In certain situations, air duct cleaning is advised by the EPA. According to its site, you should consider professional air duct cleaning services if you see any of the following dangers:
Visible mould inside your air ducts or on other components of your cooling and heating systems.
Evidence of rodents or insects inside your air ducts.
Partially or completely clogged air ducts, filled with excessive amounts of dust and debris.
Dust and debris released into your home through the supply registers.
These air pollution dangers certainly do need to be remedied with a professional cleaning, perhaps without the follow-up chemical sprays, and the larger issue is discovering and remedying the cause of mould and mildew.
Broken ducts resulting in water seepage need to be replaced to eliminate the cause of mould or mildew that can make you and your family sick.
The EPA says that it is normal for return registers to get dusty because it is pulling dust-laden air through the grates on a regular basis.
Dusty registers are not always an indicator that your entire air duct system needs cleaning. Simply vacuum — or remove and clean — your return registers at this time of year to improve your indoor air quality, cut down on allergens and provide a better appearance on these visible facets of your rooms.