Indoor Fungi

Fungal spores are a normal component of the outdoor air. The air in indoor environments also contains spores; however, indoors we think of these spores as contaminants when the fungi have colonized indoor substrates. Actually, the outdoor air may be the source of the spores whenever fresh air is introduced from open doors, windows, or mechanical ventilation systems. In addition, many indoor locations may serve as amplification sites for the growth of fungi. Such sites include carpeting, upholstered furniture, showers, shower curtains, other bathroom fixtures, potted plants, and the soil around potted plants. Anytime moisture or even high humidity is available, spores can germinate and fungi can grow and produce thousands of new spores utilizing organic material in these sites. In buildings with central HVAC (heating, ventilation, and air conditioning) systems, properly maintained in-system or in-duct filters should remove many of the spores present. Many instances are known where the HVAC system itself can serve as an amplification and dissemination site for fungal spores. In these cases fungi have been found growing on air filters as well as in the ducts. This can often be prevented by routine maintenance.

There is no agreed upon level of airborne fungi that signifies contamination, and no health-based guidelines exist. Some researchers suggest the levels should be less than 25 to 33% of outdoor levels. One factor that must be considered at all times is the intrusion of outdoor air. A home with open windows and doors may have fungal concentrations that are up to 95% of outdoor spore levels, while a large office building with central HVAC and low air exchange may have 5% of outdoor levels. In non-problem buildings the types of indoor fungi will parallel those outdoors. However, in problem buildings where there is fungal contamination, one or two spore types may dominate the air samples. Often these are not present in the outdoor air or are present in low concentrations. Cladosporium, Penicillium, and Aspergillus are commonly found growing on indoor substrates, and their conidia are often found at high concentrations in indoor air samples from contaminated buildings.

Many fungal spores are allergenic, capable of causing allergic responses in susceptible individuals. A small group of fungi are also human pathogens. While many human pathogens just cause mild or annoying conditions, such as athlete's foot and ringworm, other human pathogens can cause severe and debilitating diseases.

Some fungi produce mycotoxins, which can be present within the spores and the mycelium. Mycotoxins are potent compounds, but they are only produced under certain environmental conditions. Many mycotoxins are carcinogens, and others can impair the immune system. Most of the mycotoxin research has focused on the contamination of food by toxin-forming fungi.