Air ducts, just like anything else that has air blowing through it constantly, will get dirty over time. Having your air ducts cleaned can not only help with musty smells, dust, and allergens, but can also keep your AC unit running at optimal levels.
Occasionally, your coils should be cleaned to prevent buildup. If you have a dirty duct system, this buildup can happen more quickly and cause issues with the system. Nice Air recommends that you have your ductwork cleaned at a minimum at least once every ten years.
You should have your ducts cleaned and/or inspected immediately if any of the following apply:
1.) There is visible mold growth
2.) Ducts are infested with vermin (for example, rodents or insects)
3.) Ducts are plugged with large amounts of debris and/or dust, or particles are actually being released into your home from your supply vents.