Unhealthy air pollution is elevated in subway stations

Subway stations in five US cities have surprisingly high levels of indoor air pollution (Environ. Health Perspect. 2021, DOI: 10.1289/EHP7202). These subterranean emissions may raise the risk of heart and lung disease in commuters and transit workers, says Terry Gordon, who studies environmental medicine at New York University’s Langone School of Medicine and led the work. Gordon and his colleagues measured the prevalence of airborne particles smaller than 2.5 µm in diameter (PM_2.5) inside 71 subway stations during rush hour in New York City; Jersey City, New Jersey; Boston; Philadelphia; and Washington, DC. The researchers found PM_2.5 concentrations much higher than those in the ambient outdoor air in those cities, and on average two to seven times as high as 24 h exposure limits of 35 µg/m³ set by the US Environmental Protection Agency. Subway stations in New York City consistently scored worse than those in other cities. The findings suggest the need for better ventilation, the authors say. Passenger subway trains are electric, so they don’t generate emissions from combustion. But maintenance locomotives, which run on diesel, may be a source. Another probable source is dust created by friction on the rails—by train wheels, for example. These particles have a much higher iron content than those found outdoors. Researchers need to further characterize the chemical composition and toxicity of subway particulate matter, Gordon says.