Impact of air pollution on depressive symptoms and the modifying role of physical activity: Evidence from CHARLS study

The association between air pollution and depressive symptoms has not been thoroughly investigated, and the role of physical activity (PA) is particularly intriguing. Although PA can help alleviate depression, it may also lead to increased inhalation, potentially amplifying the adverse effects of air pollution. A total of 17,332 participants aged 45 years and older from the 2018 wave of China Health and Retirement Longitudinal Study (CHARLS) were included in this study to assess the causal effect of air pollution on depressive symptoms in China and to clarify the role of PA in this relationship. Depressive symptoms were assessed based on the Center for Epidemiological Studies Depression Scale (CES-D). Data on particulate matter (PM₁, PM_2.5, and PM₁₀), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), and carbon monoxide (CO) were retrieved from the ChinaHighAirPollutants (CHAP) dataset. PA levels were measured using a standardized questionnaire and classified into low and high categories. An instrumental variable (IV) method was used to estimate the causal effect of air pollution on depressive symptoms. Effect modification by PA was further analyzed. The IV estimates showed that all the air pollutants had significant adverse effects on depressive symptoms, with per IQR increase in PM₁, PM_2.5, PM₁₀, NO₂, SO₂, O₃, and CO associated with 1.57 (95% confidence interval (CI): 1.15, 1.99), 1.49 (95% CI: 1.10, 1.89), 1.71 (95% CI: 1.26, 2.17), 2.22 (95% CI: 1.62, 2.81), 1.30 (95% CI: 0.96, 1.65), 4.67 (95% CI: 3.37, 5.98), and 0.97 (95% CI: 0.71, 1.22) units increase in CES-D scores, respectively. The level of PA significantly modified this association, with higher PA levels mitigating the adverse impact of air pollution on depressive symptoms.