A maze of heating and air conditioning ducts runs inside the walls and floors of 80 percent of American homes. 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.
To find out if your ducts need cleaning, pull off some supply and return registers and take a look. If a new furnace is being installed, you should probably invest in a duct cleaning at the same time, because chances are the new blower will be more powerful than the old one and will stir up a lot of dust.