Dust consisting of dirt, pollen, germs,  fibers, mold spores, hair and more gets drawn into your ventilation system. About 95% of these particles will pass through the filter and back into the room  air, or it accumulates on the fan blades, grills, or in the ductwork.

This buildup can reduce air flow up to 40%. It is also a breeding ground for bacteria and molds  that can cause allergic reactions in many people and is also a source of musty odors. Remember -- most problems with indoor air quality are invisible and odorless!  Ductwork should be cleaned every three years.