There are a few things that complicate matters. First, dust isn't just your average dirt; it can consist of a variety of materials such as pet dander, animal fur, bird feathers and droppings, plant pollens and spores, along with human skin cells. Second people react differently to certain forms of airborne particle matter. According to HVAC equipment manufacturers, the frequency of re-cleaning varies from once every five years for residential homes, to one year for businesses with eight or fewer employees. Commercial buildings that use in-duct heating and cooling may need duct work cleaning more often than non-commercial buildings. The EPA recommends every 3-5 years.