To examine the air quality in metropolitan subway systems in the United States, investigators collected air samples from 71 stations across 12 transit lines in New York City (NYC), Boston, Philadelphia, and Washington, DC during rush-hour periods. Samples of particulate matter (PM) and black carbon (BC) were collected from the Philadelphia transit system in 2015; all others were collected between June and September 2019. The NYC transit lines included the MTA, PATH, and LIRR systems—35 platforms in 30 stations with known elevated concentrations of PM2.5 were selected as sampling sites. Sites in Boston and Washington, DC were chosen based on a combination of factors, including ridership, the number of underground stations, and delivery of electricity (i.e., overhead or third rail).

Real-time PM2.5, BC2.5, and relative humidity (RH) were measured on each subway platform and approximately 0 to 8 meters from an aboveground station entrance for 5 to 10 minutes during morning and evening rush hours (with the exception of Philadelphia stations, where data was only collected in the evening). The investigators also obtained data on particle composition (i.e., total carbon, organic carbon, elemental carbon, Fe, and Si) at each station. Measurements of air particle concentration were collected onboard trains as the investigators traveled between stops and averaged for each run. Gravimetric data and RH measurements were used to correct and adjust real-time PM2.5 concentrations.