There are over 200 species of fungi to which people are routinely exposed indoors and outdoors. These include mold-like fungi, as well as other fungi such as yeasts (unicellular fungi forming pasty colonies) and mushrooms, which are characterized by the familiar fruiting bodies people think of as “mushrooms.” The terms “mold” and “mildew” are non-technical names commonly used to refer to any fungus that is growing in the indoor environment. These names are used interchangeably, although mildew is often applied to growths on fabrics, window sills or bathroom tiles. Because molds and mildews may be any of several natural classes of fungi, these names are not interchangeable with the nomenclature used in biological classification systems.
In general, molds are characterized by a visible vegetative body, or colony, composed of a network (mycelium) of threadlike filaments (hyphae), which infiltrate the mold’s food or habitat. Mold colonies may appear cottony, velvety, granular, or leathery, and may be white, gray, black, brown, yellow, greenish, or other colors. Many reproduce via the production and dispersion of spores. They are usually saprophytes (i.e., they feed on dead organic matter) and, provided with sufficient moisture, can live off of many materials found in homes, such as wood, cellulose in the paper backing on drywall, insulation, wallpaper, glues used to bond carpet to its backing, and everyday dust and dirt.
Research indicates that certain molds can cause a variety of adverse human health effects, including allergic reactions and immune responses (e.g., asthma), infectious disease (e.g., histoplasmosis-a disease caused by the inhalation of spores of the fungus Histoplasma capsulatum (associated with bird or bat droppings); disease is most often asymptomatic but occasionally produces acute pneumonia or an influenzalike illness and spreading to other organs and systems in the body.), and toxic effects (e.g., aflatoxin-induced liver cancer). Molds are thought to play a role in asthma in several ways. They are known to produce a large number of proteins that are potentially allergenic, and there is sufficient evidence to support associations between fungal allergen exposure and asthma exacerbation and upper respiratory disease. In addition, molds may play a role in asthma via release of irritants that increase potential for sensitization, or release of toxins (mycotoxins) that affect immune response. Finally, mold toxins can cause direct lung damage leading to pulmonary diseases other than asthma.