NYU Grossman research (published on Dec. 30 in the journal Transportation Research Part D: Transport and Environment) searched for answers why the New York's subway stations near river tunnels had much more polluted air compared to other subway stations. They found that "stations neighboring river tunnels had 80% to 130% higher concentrations of potentially dangerous particles in the air compared with stations only two or three stops further away from rivers".

They have concluded that the subway tunnels beneath the river have limited ventilation compared to other tunnels therefore debris accumulates in time and gets airborne, creating an unhealthy situation for the passengers. One solution is more often cleaning of debris.

https://finance.yahoo.com/news/subway-stations-near-river-tunnels-123000976.html

the research paper at Transportation Research Part D: Transport and Environment:

https://www.sciencedirect.com/journal/transportation-research-part-d-transport-and-environment