"After taking samples last year at 54 subway and PATH stations, researchers found toxins like iron and carbon get trapped as the trains pass under the river. They are thrown to the walls and since there is no air exchange, they remain in the stations.

The toxins have been linked to serious health problems. including asthma, lung cancer, and heart disease, according to the state Department of Health."

Via https://pix11.com/news/morning/nyc-subway-stations-near-rivers-have-the-worst-air-quality-study-shows/