Molds are microscopic fungal organisms. They grow as networks of interlocking filaments that spread on and into organic matter, leading to its decomposition. When clusters of these filaments become large enough, they are visible as fuzzy growths of mold or mildew. Bread mold is a familiar example.

Mold lives by secreting enzymes that break down the organic matter on which the mold is living, making it available as a nutrient for the mold. This breaking-down action is what makes mold damaging to household items such as paper, fabric, or leather. Molds reproduce by forming spores, which travel through the air, settle on other plant or animal organic material, and grow into new clusters of filaments. The mold spores travel over large distances, and are often more numerous than the pollen grains of plants. Proteins in the secretions of the mold filaments, and possibly in the mold spores, are capable of causing allergy in some people.

WHERE ARE MOLDS FOUND?
Molds are found primarily in warm, dark, and damp locations. Unlike plants, which use energy from the sun to produce food, molds obtain their energy by digesting other organic matter. To do so, they need moisture. Molds grow out of doors, and, if the humidity is high enough, indoors as well. They do not have the clearly defined seasons that pollens do, but are at their peak during months of high humidity, and are absent in outdoor air only if there is snow on the ground. They can grow on grass and on the bark of trees, and are plentiful in fallen leaves and other decaying vegetation. Indoors, they live in areas of high humidity, such as basements or poorly ventilated bathrooms.