Fungus and mold
Fungi comprise a vast world of organisms, perhaps as many as 300,000 species. The U.S. Environmental Protection Agency defines funguses, or fungi, as "types of plants that have no leaves, flowers or roots."3Fungi include such seemingly unrelated substances as poisonous and non-poisonous mushrooms; organisms that can cause athlete’s foot, fingernail infections, and some types of pneumonia; molds found in cheese, peanut butter, mulch, hay, grains, and spoiled foods; and the black material growing in bathroom grout.
Fungi reproduce by means of spores which are spread through the air but land and survive on surfaces. Many spores can remain dormant for long periods under dry conditions, but typically develop into fungi in the presence of moisture.
Outdoors, fungi break down organic matter, including leaves, grass clippings, and dead trees. The fungi themselves constitute a large mass of material with many types of spores. These spores vary with the material on which they are found, the season, and the weather. At any given time, the same types of spores are found indoors because they enter through doors and windows and on clothing and shoes.
Molds are fungi. Homes and structures often provide many opportunities for mold spores to grow, even in the absence of frank water leaks: seepage through foundation walls and cellar floors, dehumidifiers and air conditioners, window condensation, defective plumbing, damp bathrooms, air filters, and potted plants.4 Different types of mold spores thrive on different surfaces; for example, the "yellow slime" found on hardwood mulch won’t be found growing in a tiled bath enclosure.
Common indoor mold species include Aspergillus, Alternaria, Acremonium, Cladosporum, Dreschslera, Epicoccum, Penicillium, Stachybotrys, and Trichoderma.2 Specific types of molds can be tested for and identified. This allows comparison of indoor and outdoor mold species at a given location and time. If the two don't correlate, at least roughly, it is possible that indoor mold colonies have developed. Even if they're not in a visible location, such molds can release spores and other material into the indoor air.
The presence of molds or mold metabolites does not necessarily correlate with human illness, though. Tests identify the presence of these substances at a moment in time, and not necessarily the time frame in which individuals are exposed and illness develops. Also, the presence of these substances does not necessarily mean exposure: the fact that they are present doesn't necessarily mean they were inhaled.5
Note that identification of specific mold spores is not necessary when cleaning up indoor mold colonies. It may or may not be useful when treating health effects of mold exposure, depending on the circumstances. In any case, the role of testing for indoor mold is undefined, because as yet there are no standards for interpreting these tests.6