(Fig. 7a) indicates that PM2.5 and coarse PM have a common origin. Although this common PM source cannot be determined based on the data presented in this manuscript, previous studies have attributed metallic components of PM to originate from the friction of the wheels on the steel rails, the vaporization of metals due to sparking, wear of brakes, and particulate resuspension and dispersion from train and passenger movement. The upcoming chemical analysis of fine and coarse PM will help determine the degree to which the aforementioned sources may contribute to PM exposure in the underground environment.