The material settling in your ducts is comparable to that collecting in more visible locations. Your indoor airstream moves particulates through the house, and the ductwork is part of that path. Your air filter provides some defense against these materials, but it may not collect finer particles to the degree that you would like. Additionally, many homeowners tend to forget their air filters.
You may install a fresh filter at the start of heating or cooling season, failing to check again until the next season change. A clogged filter can collapse, allowing unfiltered air to move through the system. The result is extra dirt on your equipment and in your ducts. You can address this by making frequent checks of your filter throughout the heating and cooling seasons. Additionally, you will need to acknowledge that you can’t perfectly eliminate the infiltration of dirt into your system. HVAC maintenance and duct cleaning are important for addressing the residues that result from the movement of particles in your airstream. If you notice material visibly exiting your vents in connection with HVAC activity, a duct inspection is warranted.