The answer to 'why is my air duct moldy' is that mold growth in air ducts is usually caused by excess moisture or water intrusion. Mold can thrive in the dark, humid environment inside ductwork and spread quickly if left untreated. Common symptoms include a musty odor, visible mold growth, and poor indoor air quality.
The most effective solutions are to identify and address the root cause of the moisture problem, then thoroughly clean and disinfect the affected ductwork. This often requires the expertise of a professional HVAC or mold remediation specialist, as improper cleaning can disturb the mold and make the problem worse. DIY attempts at cleaning moldy ducts are generally not recommended unless the issue is caught very early.