New York's subway stations are frequented by 4.3 million people every day and with so many people passing through, it's understandable that the air may not be so clear.
For a study, scientists at the University of Colorado took samples of air from the subway system in 2007 and 2008 and found that 15% of it consisted of human skin.
Most of it was from the head or feet of passengers, but 12% of the skin came from belly buttons, ear canals, armpits, and even rear ends.
Interestingly, the researchers found that there wasn't much difference between the air inside the stations to the air outside at Union Square Park. The only difference being that the stations had higher levels of fungi, which the researchers put down to wood rot.
Fortunately, they found no organisms that are cause for alarm.