The paper throwaway filters found in residential furnaces and air conditioners do not improve the air quality in the building. They are there to keep dirt out of system components such as fans and coils. On the other hand, the banks of filters found in commercial installations remove pollutants from the air as well as protect the equipment. The efficiency of a filter determines both the size of particles and percentage of particles that can be removed. Dirty filters release pollutants into the ventilation air and allow dirt to build up on coils and fans. Replace dirty filters carefully so the trapped pollutants are not released into the air again: