Public transit reduces the number of single occupancy vehicles on the road, which reduces the amount of pollutants entering the air.
Increased use of public transportation can help reduce air pollution and the health benefits that accompany it.  Public transportation produces far fewer quantities of air pollutants, like carbon monoxide, carbon dioxide, and nitrogen oxides, per passenger mile.  Public transit reduces the number of single occupancy vehicles on the road, which reduces the amount of pollutants entering the air.  In addition, most public transit does not have door to door service, meaning that riders have to walk a few blocks to get to a transit stop and to their final destination.  Both of which reduce vehicle emissions.