While it's a well-known saying, it's not a wise way to look at your ducts. Your ducts can be littered with all sorts of debris. Even as you're interpreting this, there could be years of increase from debris, dust, dirt, pollen, hair, dander, and other things, all of which can interfere with other cleaning efforts. Dirty ducts can also concession the health of some of the occupants on your property, mostly those with allergies and other respiratory problems.