Mold is a type of fungus that consists of small organisms found almost everywhere. They can be black, white, orange, green, or purple. Outdoors, molds play an important role in nature, breaking down dead leaves, plants, and trees. Molds thrive on moisture and reproduce by means of tiny, lightweight spores that travel through the air. You’re exposed to mold every day. In small amounts, mold spores are usually harmless, but when they land on a damp spot in your home, they can start to grow. When mold is growing on a surface, spores can be released into the air where they can be easily inhaled. If you're sensitive to mold and inhale a large number of spores, you could experience health problems.