We estimated the effect of predicted BC on lung function by linear regression while adjust- ing for standard control variables, which included year of assessment, age, race/ethnicity (white vs. other race/ethnicity), and education as a marker of individual-level SES (model 1). To assess the potential for confounding, we examined the sensitivity of those results to fur- ther adjustment for smoking status (model 2) and a diagnosis of asthma and/or chronic bron- chitis (model 3). Finally, we used multilevel modeling to take into account the neighbor- hood-level SES markers (Goldstein 2003). We also considered tertiles of BC exposure to assess for a threshold effect or an exposure–response relationship. We next ran a series of stratified regres- sions as further sensitivity analyses. We first stratiﬁed our analyses by length of residence. Second, we conducted separate regression models for the warm and cold seasons. Third, we conducted a sensitivity analyses among the healthiest group of women, excluding those with an asthma diagnosis or chronic bronchi- tis. Finally, we ran the models stratified by smoking status. We deﬁned statistical signiﬁ- cance as a p-value < 0.05. We conducted all analyses in SAS version 9.0 (SAS Institute Inc., Cary, NC, USA). Results The sample of 272 women 18–42 years of age included 57% who self-identiﬁed as nonwhite (144 Hispanics, 2 blacks, 1 Asian, and 7 other race/ethnicity), 41% with less than high school education, and 18% who were current smokers (Table 1). Mean ± SD predicted annual BC exposure level was 0.62 ± 0.2 µg/m3; mean FEV1 was 2.62 ± 0.4 L; mean FVC was 3.10 ± 0.4 L, and mean FEF25–75% was 3.26 ± 0.8 L/sec. The IQR of BC was 0.22 µg/m3 (Table 1). BC was associated with race/ethnicity and education in bivariate analyses and with smok- ing status. Hispanics and women with a high school education or less had higher BC levels than did their counterparts. Nonsmokers also had higher levels of predicted BC exposure than did current or former smokers. Lower d 15 e itdc n10 rpe itco 5 rom fun 0 f g–5 t nu en l–10 c r eP –15 1st tertile 2nd tertile 3rd tertile FEV1 FVC FEF25–75% Figure 1. Linear regression models of predicted ter- tiles of BC by lung function measures (n = 272). All models were adjusted for age, race/ethnicity, edu- cation level, smoking history, asthma and chronic bronchitis, and year of assessment. Trend test: FEV1, p = 0.2; FVC, p = 0.3; FEF25–75%, p = 0.10. Black carbon and lung function among women levels of lung function were associated with a diagnosis of asthma or chronic bronchitis. In adjusted analysis, BC (per IQR increase) was associated with a 1.1% decrease [95% conﬁdence interval (CI), –2.5 to 0.3%] in FEV1, a 0.6% decrease (95% CI, –1.9 to 0.6%) in FVC, and a 3.0% decrease (95% CI, –5.8 to –0.2%) in FEF25–75% (Table 2). Adjustment for smoking history, asthma, and chronic bronchitis slightly increased the effect estimates. Further adjustment for environ- mental tobacco smoke exposure, use of a gas stove for cooking, and occupational dust exposure did not change the effect estimates. In multilevel models adjusting for percent unemployed within census tract and living in a census tract where > 20% of households earned less than the federal poverty guide- lines, the previously reported effect estimates actually increased slightly: FEV1, –1.44% (95% CI, –3.2 to 0.3%), FVC, –1.06% (95% CI, –2.7 to 0.5%), and FEF25–75%, –3.58% (95% CI, –7.2 to 0.06%). To explore the potential for an exposure–response relationship, we conducted additional analyses using tertiles of BC expo- sure. In analyses adjusted for age, race/ethnic- ity, education level, smoking history, asthma and chronic bronchitis, and year of assess- ment (Figure 1), the highest BC exposure group (mean, 0.8 µg/m3) had decreases in FEV1, FVC, and FEF25–75% compared with the lowest tertile group (mean, 0.5 µg/m3), although these differences were not statisti- cally significant (p > 0.05). Again, further adjustment for all other covariates did not change these ﬁndings signiﬁcantly. As sensitivity analyses, we conducted analy- ses stratiﬁed by length of residence (data not shown). Among women who had lived in their residence for > 2 years, effect estimates did not change from those previously reported for the entire group. We also conducted analyses strat- iﬁed by season. The effect estimates and 95% CIs for a one-IQR-range increase in BC during the summer months are, for FEV1, –1.32% (95% CI, –2.8 to 0.2%); FVC, –0.70% (95% CI, –2.1 to 0.2%); and FEF25–75%, –3.34% (95% CI, –6.3 to –0.4%); and for the winter are, for FEV1, –0.83% (95% CI, –2.2 to 0.5%); FVC, –0.52% (95% CI, –1.7 to 0.7%); and FEF25–75%, –2.73% (95% CI, –5.4 to –0.1%). In regression analyses on the healthiest group of women, those without an asthma diagnosis or chronic bronchitis, the effect esti- mates of BC on lung function were very similar to the effect estimates found in the full cohort. The group with the highest BC exposure had decreases in FEV1 (–2.6%; 95% CI, –6.6 to 1.4%), FVC (–1.7%; 95% CI, –5.4 to 2.1%), and FEF25–75% (–6.9%; 95% CI, –15.0 to 1.2%) compared with the lowest tertile group. We explored the potential of smoking his- tory to act as a modiﬁer of the association of interest. We noted significant interactions (p < 0.05) between BC and current smoking sta- tus and former smoking status for both FEV1 and FEF25–75%, so we repeated our adjusted linear regression models stratifying by smoking history (Table 3). We noted no significant effects of BC exposure among current smokers, but signiﬁcant reductions in FEV1, FVC, and FEF25–75% among former smokers (–4.4%, –3.1%, and –8.9%, respectively; all p < 0.05). Among never smokers, BC predicted a signiﬁ- cant decrease in FEF25–75% (4.4% decrease, p < 0.05) and decreases in FEV1 and FVC, but these did not reach statistical signiﬁcance. Discussion In this urban community–based cohort study, we found that predicted long-term BC expo- sure was associated with decreased lung func- tion among younger women, after adjusting for smoking history, sociodemographics, and past medical history. We noted decreases across all three measures of lung function even after excluding women with a diagnosis of asthma or chronic bronchitis. In the overall sample, the most robust ﬁnding was a signiﬁcant associa- tion between increased BC exposure and reduc- tions in FEF25–75%, which is an indicator of small-airway disease leading to airway obstruc- tion. This corroborates recent findings from Churg et al. (2003), who examined lung biopsy specimens under light microscopy comparing those from nonsmoking adult subjects with high and low air pollution exposure, document- ing that long-term exposure to ambient PM was associated with small airway remodeling. Notably, the observed effects on FEF25–75% were even stronger among nonsmokers in the stratified analysis. We also noted stronger effects in summer compared with winter months. Because windows are more likely to be closed and ventilation lower in the winter than in the summer, we think that the observed difference can most likely be explained by exposure differences. Table 3. Linear regression models of predicted BC (average summer and winter) by lung function meas- ures, stratiﬁed by smoking status (n = 272): effect estimate [% (95% CI)] for change in percent predicted lung function per IQR increase in BC (0.22 μg/m3). BC model Current smokers (n = 48) Former smokers (n = 61) Nonsmokers (n = 163) FEV1 0.62 (–2.1 to 3.4) –4.40 (–7.8 to –1.0) –0.98 (–2.9 to 0.9) FVC 0.64 (–2.0 to 3.3) –3.11 (–6.1 to –0.2) –0.32 (–2.0 to 1.4) FEF25–75% 2.63 (–3.7 to 8.9) –8.78 (–14.7 to –2.9) –4.39 (–8.1 to –0.6) aWe adjusted all models for age, race/ethnicity, education level, asthma/chronic bronchitis diagnosis, and year of assessment. Environmental Health Perspectives • VOLUME 116 | NUMBER 10 | October 2008 1335