Concentration-Dose Decoupling and Nonlinear Health Risks of Dynamic PM2.5 Inhaled Doses in Public Transit Microenvironments

Fine particulate matter (PM2.5) exposure in public transport microenvironments has important implications for commuter health, yet concentration-based assessments may not adequately reflect the dose actually inhaled by passengers. This study quantified dynamic PM2.5 inhaled doses in Taiyuan, China, using 1 Hz portable monitoring and matched travel surveys across 19 bus and metro routes during summer and winter 2025. After data screening, 1103 valid commuter samples were retained. We combined dose estimation with DML, XGBoost-SHAP, SEM, and Random Forest analysis to examine adjusted associations, explore potential nonlinear patterns, and characterize behavioral responses. Trip-averaged PM2.5 concentrations exceeded the WHO 24 h guideline on most monitored routes when interpreted as a health-based reference benchmark for short commuting exposures rather than as a direct regulatory exceedance metric. More importantly, a clear concentration-dose decoupling pattern was observed: 6.6% of trips fell into a low-concentration but high-dose category, indicating that prolonged in-vehicle exposure could substantially elevate inhaled dose even when PM2.5 concentrations remained below the sample median. The mean inhaled dose in the longer observed-duration group (top 20% by observed in-vehicle duration) reached 612.26 ± 412.21 μg, which was 7.2 times that of the remaining trips (84.87 ± 115.71 μg). DML results showed that inhaled dose, rather than PM2.5 concentration alone, was significantly associated with psychological distress. SHAP analysis suggested an exploratory threshold-like pattern at approximately 300 μg per trip, above which health-risk attribution increased rapidly. SEM results indicated that inhaled dose was associated with higher self-reported somatic burden, whereas PM2.5 concentration mainly influenced health indirectly through risk perception. These findings suggest that public transport exposure assessment should move beyond static concentration metrics and incorporate dynamic inhaled dose to better identify high-risk commuting scenarios and support more targeted health-oriented transit management.