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

Abstract

Background: Black carbon (BC), a component of fine particulate matter [particles with an aerodynamic diameter $\le 2.5\mu m$ (${\text{PM}}_{2.5}$)], 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 ${\text{PM}}_{2.5}$, we regressed BC on ${\text{PM}}_{2.5}$ 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 baseline 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 ${\text{PM}}_{2.5}$ and lung cancer. Additional studies are needed to confirm our results and further disentangle the effects of BC, total ${\text{PM}}_{2.5}$, and other constituents. https://doi.org/10.1289/EHP8719.

Figure 1.

Black carbon (top) and ${\text{PM}}_{2.5}$ (bottom) concentrations between 1989 and 2015 at residential addresses of 19,348 participants of the French Gazel cohort. Black carbon (${10}^{-5}/\mathrm{m}$) and ${\text{PM}}_{2.5}$ (${\mu \mathrm{g}/\mathrm{m}}^3$) are depicted by yearly boxplots in black (minimum, 25th percentile, median, 75th percentile, outliers calculated as 75th percentile plus 1.5 times the interquartile range, and maximum) and violin plots in gray (two rotated kernel density plots depicting the probability of each exposure level and informing on the skewedness of the distribution).

Figure 2.

Associations between cumulative black carbon (left) and ${\text{PM}}_{2.5}$ (right) and all-site incident cancer in the main, sensitivity, and stratified analyses in the Gazel cohort, with the number of identified cancer cases among the number of participant-year over the follow-up. Hazard ratios and confidence intervals expressed for one IQR increase in ln-transformed cumulative exposure to black carbon or ${\text{PM}}_{2.5}$ in separate single-pollutant Cox model with attained age as underlying time-scale and time-dependent variables, adjusted for sex, cumulative smoking pack-years, passive smoking, alcohol use, BMI, education, socioeconomic status, family status, fruit and vegetable consumption, occupational exposure to lung carcinogens, age at inclusion, and calendar time. Exposures were lagged 10 y. Participants were excluded from the analysis if they were diagnosed with cancer before 1999. See Table S4 for corresponding numeric data. Unless specified otherwise, these model-based estimates were computed using MICE to address missing data and were pooled following Rubin’s rules. Note: BMI, body mass index; IQR, interquartile range.

Figure 3.

Associations between cumulative black carbon (left) and ${\text{PM}}_{2.5}$ (right) and lung incident cancer in the main and sensitivity analyses in the Gazel cohort, with the number of identified cancer cases among the number of participant-year over the follow-up. Hazard ratios and confidence intervals expressed for one IQR increase in ln-transformed cumulative exposure to black carbon or ${\text{PM}}_{2.5}$ in separate single-pollutant Cox model with attained age as underlying time-scale and time-dependent variables, adjusted for sex, cumulative smoking pack-years, passive smoking, alcohol use, BMI, education, socioeconomic status, family status, fruit and vegetable consumption, occupational exposure to lung carcinogens, age at inclusion and calendar time. Exposures were lagged 10 y. Participants were excluded from the analysis if they were diagnosed with cancer before 1999. See Table S5 for corresponding numeric data. Unless specified otherwise, these model-based estimates were computed using MICE to address missing data and were pooled following Rubin’s rules. Note: BMI, body mass index; IQR, interquartile range.