Particulate matter across subway microenvironments worldwide: A systematic review and quantitative health-risk assessment

Subway particulate matter (PM) pollution represents an increasingly important environmental health issue, yet its spatial variability and exposure implications across subway microenvironments remain insufficiently synthesized. This study reviewed 196 studies from 24 countries spanning a 15-year period, to characterize mass concentrations and chemical composition of PM₁₀, PM_2.5, and PM₁ across key subway spaces, including station offices, concourses, platforms, tunnels, carriages, and commuting exposure, and to quantify associated health risks. Subway PM exhibited pronounced spatial heterogeneity: concentrations were highest in tunnels, followed by platforms and commuting exposure, whereas concourses and carriages showed comparatively lower but still elevated levels. Limited evidence for office environments nevertheless indicated severe pollution, with peak concentrations reaching 405.1 µg/m3 for PM₁₀ and 212.1 µg/m3 for PM_2.5, suggesting an overlooked occupational exposure scenario. Strong enrichment of metal components was observed in subway aerosols. Fe showed the highest indoor-to-outdoor ratios, with mean values up to 66.5 for PM_2.5 and 41.5 for PM₁₀. Multi-study synthesis identified key drivers of PM distribution, including station depth, platform door configuration, operation frequency, ventilation, season, and outdoor pollution, reflecting combined internal generation and external infiltration. Health-risk assessment revealed that carriages constituted the highest-risk microenvironment. Mean cancer risks reached 8.4 × 10−4 for PM_2.5 and 4.5 × 10−4 for PM₁₀, while non-cancer risks were 28.0 and 2.4, respectively. Subway employees experienced greater risks than passengers. Element-specific analysis showed carcinogenic risk was primarily driven by Cr and Co, whereas non-carcinogenic risk was mainly associated with Co, Ni, and Mn, providing a basis for microenvironment-targeted air quality control in subway systems.