Association between long-term exposure to outdoor black carbon and mortality in the French Gazel Cohort

Abstract

Background: Fine particulate matter (PM2.5) is associated with mortality, and recent evidence identified black carbon (BC) as one component of PM2.5 that may at least partly explain the health effects. Yet, this remains poorly documented. This study aimed to examine the association between long-term exposure to BC and mortality in a population-based French cohort. Methods: We used data from the Gazel cohort collected between 1989-2015, including geocoded residential address history. We used land use regression models with temporal extrapolation to estimate the exposure to BC and PM2.5 for 19,850 participants. We used extended Cox models with attained age as time-scale and time-varying average exposure to BC, adjusted for relevant covariates including sex, smoking status and cumulative pack-years, and including a 10-year lag, to estimate the association between long-term exposure to BC and all-cause and cardiovascular mortality. To handle confounding by PM2.5, we regressed BC against PM2.5 and used the residuals as the exposure variable in a sensitivity analysis. Separate analyses by sex and smoking status were also done to examine effect modification for all-cause mortality. Results: The median long-term BC exposure was 2.34 10-5/m (inter-quartile range (IQR): 1.09). We found a significant association between BC and all-cause mortality (n=1794) using long-term average BC and residuals, with respective hazard ratios (HR) of 1.10 (95%CI: 1.04-1.17) and 1.19 (1.11-1.26) per IQR increase. We found a similar association between BC and cardiovascular mortality (n=271) with a HR of 1.15 (0.99-1.35). The association with all-cause mortality slightly varied across population subsets with HRs of 1.10 (1.03-1.18) and 1.09 (0.96-1.24) for men and women, and 1.10 (0.97-1.25) and 1.14 (1.07-1.23) for never- and ever-smokers, respectively.Conclusions: We found positive associations between long-term exposure to BC and increased mortality, reinforcing the emerging evidence that BC is a harmful component of PM.