Effects of physical activity in polluted air on physiological health in healthy populations: A systematic review and meta-analysis of controlled trials.

The health consequences associated with exercising in polluted air are not fully understood. Therefore, we synthesized controlled trials that compared exercise performed under higher air-pollution exposure with a corresponding lower-exposure condition (e.g., filtered air or a lower concentration) in healthy participants, and explored potential effect modifiers. We searched PubMed, Embase, the Cochrane Library, Web of Science Core Collection, and Scopus for studies published between 2000 and 2024, and included 35 eligible controlled trials. Risk of bias was assessed using RoB 2 for randomized crossover trials and ROBINS-E for non-randomized studies, and the overall certainty of evidence using the GRADE framework. Random-effects meta-analyses using Hedges' g as a pooled effect size were conducted, with subgroup and meta-regression analyses exploring moderators. Results for pulmonary function showed that FEF25-75 declined significantly (g = -0.323, 95% CI -0.633 to -0.014, I2 = 19%, k = 5, certainty of evidence (CoE)), with smaller reductions in FEV1 (g = -0.183, 95% CI -0.334 to -0.032, I2 = 0%, k = 15, high CoE) and FVC (g = -0.131, 95% CI -0.280 to 0.018, I2 = 0%, k = 14, moderate CoE). For cardiovascular function, there was a modest rise in systolic blood pressure (g = 0.279, 95% CI 0.093 to 0.464, I2 = 0%, k = 9, high CoE), while diastolic pressure and heart rate were largely unaffected. Results for hematological outcomes showed that white blood cells (g = 0.479, 95% CI -0.002 to 0.960, I2 = 73%, k = 6, very low CoE) increased, and red blood cells (g = -0.791, 95% CI -1.353 to -0.230, I2 = 74%, k = 5, very low CoE) decreased, whereas results for inflammatory cytokines were inconsistent. Findings for hematological and inflammatory markers should be interpreted cautiously given heterogeneity and generally low certainty of evidence. Overall, these modest but consistent changes suggest that repeated exposure during exercise may accumulate respiratory and cardiovascular risks, underscoring the need to promote activity in cleaner environments to safeguard health benefits.