The "green exercise" paradox: Quantifying the estimated breakpoint of acute physiological perturbation from ambient PM2.5 in urban runners using wearable biosensors.

Exercise in polluted urban environments creates a paradox: physical activity benefits health, yet elevated PM2.5 may simultaneously cause acute physiological perturbation. We conducted a prospective panel study of 72 runners in Nanchang, China, each completing 8 monitored sessions across four seasons (576 total sessions; March 2024-January 2025). Personal PM2.5 was measured using calibrated Plantower PMS7003 monitors (R2 = 0.93 versus BAM 1022). Outcomes included heart rate variability (RMSSD), pulse oximetry (SpO2), spirometry (FEV1/FVC), and circulating biomarkers (hsCRP, IL-6, 8-OHdG) in a subset (n = 40). PM2.5 averaged 41.2 ± 26.2 μg/m3 (range: 6.6-212.5). In cluster-robust models (validated by fixed-effects within-estimation, ICC = 0.13), PM2.5 independently predicted reduced RMSSD (β = -0.070, p < 0.001), decreased SpO2 (β = -0.006, p = 0.002), and FEV1 decline (β = -0.038, p < 0.001; all FDR q < 0.01). Segmented regression identified estimated breakpoints in the approximate 18-47 μg/m3 range, with the FEV1 breakpoint at 18.2 μg/m3 (CI: 11.9-24.5; Davies p = 0.021) reaching statistical significance and the RMSSD breakpoint at 31.6 μg/m3 (95% CI: 16.3-46.9; Davies p = 0.085) serving as an approximate guide given the wide confidence interval and marginal significance. At PM2.5 ≥ 75 μg/m3, FEV1 declined 3.06%, remaining below the exercise-induced bronchoconstriction threshold (≥10%). Temperature-adjusted biomarker models confirmed PM2.5 as the dominant predictor (hsCRP β = 0.236, p < 0.001; temperature p = 0.38). Green space routes reduced PM2.5 by 34% and showed a buffering trend for HRV (interaction p = 0.084). Shortening exercise duration by one-third on street routes achieves inhaled dose equivalence with green routes. These findings support integrating air quality monitoring into exercise guidance while recognizing that chronic exercise benefits likely outweigh acute perturbations for healthy adults.