The answer to 'air duct cleaning smells bad' is that a foul odor coming from your air ducts is often a sign of a deeper issue that requires professional attention. Common causes include mold growth, pest infestations, or a buildup of dust, debris, and other contaminants.
A smelly air duct system can be a health hazard, as it can circulate unpleasant and potentially harmful particles throughout your home. While some minor issues may be addressed with DIY cleaning, significant odor problems usually require a thorough inspection and cleaning by a licensed HVAC technician to identify and resolve the root cause.