However, in response to increasing scientific and public awareness regarding the importance of clean air to human health, several studies have revealed unacceptably high levels of inhalable particulate matter (PM) in some subway systems. On top of being crucial to the mobility of city dwellers, subway systems can also play a pivotal role in reducing outdoor air pollution in large metropolises by helping to reduce motor-vehicle use. Internationally, more than 120 million people commute by subway every day, and this number will keep increasing in the future as the United Nations predicts that 75% of the world’s population will be urban by 2050.