Long-term air pollution exposure and mental health in French adults of the CONSTANCES cohort: Role of black carbon independently of PM2.5

Ambient air pollution could be associated with poor mental health. Black carbon (BC) has been highlighted as a crucial component of particulate matter; however, its isolated role independent from the total particulate matter mass has been poorly studied. Our study aimed to examine the associations between long-term exposure to particulate matter with a diameter <2.5 μm (PM_2.5), BC and nitrogen dioxide (NO₂) and psychological distress in the French CONSTANCES cohort and to assess the role of BC independently of PM_2.5. This cross-sectional study included 104,146 adults. Psychological distress was assessed in 2019 using the General Health Questionnaire-12 (GHQ-12). Annual concentrations of PM_2.5, BC, and NO₂ estimated from land-use regression models at each participant's residential address. Negative binomial models with different covariate adjustments were used. A residuals method was used to assess the independent role of BC. Incident rate ratios (IRR) per an interquartile range (IQR) increase in exposure to each pollutant were calculated. Stratified analyses by age, sex, education and season were also conducted. Mean exposures were 9.38 μg/m³ for PM_2.5 (IQR = 2.6), 1.15 × 10^-5/m for BC (IQR = 0.5) and 19.1 μg/m³ for NO₂ (IQR = 11.5). Exposure to each pollutant was significantly associated with higher psychological distress (IRR (95 % CI): 1.052 (1.014-1.092) for PM_2.5, 1.078 (1.055-1.101) for BC, and 1.082(1.057-1.109) for NO₂). Stronger associations were found for men, elderly, lower-educated, and during warm season. BC residuals were significantly associated with higher psychological distress when regressed on PM_2.5 (1.055 (1.039-1.071)) and when regressed on NO₂ (1.067(1.041-1.093)). Exposure to ambient air pollution was associated with psychological distress, with BC showing a deleterious role independently of PM_2.5 and NO₂.