Figure 1.

Mean (horizontal lines) gravimetric-based ${\mathrm{PM}}_{2.5}$ concentrations on underground subway platforms in four urban transit systems. The open circles represent the individual measurements. Stations represented in the Boston transit system: Broadway ($n=2$), Government Center–Blue Line ($n=2$), and Tufts Medical Center ($n=2$); in the PATH-NYC/NJ transit system: Newport ($n=2$) and Christopher Street ($n=4$); in the Philadelphia transit system: Race/Vine ($n=1$), City Hall ($n=1$), 13th Street ($n=1$), 15th Street ($n=1$), and 30th Street ($n=1$); and in the Washington, DC, transit system: Van Ness ($n=2$) and Capitol South ($n=2$). Filtered-based ${\mathrm{PM}}_{2.5}$ was collected for 30 to 60 min at $10\mathrm{L}/\text{min}$.