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Mold gets in vents too!

Black mold can be found in any environment that provides food for the mold to thrive, including inside walls and inside vents and other duct work. When you experience black mold in air vents cleaning the vents out is the first step, but you will also need to eliminate the source of the mold to eliminate it completely.  Mold spores are generally air borne, or spread through air. Spores can enter the duct work by simply blowing in from another area of the home, or from outside. The conditions need to be right for the spores to develop, however. A single mold spore can blow inside your home and begin to develop into a colony if the conditions are right. Warmer, moist environments are ideal for mildew spores to grow.

Souce:  https://blackmoldcontrol.com/remove-black-mold-in-air-ducts-vents/