Longitudinal Associations Between PM2.5 Chemical Components and Allostatic Load: An Individual Fixed-Effects Analysis in a Nationwide Panel Study

The longitudinal impact of fine particulate matter (PM2.5) and its specific chemical drivers on multi-system physiological dysregulation, quantified by the allostatic load index (ALI), remains unclear. Leveraging panel data from 7421 middle-aged and older adults in the China Health and Retirement Longitudinal Study (CHARLS), we employed individual fixed-effects models—strictly controlling for time-invariant confounding—to evaluate the associations of PM2.5 and five components with ALI. Within-person increases in PM2.5 and all evaluated components significantly elevated ALI scores. Notably, secondary inorganic aerosols (SIAs: nitrate, ammonium, and sulfate) exerted the most pronounced effects, exhibiting J-shaped non-linear dose-response curves that suggest high-concentration exposures may overwhelm homeostatic buffering thresholds. This study provides the first longitudinal evidence identifying SIAs as the primary pollutants driving cumulative multi-system physiological decline in aging populations, offering critical support for component-specific air quality management.