Exposure to Particulate Matter in Philadelphia Subway

Subways are an efficient mode of public transport in urban cities, reducing both traffic congestion and urban traffic emissions. However, a high level of particulate matter (PM) in underground subways is an environmental health concern for both subway commuters and workers. This study presents the concentrations of PM₁, PM_2.5, PM₁₀, ultra-fine particles (UFP), and black carbon (BC) from the Philadelphia-15^th Street station, one of the busiest subway lines in the city. The measurements were carried out concurrently from both the underground subway platform and the aboveground roadside level on five different working days (7/13/2022 to 7/21/2022) for 6 hours (9 am to 3 pm). The mean concentrations of PM₁, PM_2.5, and PM₁₀ from underground subway station were 117.2 ± 63 µg/m³, 125.3 ± 67 µg/m³, and 191.2 ± 134.3 µg/m³, respectively. Compared to the aboveground roadside level, the underground subway station’s PM₁, PM_2.5, and PM₁₀ were 5.4, 5.7, and 7.6 times higher, respectively. The lung-deposited surface area (LDSA) of UFP (63.6 ± 39.5 µm²/cm³) and BC (9.97 ± 6.3 µg/m³) at the subway platform was 3 times and 11.7 times higher, respectively, than the aboveground roadside level. PM and BC for underground subway platform were positively correlated, suggesting the source of BC was from both the outside road traffic and inside the platform from the diesel-operated cars used for the maintenance in the underground subways. This study shows the need for proper maintenance and control methods, such as improved ventilation and frequent cleaning, to reduce PM exposure at this subway station.