The concern over the presence of certain types of bacteria and moulds in indoor air is three-fold: pathogenicity, toxicity, and allergenicity.

Pathogens:

Moulds are known to cause a wide variety of diseases (mycoses) in humans, including aspergillosis, blastomycosis, and zygomycosis.

Aspergillosis is caused by certain species of Aspergillus . Infection can occur in the respiratory tract, sinuses, skin, or kidneys. This is considered a secondary infection and is of particular concern in immuno-compromised individuals. Blastomyces dermatitidis causes blastomycosis, initiated by inhalation of conidia into the lungs. Lung infections can be similar to bacterial pneumonia. Certain members of the zygomycetes, such as Absidia corymbifera , cause zygomycosis by invading the blood vessels of susceptible individuals. Infection can occur in the lungs, skin, gastrointestinal tract, sinuses, heart, bone, kidneys, bladder, and trachea.

Airborne bacteria also cause serious infections. Legionnaire's disease and Pontiac fever are both caused by Legionella pneumophila . The diseases are usually acquired through breathing water vapours containing the bacteria (ex. air conditioning units or whirlpool spas).

Toxins:

Many moulds are capable of producing mycotoxins which are harmful to humans. Mycotoxins are secondary metabolites that can produce acute or chronic disease, even in small concentrations. Most mycotoxins are produced by Aspergillus , Penicillium , Fusarium , and Alternaria . Stachybotrys is also well-known for its ability to produce satratoxin. Exposure to this toxin can cause flu-like symptoms and immune suppression. Satratoxin has been implicated in a cluster of fatal pulmonary hemorrhage in infants.

Allergens:

Many moulds are capable of inducing an allergic response in susceptible individuals. The response can be immediate hypersensitivity (ex. asthma), immune complex disorder (ex. Farmer's Lung), and hypersensitivity pneumonitis (ex. Humidifier Lung).