Inside the walls and floors of 80 percent of Oklahoma homes run a maze of heating and air conditioning ducts that connect each room to the furnace. As the supply ducts blow air into the rooms, return ducts inhale airborne dust and suck it back into the blower. Add moisture to this mixture and you've got a breeding ground for allergy-inducing molds, mites and bacteria. Many filters commonly used today can't keep dust and debris from streaming into the air and over time sizable accumulations can form - think dust bunnies, but bigger.