Molds are a natural part of the environment. They are crucial to nature in the outdoors, as they are responsible for breaking down dead organic matter such as fallen leaves and dead trees. Molds are a type of fungi, which is the kingdom of organisms that also include mushrooms, mildews and yeasts. Molds reproduce via tiny spores that are invisible to the naked eye and float through outdoor and indoor air. Mold spores range in size from 10 to 30 microns. How small is that? A human hair is typically about 100 microns.
More than 1,000 different kinds of indoor molds have been found in U.S. homes and there is always going to be some level of mold present in the air both indoors and out. They can be found just about anywhere and can grow on any pretty much any organic substance such as wood, paper, foods and insulation. Although spores are capable of resisting dry periods and can survive for long periods of time in adverse environmental conditions, none of them will grow without water or moisture. The other necessary component is oxygen. Since "mold food" and oxygen are always abundant, excess moisture is the one thing that can be controlled.