OBJECTIVES: The goals of this study were to assess the air quality in subway systems in the northeastern United States and estimate the health risks for transit workers and commuters. METHODS: We report real-time and gravimetric PM2:5 concentrations and particle composition from area samples collected in the subways of Philadelphia, Pennsylvania; Boston, Massachusetts; New York City, New York/New Jersey (NYC/NJ); and Washington, District of Columbia. A total of 71 stations across 12 transit lines were monitored during morning and evening rush hours. RESULTS: We observed variable and high PM2:5 concentrations for on-train and on-platform measurements during morning (from 0600 hours to 1000 hours) and evening (from 1500 hours to 1900 hours) rush hour across cities. Mean real-time PM2:5 concentrations in underground stations were 779 ± 249, 548 ± 207, 341 ± 147, 327 ± 136, and 112 ± 46:7 lg=m3 for the PATH-NYC/NJ; MTA-NYC; Washington, DC; Boston; and Philadelphia transit systems, respectively. In contrast, the mean real-time ambient PM2:5 concentration taken above ground outside the subway stations of PATH-NYC/NJ; MTANYC; Washington, DC; Boston; and Philadelphia were 20:8±9:3, 24:1±9:3, 12:01 ± 7:8, 10:0±2:7, and 12:6±12:6 lg=m3, respectively. Stations serviced by the PATH-NYC/NJ system had the highest mean gravimetric PM2:5 concentration, 1,020 lg=m3, ever reported for a subway system, including two 1-h gravimetric PM2:5 values of approximately 1,700 lg=m3 during rush hour at one PATH-NYC/NJ subway station. Iron and total carbon accounted for approximately 80% of the PM2:5 mass in a targeted subset of systems and stations. DISCUSSION: Our results document that there is an elevation in the PM2:5 concentrations across subway systems in the major urban centers of Northeastern United States during rush hours. Concentrations in some subway stations suggest that transit workers and commuters may be at increased risk according to U.S. federal environmental and occupational guidelines, depending on duration of exposure. This concern is highest for the PM2:5 concentrations encountered in the PATH-NYC/NJ transit system. Further research is urgently needed to identify the sources of PM2:5 and factors that contribute to high levels in individual stations and lines and to assess their potential health impacts on workers and/or commuters.