Airborne Fungi in Indoor Environments

Estelle Levetin, PhD
University of Tulsa

Fungal spores, commonly called mold spores, are a normal component of the outdoor air. They are present in the atmosphere anytime that the ground is not covered with ice or snow. The spores are discharged from fungi growing as saprophytes (existing on dead or decaying organic matter is the soil or elsewhere in the environment) or parasites (infecting living tissues - most are plant pathogens). Many species of fungi are found as leaf surface microorganisms where they exist on organic matter produced by the plant. Concentrations outdoors can be high, especially in the late summer or fall. We have occasionally recorded hourly concentrations in Tulsa above 100,000 spores per cubic meter of air.

The air in many indoor environments also contains spores; however, indoors we speak 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. 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 served 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 or even less. 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 present in low concentrations. Until more data become available on dose-response relationships to specific fungi, investigators must rely on their knowledge and experience to interpret the sampling data.

Many fungal spores (possibly all) 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 are only produced under certain environmental conditions. Many are carcinogens, and others can impare the immune system. There has been a great deal of interest in the past few years on mycotoxins in indoor air; however, the medical literature does not currently support the negative health effects found in the news media.

The following provides some background information on fungi commonly found in air samples:

· Acremonium - common soil fungus. Occurs indoors in wet environments on a variety of substrates. Considered allergenic but not well studied.
Alternaria - common fungal genus especially in outdoor environments; worldwide it is often the second most common mold spore genus. It is a common saprophyte, leaf surface organism, and plant pathogens. Alternaria spores are highly allergenic. Can occur on a number of indoor substrates, but it is usually not found at high levels indoors.
Ascospores - spores produced by the sexual stage of many fungi. Usually from outdoor sources with the exception being Chaetomium which readily occurs on indoor substrates. Suspect allergens but not well studied.
Aspergillus - common fungal genus, especially in indoor environments. In nature this typically occurs in the soil. Various species of Aspergillus have been implicated in aspergillosis, a serious and persistent lung infection. Other species of Aspergillus are known to produce mycotoxins. Also known to be allergenic.
Aureobasidium - commonly occurring black yeast. In the natural environment it is found on leaf surfaces, indoors it is common where moisture accumulates such as kitchens and bathrooms.
Basidiospores - spore produced by mushrooms, bracket fungi, puffballs and similar higher fungi. Common in the outdoor environment and introduced indoors with outdoor air. Indoor sources of basidiospores are from dry rot. Many basidiospores are allergenic.
Chaetomium - common fungus found indoors and outdoors. Readily colonizes cellulose-based materials and is commonly found on sheet rock and other indoor substrates. Allergenicity is not known. Some species produce mycotoxins but these have not been well studied.
Cladosporium - common fungal genus occurring both indoors and outdoors. Spores of Cladosporium are the most abundant outdoor spore type and have a worldwide distribution. The fungus normally exists as a saprophyte or weak plant pathogen, but a few species of Cladosporium have been reported as human pathogens. The spores are known to be allergenic.
Curvularia - a fungus whose spores are often present in the outdoor air. It occurs as a saprophyte and plant pathogen. Although little is known, it is assumed to be allergenic and also can be a cause of fungal sinusitis.
Drechslera-type - several genera of very similar fungi including the genera Drechslera, Bipolaris, Helminthosporium, and Exserohilum. The fungi are either plant pathogens or saprobes in the natural environment. They are known to be allergenic and can also cause fungal sinusitis.
Epicoccum - a common saprophyte both indoors and outdoors. Epicoccum can grow on many substrates indoors. At times the outdoor concentrations are high in agricultural areas. Known to be allergenic and possibly cross react with Alternaria.
Fusarium - a common saprophyte and important plant pathogen. It normally is found in the soil but can be introduced indoors. Indoors it is often found in the bathroom or other areas with moisture. Some species of Fusarium produce mycotoxins.
Memnoniella - a soil fungus in the natural environment. Indoors it is found on cellulose-based materials that have gotten wet. Allergenic status not known. It is closely related to Stachybotrys and often occurs with it. It is known to produce mycotoxins.
Myrothecium - a soil fungus in the natural environment occurring on cellulose-based materials. Capable of forming mycotoxins similar to those from Stachybotrys
Non-sporulating - many fungi do not reproduce in culture because they require specialized environmental conditions. Without spores it is not possible to identify fungal colonies.
Paecilomyces - widespread genus occasionally found indoors. Able to grow on a number of indoor substrates. Similar to Penicillium. Considered to be allergenic and able to form mycotoxins. Not distinguishable from Penicillium on total spore samples
Penicillium - common fungal genus, especially in indoor environments. Found in water damaged buildings and homes. Also common on foodstuffs. In nature this typically occurs in the soil. Known to be allergenic. Some species of Penicillium are also known to produce mycotoxins.
Phoma - soil fungus, common on plant material. Indoors found on walls, ceiling tiles, shower curtains. Known to be allergenic but the spores are not readily dispersed by air currents.
Pithomyces - common saprophyte that is prevalent outdoors especially during summer and fall. Occasionally found indoors.
Rhizopus - common saprophyte that grows on a variety of substrates, especially spoiled food. Allergenic
Smut spores - spores produced by smut fungi, common plant pathogens that are especially prominent on cereals and other grasses. Very common in the outdoor atmosphere almost year round. These can be introduced indoors with outdoor air. Spores are allergenic but not well studied.
Stachybotrys - a soil fungus in the natural environment. Commonly found indoors on wet materials containing cellulose, such as wallboard, jute, wicker, straw baskets, and other paper materials. Some consider it to be allergenic although little is known. Able to produce mycotoxins.

General References on Bioaerosols

1. Burge, H. (Ed). 1995. Bioaerosols, Lewis Publishers, Boca Raton.

2. Committee on Damp Indoor Spaces and Health. 2004. Damp Indoor Spaces and Health. National Academy Press, Washington DC.

3. Cox, C.S. 1987. The Aerobiological Pathway of Microorganisms, John Wiley & Sons, New York.

4. Flannigan B, Samson RA, Miller JD. 2002. Microorganisms in Home and Indoor Work Environments: Diversity, Health Impacts, Investigation and Control, Taylor and Francis, NY.

5. Gregory, P.H. 1973. The Microbiology of the Atmosphere, 2nd ed., Halstead Press, New York

6. Levetin, E. 1995. "Fungi". In: Bioaerosols, ed. H. Burge, Lewis Publishers, Boca Raton pp: 87-120.

7. Macher, J. (Ed). 1999. Bioaerosols: Assessment and Control, ACGIH, Cincinnati

8. Storey E, Dangman KH, Schenck P, DeBernardo RL, Yang CS, Bracker A, Hodgson MJ. 2004. Guidance for the Clinicians on the Recognition and Management of Health Effects Related to Mold Exposure and Moisture Indoors. University of Connecticut Health Center, Division of Occupational and Environmental Medicine, Center for Indoor Environments and Health, Farmington, CT.

Useful Links

CDC- Center for Disease Control

EPA - Environmental Protection Agency

Indoor Air Quality Association mold resources

California Dept of Health Services

Indoor Air Quality Info Sheet

Stachybotrys Info Sheet

Washington Dept of Health

Dr. David Malloch - University of Toronto

Doctor Fungus Home Page

Environmental Microbiology Lab - Description of commonly occurring fungal genera