“…Studies have shown that the concentrations of particular matter (PM) 10 and PM2.5 in subway systems were âŒ20-50% higher than those in the aboveground environment (38,39) and were affected by train running, crowd activity, season, and outdoor climate (38,(40)(41)(42). Atmospheric particles in subway systems contain high concentrations of metal compounds with the potential for oxidation, including manganese (Mn), chromium (Cr), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn), nickel (Ni), and molybdenum (Mo), as well as organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), which come from subway tracks, wheels, chain rings, brake pads, and pantographs (33,35,37,43,44). The elevated particles increased commuter exposure to them; for example, subway personal PM exposure increased by 3%, Fe exposure increased by nearly 200%, Mn exposure increased by 60%, and Cu exposure increased by 40% (45,46).…”