Indoor Molds


Fate and Transport in the Environment

Methods for Monitoring in the Environment

Exposure Pathways

Methods for Measuring Human Exposure

Strategies for Preventing or Controlling Mold Exposure

Harmful Effects

Absorption, distribution, metabolism, and sites of toxicity


Molecular mechanism of action

Risk Assessment

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Characteristics of Molds

Molds are microscopic organisms, present virtually everywhere. Molds, along with mushrooms and yeasts, are fungi and are needed to break down organic material and recycle nutrients in the environment. Molds release countless tiny microscopic cells called spores, which spread easily through the air and form new colonies where they find the right conditions. The most common indoor molds are Cladosporium, Penicillium, Aspergillus, and Alternaria.

For molds to grow and reproduce, they need only a food source – any organic material, such as leaves, wood, paper, or dirt – and moisture (relative humidity > 60%). Growth occurs when there is moisture from water damage, excessive humidity, water leaks, condensation, water infiltration, or flooding. Because molds grow by digesting the organic material, they gradually destroy whatever they grow on. Sometimes, new molds grow on old mold colonies. (Photo: magnified mold spores)

Mold growth on surfaces can often be seen in the form of discoloration and/or staining, frequently green, gray, brown, or black but also white and other colors. It is also evident by an earthy or musty smell in the suspected area of growth (Photo: mold growing on wet ceiling tiles)

Certain types of molds may produce compounds that have toxic properties, which are called mycotoxins. They are not always produced, and whether a mold produces mycotoxins while growing in a building depends on what the mold is growing on, conditions such as temperature, pH, humidity or other unknown factors. When mycotoxins are present, they occur in both living and dead mold spores and may be present in materials that have become contaminated with molds.

Stachybotrys chartarum (also known as Stachybotrys atra) is a greenish-black toxigenic mold that has received increasing attention recently among indoor air researchers. It can grow on material with a high cellulose and low nitrogen content, such as fiberboard, gypsum board, paper, dust, and lint. While Stachybotrys is growing, a wet slime layer covers its spores, preventing them from becoming airborne. However, when the mold dies and dries up, air currents or physical handling can cause spores to become airborne.

Memnoniella is another mold found indoor most often with Stachybotrys on wet cellulose. It forms in chains and certain species do produce toxins very similar to the ones produced by Stachybotrys chartarum. It is commonly found as a dry spore that gets carried by the wind.