Association between air pollution exposure, physical activity, and risk for cardiometabolic multimorbidity incidence: a cohort study from China

Cardiometabolic multimorbidity (CMM) has emerged as a major threat to health among older adults, yet the effects of air pollution and physical activity on CMM remain insufficiently investigated. This study aimed to examine the associations between air pollutant exposure, physical activity, and CMM risk, and to analyze the potential moderating and mediating roles of physical activity in the relationship between pollutants and CMM. This study utilized data from the China Health and Retirement Longitudinal Study (CHARLS) from 2015-2020, including 17,718 participants. We assessed exposure levels to PM_2.5, PM₁₀, NO₂, SO₂, CO, and O₃, and categorized physical activity into four levels (Q1-Q4) based on metabolic equivalent quartiles. Cox regression models were employed to analyze the effects of pollutants and physical activity metabolic equivalents on CMM, with restricted cubic splines for dose-response analysis. Additionally, Baron & Kenny's method was applied to evaluate the mediating effect of physical activity. During the follow-up period, 741 participants developed CMM. After multivariate adjustment, all pollutants were significantly associated with increased CMM risk, with SO₂ (HR=2.768, 95% CI: 2.526-3.033) showing the most significant impact. Higher levels of physical activity demonstrated pronounced protective effects. Dose-response analyses revealed non-linear relationships between pollutants and CMM (P for nonlinear < 0.001), while physical activity exhibited a protective non-linear relationship with CMM (P for nonlinear = 0.037). Mediation analysis identified significant partial mediating effects of physical activity in the relationships between pollutants (PM_2.5, PM₁₀, NO₂, and O₃) and CMM, with mediation proportions ranging from 5.71% to 19.88%, with NO₂ showing the highest mediation proportion (19.88%). Long-term exposure to air pollutants is significantly associated with increased CMM risk, while higher levels of physical activity confer substantial protective effects. Pollutants may partially increase CMM risk indirectly by inhibiting physical activity.