The Clean Transit Paradox: Elevated Particle Exposure and Health Impacts in Subway Microenvironments

Subway systems, integral to urban transit, are perceived as clean yet pose unique health challenges from elevated particulate matter (PM) in enclosed spaces. This comprehensive Review synthesizes global research on the characteristics, health risks, and control strategies of subway PM. Findings indicate that subway PM concentrations consistently exceed ambient levels with fine particles (PM_2.5) dominating the profile. Key components are iron-rich particles from mechanical wear, soluble metals, carbonaceous matter, microplastics, and biological agents, which confer distinct properties and heightened bioactivity. Major sources encompass internal processes such as wheel-rail friction, brake wear, material release, and passenger activities and infiltration of outdoor air pollutants. Exposure to subway PM triggers multifaceted health effects via oxidative stress, inflammation, genotoxicity, immune dysregulation, and vascular dysfunction, affecting respiratory, cardiovascular, and neurological systems in both commuters and occupational workers. Mitigation focuses on source control, engineering interventions, and personal protection. However, critical challenges persist, including incomplete composition characterization, limited epidemiological evidence, elusive toxicity mechanisms, and insufficient health risk-based standards. Future research should prioritize advanced compositional analysis, large-scale prospective health studies, mechanistic insights into key toxic components, and the development of integrated exposure-control frameworks. This Review underscores the urgency of addressing subway air quality to ensure public health and sustainable urban mobility.