Dirty Air Ducts are one of many places that trap dust in a home. Pollutants that enter the home both from outdoor and indoor activities such as cooking, cleaning and smoking can cause additional contamination in the Air Ducts in a home. Dust enters the home as the doors and windows are opened due to entry and exiting of persons living in a home. As the dust settles in a home it gets re-circulated through the HVAC system and settles further in the Air Duct system. Part of having a healthy and clean home is having clean Air Ducts. If your ducts look dirty, they probably are. Consider this, if one does not vacuum or sweep their floors, would there be dust build up? Absolutely, and thus the same for the Air Ducts in your home.
Below is a basic diagram of a residential air duct and HVAC system. Picture credit: www.epa.gov