Subjects with missing values were assigned to a “missing information” category. Only age at diagnosis and the two neighborhood ecologic covariates were treated as continuous variables. Fully adjusted models included accepted and suspected risk factors for postmenopausal breast cancer (Table 1): age at diagnosis, fam- ily history, education, ethnicity, age at bilat- eral oophorectomy, age at menarche, age at first full-term pregnancy, alcohol consump- tion, and duration of hormonal replacement therapy. In addition, factors whose causal association with breast cancer is still uncertain were included as covariates: oral contraceptive use, smoking, total duration of breast-feeding, body mass index, neighborhood ecologic cova- riates, and the selected occupational exposures before 36 years of age. We also adjusted for the design variables proxy respondent status and for the hospital where subjects were diag- nosed. Standard regression diagnostics were applied to identify possible influential subjects and to ensure that the models did not violate the assumptions of logistic regression. We included NO2 as a continuous, linear variable after verifying this assumption through the use of natural cubic spline functions (2–3 degrees of freedom) and visual inspection of the fitted exposure–response curves. ORs are presented for each increase in exposure to NO2 of 5 ppb (referred to as OR5ppb) and for an increase across the interquartile range for each exposure period [presented in the Supplemental Material, Table 2 (doi:10.1289/ehp.1002221)]. The sensitivity of the results was assessed by analyses limited to those subjects for whom we had information on duration of residence and who had been residents at the same address for ≥ 10 years before diagnosis, by excluding proxy respondents, and by excluding controls with bladder cancer, because there is evidence that exposure to diesel exhaust may increase the risk of incidence of bladder cancer (Boffetta and Silverman 2001). Results Exposure surfaces. Concentrations of NO2 decreased over time, with the highest mean value observed in 1985 (20.1 ppb), and the lowest in 2006 (11.3 ppb) (Table 2). The trend of decreasing concentrations is consistent with observations from Environment Canada’s fixed-site stations [see Supplemental Material, Figure 2 (doi:10.1289/ehp.1002221)]. The seven surfaces reflected also a narrowing of the distributions of NO2 over time. We found positive correlations among 1,000 randomly sampled locations on the different exposure surfaces (r = 0.96–0.99) and among observed concentrations of annual mean concentrations of NO2 at the fixed-site stations for 1985, 1996, and 2005 (see Supplemental Material, Table 1), suggesting that the spatial patterns of NO2 did not vary importantly during the 20-year period between 1985 and 2006. Furthermore, the observed concentrations at the locations of the fixed-site monitoring sta- tions in 2006 were correlated with those in 1996 (r = 0.89; 95% CI, 0.60–0.97) and in 1985 (r = 0.72; 95% CI, 0.12–0.94). Description of the cases and controls. A total of 1,631 subjects were potentially eli- gible for this study. Interviews were conducted among 608 cases and 667 control subjects, for response rates of 81.1% for cases and 75.7% for controls. Of these 1,275 participants, 106 were deemed to be premenopausal and were therefore excluded; a further 79 participants had incomplete or inaccurate address informa- tion, and 291 subjects resided outside of the Island of Montreal. Therefore, these analyses included 799 subjects: 383 cases and 416 con- trols (Figure 1). The most frequent sites of cancer in the 416 controls were colon (21.6%), uterus (19.0%), ovaries (9.1%), rectum (6.7%), and bladder (6.0%). We found associations for the generally accepted risk factors for Breast cancer and air pollution in Montreal postmenopausal breast cancer (i.e., family his- tory, benign breast disease, education, age at menarche, duration of hormonal replace- ment therapy) (Table 2). We found essen- tially no differences in characteristics between the home neighborhoods of the cases and controls (i.e., median household income and percentage of adults ≥ 15 years of age with- out high school diploma were C$34,239 and C$34,436, and 32.6% and 32.8%, respec- tively, for cases and controls). Associations between postmenopausal breast cancer and air pollution. In the age- adjusted models, the OR5ppb ranged from a low of 1.05 (95% CI, 0.91–1.22) using esti- mates of exposure for 1985 to a high of 1.15 (95% CI, 0.89–1.48) in 2006 (Table 3). In the fully adjusted model using estimates of exposure from 1996, the OR5ppb was 1.31 (95% CI, 1.00–1.71). The ORs calculated per interquartile range were less variable between exposure periods compared with those com- puted per 5 ppb, varying from 1.19 in 1985 to 1.30 in 1996 [see Supplemental Material, Table 2. Distributions of concentrations of NO2 (ppb) in seven different exposure surfaces, Island of Montreal, Canada. Model year Minimum 25th percentile Mean 75th percentile Maximum Median 2006 4.3 9.2 11.3 12.9 37.4 10.8 1996a 6.0 12.9 15.6 17.8 44.5 15.1 Mean of 1985 and 1996a 6.9 14.8 17.9 20.3 55.6 17.2 1985a 7.8 16.5 20.1 22.8 66.8 19.3 1996b 4.9 10.3 12.7 14.7 39.5 12.2 Mean of 1985 and 1996b 5.6 11.6 14.3 16.6 49.3 13.8 1985b 6.2 12.7 15.8 18.3 59.2 15.1 aExtrapolated using observed concentrations of NO2 at each fixed-site monitoring station. bExtrapolated using predicted concentrations of NO2 derived from the land-use regression in 2006 at each fixed-site monitoring station. Control subjects Breast cancer cases Water Population density (people/km2) 0–879 880–2,379 2,380–4,189 4,190–6,465 6,466–9,258 9,259–13,188 0 1.5 3 6 9 12 Kilometers N Figure 1. Spatial distribution of residential addresses of breast cancer cases and control subjects, Montreal, 1996–1997. Environmental Health Perspectives • volume 118 | number 11 | November 2010 1581