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The Fungal Ecology of Indoor Environments

Bacteria, fungi, yeasts and algae are found wherever moisture, temperature and food sources are present. In the indoor environment, this includes virtually all structural and furnishing surfaces. Many of the organisms are helpful and occur naturally in our environment. Others can cause health problems ranging from simple discomfort and physical irritation to allergic sensitization. Because microorganisms are always present, it is the excess quantity of microbes that should be of concern. A healthy building does not support fungal growth, whereas an unhealthy building does.

There are three methods of fungal air sampling. Quiescent air sampling is conducted when the activities surrounding the air sampling are minimal to none; for example, after normal work hours. Semi-aggressive air sampling may be conducted when approximately twenty percent of the normal population and work activities are occurring. Aggressive air sampling is conducted when at least 100% of the normal population and work activities are occurring, such as when carpets and building surfaces are actively disturbed due to walking across the carpeting and actively agitating wall coverings. The best method to use depends on the type of results desired. To determine the source of contamination, highly aggressive sampling is appropriate. To characterize the fungal ecology of an average day for the indoor environment, semi-aggressive to aggressive air sampling would be appropriate.

Fungi air sampling is usually conducted in the indoor air using an Andersen Sampler with appropriate agar media for fungi sampling, in many cases Rose Bengal Agar (RBA) is utilized. Air sampling results are reported in colony Forming Units per cubic meter of air (CFU/m3). There are no OSHA, MIOSHA or ACGIH standards for "safe" levels of microbial contamination, although the US Air Force environmental lab at Brooks Air Force Base has recommended an action level of 200 CFU/m3 . In general, results obtained from bioaerosol air sampling conducted with the Single Stage Andersen Air Sampler that are below 70 CFU/m3 are extremely good (White et. al.). Microbial counts exceeding 1000 CFU/m3 may be suggestive of improvement or investigation of the indoor air quality. Actual illness or symptoms may be absent at this concentration (Morey et. al.).

Outdoor levels of fungi can be 4-10 times those found in the typical office building when heating ventilation and air-conditioning (HVAC) filters are performing properly, and when no indoor sources are present (Morey et. al.). Air samples should be taken from within the building as well as outdoors.

Depending on the type of lab analysis requested, the predominant genus and species of fungi are normally identified. Clavibacterium-like bacteria is commonly found on plant leaves. Stachybotrys atra is a fungus which hydrolyses the cellulose of ceiling tiles as a nutrient source. It is generally present on water damaged ceiling tiles. Arthrobacter-like bacteria is commonly found in soil or water from the outdoor environment. Micrococcus genus bacteria primarily inhabits human skin, therefore the source for the problem may be a person.

The fungal ecology of indoor environments is an important aspect of Indoor Air Quality, and should not be overlooked.

Written by:

Laura B. Randall, CIH
Industrial Hygienist, Enviroair Consultants

Article Provided by: The Argus Group

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