Contribution of Long-Term Exposure to Outdoor Black Carbon to the Carcinogenicity of Air Pollution: Evidence regarding Risk of Cancer in the Gazel Cohort

BACKGROUND: Black carbon (BC), a component of fine particulate matter [particles with an aerodynamic diameter ≤2:5 lm (PM_2:5)], may contribute to carcinogenic effects of air pollution. Until recently however, there has been little evidence to evaluate this hypothesis. OBJECTIVE: This study aimed to estimate the associations between long-term exposure to BC and risk of cancer. This study was conducted within the French Gazel cohort of 20,625 subjects. METHODS: We assessed exposure to BC by linking subjects’ histories of residential addresses to a map of European black carbon levels in 2010 with back-and forward-extrapolation between 1989 and 2015. We used extended Cox models, with attained age as time-scale and time-varying cumulative exposure to BC, adjusted for relevant sociodemographic and lifestyle variables. To consider latency between exposure and cancer diagnosis, we implemented a 10-y lag, and as a sensitivity analysis, a lag of 2 y. To isolate the effect of BC from that of total PM_2:5, we regressed BC on PM_2:5 and used the residuals as the exposure variable. RESULTS: During the 26-y follow-up period, there were 3,711 incident cancer cases (all sites combined) and 349 incident lung cancers. Median base-line exposure in 1989 was 2.65 10⁻⁵ /m [interquartile range (IQR): 2.23–3.33], which generally slightly decreased over time. Using 10 y as a lag-time in our models, the adjusted hazard ratio per each IQR increase of the natural log-transformed cumulative BC was 1.17 (95% confidence interval: 1.06, 1.29) for all-sites cancer combined and 1.31 (0.93, 1.83) for lung cancer. Associations with BC residuals were also positive for both outcomes. Using 2 y as a lag-time, the results were similar. DISCUSSION: Our findings for a cohort of French adults suggest that BC may partly explain the association between PM_2:5 and lung cancer. Additional studies are needed to confirm our results and further disentangle the effects of BC, total PM_2:5, and other constituents. https://doi.org/10.1289/EHP8719.