Mold growth is not likely to occur in ac ducts because ac unit air movement has to be extremely fast. If ac units are not functioning properly, mold can grow inside ac ducts. The use of an ultraviolet light can help eliminate biological growth.
When ac units are turned off, ac ducts become warm and humid, creating an ideal environment for household fungi, which can cause certain respiratory infections. Stagnant ac ducts also attract vermin such as moths and ants.