The connection between exposure to airborne fungi and disease was first established in the late 1800's. Since that time it has been clearly demonstrated that the exposure to airborne fungal spores or hyphal fragments can cause a variety of respiratory diseases, including infections (e.g. aspergillosis), and allergic reactions (e.g. hay fever, asthma, and hypersensitivity pneumonitis), as well as acute toxicosis and cancer from exposure to the metabolic products (e.g. mycotoxins).

The toxigenic mold Stachybotrys has received considerable attention because of the recent association between exposure to this mold and cases of pulmonary hemorrhage and hemosiderosis in young infants. Nine of the infants died. More recently, other health complaints, including permanent brain damage, have been associated with exposure of older children and adults to Stachybotrys.

Nevertheless, in terms of numbers, the allergic reactions, by far, outnumber other types of fungal diseases. More than 80 different fungi have been associated with symptoms of respiratory tract allergies. Thus, in terms of allergic diseases, there is not a definitive list of "good" fungi and "bad" fungi, since virtually all fungal spores are likely to be allergenic in some individuals.

In fact, the species that are the most common in outdoor air (such as Cladosporium, Alternaria, Penicillium, and Aspergillus) are some of the most common causes of allergic diseases.

Mold and Health Effects

Molds are a major source of indoor allergens. Molds can also trigger asthma. Even when dead or unable to grow, mold can cause health effects such as allergic reactions. The types and severity of health effects associated with exposure to mold depend, in part, on the type of mold present, and the extent of the occupants' exposure and existing sensitivities or allergies. Prompt and effective remediation of moisture problems is essential to minimize potential mold exposures and their potential health effects.

Because of the sporadic nature of these allergic diseases, there are no published standards of risk associated with the exposure to airborne fungi (I.e. threshold levels of mold spores). During the growing season, outdoor levels of mold spores routinely range from 1,000-10,000/m. Indoor levels of molds should generally be less than one-third of the outdoor levels, where outdoor air is the only source, and should be qualitatively similar. The presence of any one specific microorganism exceeding 500/m can also lead to the presumption of a building-related source.

Thus, while the indoor environment should provide air that is cleaner than the outdoor air in terms of mold spores, occasionally buildings become breeding grounds for fungal growth. Depending on the amount and location of the fungal growth, the concentration of airborne fungal spores may become elevated well above outdoor levels, thus exposing the building occupants to hazardous levels of these spores. For example, molds growing on wall surfaces, ceiling tile, etc. may release spores into the air from the movement of the air due to human activity. Moisture is generally the limiting factor for mold growth in these situations, with little growth on surfaces if the relative humidity is less than 70%. Wet surfaces from roof leaks, broken pipes, condensation, or over-watered plants are also conditions that promote mold growth.

http://www.epa.gov/mold/moldguide