Transit authorities have often addressed the quality of the air outside the bus, but only recently have people began discussing the quality of the air inside the bus.
Filter media efficiency is very low and although not technically simple, increasing the MERV rating is not practical as the compact systems are not able to handle the back pressure.
Dust, odors, VOC’s and microbiological contaminants are in the air space and pass through the HVAC filter. Much of this is recycled again and again in a suspended state during operation of the vehicle.
Perhaps this is one of the key reasons health advocates recommend avoiding public transit during the flu season.