Mold and mildew are simple, microscopic organisms that can grow virtually anywhere if they have adequate moisture, nutrients and appropriate temperatures. Depending on the particular mold or mildew, growing colonies can be almost any color from white to black. Most household molds and mildews are black, grey, or charcoal colored. Spores of dozens of kinds of mold and mildew are present at all times in indoor and outdoor air. These spores can settle, germinate and grow wherever good growth conditions are found. They can grow on soil, plants, dead plant materials, foods, fabrics, paper, wood and many other materials. Most molds are not harmful. In fact, molds have important roles in the environment and in living systems. In soil molds play a crucial part in decomposition of organic matter and in making nutrients available to plants.

Molds and mildews can be very destructive to materials on which they grow, and are usually unwelcome and unhelpful in homes. They cause staining, decomposition (rotting of materials) and objectionable, musty odors. Where colonies are extensive they can also produce enough spores, and by-products to be harmful to health. Many of the by-products of mold and mildew are irritating to skin, eyes and respiratory tracts. Some molds produce true allergic sensitization and allergic reactions in susceptible people. Some molds produce toxic by-products that could be harmful to skin, and poisonous if ingested or inhaled in quantity.