Yea nobody is walking borough to borough,that isn't feasible.
The air pollution in the subway is TERRIBLE,people are inhaling steel dust amongst other things,the ventilation is terrible,and this is not even mentioning the experiments your own government has been conducting on you by pumping chemicals into the system.
I no longer operate trains but I can recall many a time smelling odd chemicals down there and feeling light headed,I tell all the new hires to wear respiratory masks when they are at work nowadays.