The Facts About Mold: A Glossary

Allergen: A substance that elicits an antibody response and is responsible for producing allergic reactions by inducing formation of IgE. IgE is one of a group of immune system mediators. IgE antibodies, when bound to basophiles in circulation or mast cells in tissue, cause these cells to release chemicals when they come into contact with an allergen. These chemicals can cause injury to surrounding tissue—the visible signs of an allergy. Fungal allergens are proteins found in either the mycelium or spores. Only a few fungal allergens have been characterized, but all fungi are thought to be potentially allergenic.

Biocide/Fungicide: Chemicals that limit the growth of or kill microorganisms such as fungi.

“Black mold”: This poorly defined term, which has no scientific meaning (also called “toxic black mold”), has been associated with Stachybotrys chartarum. While only a few molds are truly black, many appear black. Not all molds that appear to be black are Stachybotrys.

Fungi: Neither animals nor plants, fungi are classified in their own kingdom. The fungi kingdom includes a very large group of organisms, including molds, yeasts, mushrooms, and puffballs. There are more than 100,000 accepted fungal species—but current estimates range up to 10 million species. Mycologists (people who study fungi) group fungi into four large groups according to their reproduction method.

Hidden mold: Visible mold growth on building structures that is not easily seen. For example: above drop ceilings, within a wall cavity (the space between the inner and outer structure of a wall), inside air handlers, or within the ducting of a ventilation system. Visible mold within a ventilation duct is in immediate contact with the occupied space. Spores released from such growths are affected by air movement and relative humidity. Spores of mold growth in wall cavities are released by the air exchange between the wall cavity and occupied space. The rate of spore movement between such spaces is typically slow. Volatile gases produced by visible mold growth in wall cavities are also known to occur and migrate to occupied spaces even through air barriers.

Microbial volatile organic compounds (MVOCs): Chemicals produced by fungi as a result of their metabolism. Some of these chemicals are responsible for the characteristic moldy, musty, or earthy smell of fungi, whether mushrooms or molds. Some MVOCs are considered offensive or annoying. Specific MVOCs are thought to be characteristic of wood rot and mold growth on building materials. The human nose is very sensitive to mold odors, sometimes more so than current analytical instruments.

Mold: A group of organisms that belong to the fungi kingdom (see Fungi). Although the terms mold and fungi have been commonly referred to interchangeably, all molds are fungi, but not all fungi are molds.

Mycotoxin: Compounds produced by “toxigenic fungi” that are toxic to humans or animals. By convention, the term “mycotoxin” excludes mushroom toxins and compounds of low potency or toxicity only in in vitro systems. The ordinary use of the term refers to compounds of importance in agriculture. This includes a small number of very potent compounds such as deoxynivalenol, aflatoxin, fumonisin, ochratoxin, and zearalenone. It also includes the much less common nivalenol, T-2/HT-2 toxins, as well as some other Penicillium and Aspergillus toxins and toxins from S. chartarum and Pithomyces chartarum. The biochemical targets of mycotoxins are usually many but the mechanisms of toxicity, even within families of toxins, are typically different.

The genetic property to produce mycotoxins is particular to given species. Some species including Fusarium graminearum and S. chartarum have genetic subpopulations called chemotypes that produce different mixtures of compounds. In the case of F. graminearum, these chemotypes are distributed by continent. In the case of S. chartarum, both chemotypes occur together.

Remediate: To fix a problem. Related to mold contamination, remediation includes fixing the water/moisture problem and the cleaning, removal, and/or replacement of damaged or contaminated materials.

Spore: General term for a reproductive structure in fungi, bacteria, and some plants. In fungi, the spore is the structure that may be used for dissemination and may be resistant to adverse environmental conditions.

Stachybotrys: Genus that includes approximately 10 species and occurs mainly on dead plant materials. Of these, Stachybotrys chartarum is the most common. This species is widespread and typically grows on straw. In the indoor environment, it is commonly found on cellulosic materials including paper, canvas, and jute that are wetted to a water activity > 0.98. This is a toxigenic mold. There are two chemotypes of this species that produce trichothecenes plus spirolactones or atranones plus spirolactones; these toxins have been demonstrated on mold-damaged building materials. The closely related species Memnoniella echinata occurs on the same materials but does not produce potent trichothecenes. Both chemotypes of S. chartarum and M. echinata typically occur together on samples of very wet cellulosic materials with M. echinata being more important in warmer climates. This fungus does not cause invasive disease. Antigens to S. chartarum have been identified.

“Toxic mold”: This has no scientific meaning, since the mold itself is not toxic. The metabolic byproducts of some molds may be toxic (see Mycotoxin).

Toxigenic fungi: Fungi that can produce mycotoxins (see Mycotoxin).

Common Indoor Fungi:

Alternaria: A genus comprised of approximately 50 species, most of which are saprophytes or plant pathogens. Alternaria alternata is an extremely common saprophyte found worldwide on plants, wood, wood pulp, textiles, and food. A. alternata grows on the surfaces of leaves (phylloplane) and occurs in outdoor air at modest levels, peaking in July or August depending on the location (reaching perhaps 500 spores/m3). The allergens of A. alternata can induce reactions at very low concentrations in sensitized individuals. Phylloplane strains of A. alternata that are found in air do not produce AAL toxin. Some produce the phytotoxin (a compound toxic to plants) alternariol and related metabolites.

Aspergillus: The asexual stage of a number of ascomycetes. Species of Aspergillus are distributed worldwide, although they are more common in warmer climates. These species grow on a vast array of organic materials. There are 182 accepted species, although only 40 occur with any frequency. Species of Aspergillus include several of considerable economic importance: A. flavus is the main producer of the potent carcinogen aflatoxin and A. fumigatus is an important cause of the invasive disease aspergillosis. Several species are common on building materials, including A. versicolor. A. fumigatus is common in outdoor air in some regions during the fall, occurring on composting materials.

Cladosporium: A genus comprised of approximately 500 species, most of which are saprophytes or plant pathogens; perhaps 20 are common. Cladosporium sphaerospermum, C. cladosporoioides, and C. herbarum are the most common species. All are found on plants, wood, wood pulp, textiles, and food. Of the three, C. sphaerospermum is the species typically found on building materials. The other two are phylloplane species that occur in outdoor air at high levels, peaking in June, July, or August depending on the location (reaching perhaps 10,000 spores/m3). C. herbarum produces a wide variety of allergens, and approximately 10 percent of the population is sensitized to Cladosporium. Phylloplane strains of Cladosporium do not produce metabolites with material toxicity.

Penicillium: The asexual stage of a number of ascomycetes. The species of Penicillium are found worldwide but are more common in temperate climates. These species grow on a vast array of organic materials. There are 225 accepted species, although only 70 occur with any frequency. Species of Penicillium include several of considerable economic importance: P. verrucosum produces ochratoxin on cereals and P. chrysogenum produces penicillin. Many Penicillium species cause damage in damp building materials, including the toxigenic species P. aurantiogriseum.