The Washington Post looks at a recent study of the air in subway stations, etc. Not to anyone's susprise, the air is filled with contaminents. In one PATH station, the air was equivalent to that of a nearby forest fire.
The breathed materials include metals and organic toxins.
Paywall possible:
https://www.washingtonpost.com/local/trafficandcommuting/subway-air-contamination/2021/02/12/68ab3c72-6d3d-11eb-9ead-673168d5b874_story.html