NO2, we found increasing risks of breast cancer for all subjects combined and stronger associations when analyses were restricted to those women who had lived at their current address for 10 years or more. Specifically, the OR, adjusted for personal covariates, per increase in the interquartile range (IQR = 3.75 ppb) of NO2 was 1.08 (95%CI: 0.92-1.27). For women living in their homes for 10 years or more, the adjusted OR was 1.17 (95%Cl: 0.93-1.46; IQR= 3.84 ppb); for those not living at that home 10 years before the study, it was 0.93 (95%CI: 0.64, 1.36; IQR = 3.65 ppb). For UFPs, the ORs were lower than for NO2, with little evidence of association in any of the models or sub-analyses and little variability in the ORs (about 1.02 for an IQR of similar to 3500 cm (-3)). On the other hand, we found higher ORs amongst cases with positive oestrogen and progesterone receptor status; namely for NO2, the OR was 1.13 (95%CI: 0.94-1.35) and for UFPs it was 1.05 (95%CI: 0.96-1.14). Conclusions: Our findings suggest that exposure to ambient NO2 and UFPs may increase the risk of incident postmenopausal breast cancer especially amongst cases with positive oestrogen and progesterone receptor status.