Airborne Fungal Spores: What You Need to Know

Fungal spores and especially mold spores are a normal component of the outdoor air. They are microscopic structures that function as “seeds” to allow fungi to reproduce. Apart from acting as “seeds” for reproduction, many fungal spores are adapted for dispersal and for survival, often for extended periods of time, in unfavorable conditions.

Airborne Fungal Spores In the Outdoor Environment

Spores are released into the environment from fungi growing as saprophytes or parasites. Saprophytes are fungi that grow on dead or decaying organic matter in the soil or elsewhere in the environment. Parasites, on the other hand, infect living host plants. Fungal spore concentrations outdoors are usually high in the late summer or fall.

Airborne Fungal Spores In the Indoor Environment

The air in many indoor environments also contain fungal spores. Many of the indoor air spores infiltrating from outdoors whenever doors or windows are opened or fresh air is introduced. If the indoor spores originated from outdoors the spore composition in indoor and outdoor air are similar but the concentration indoors is lower than outdoors.

Many indoor environments have fungal growth if they are wet or damp.  Fungal growth is often found on wet or damp carpets, upholstered furniture, showers, shower curtains, other bathroom fixtures, potted plants, and the soil around potted plants. With indoor sources, the spore composition indoors and outdoors is different and some spore types indoors are significantly higher indoors than outdoors.

Causes of Fungal Growth

Fungal spores often settle on surfaces and will not germinate unless there is enough moisture.  When moisture or even high humidity is available, spores can germinate and form fungal colonies from which thousands of new spores are produced. If the new spores settle on wet or damp organic materials then they develop into new fungal colonies and the process continues.

Health Effects Associated with Fungal Spores

Many fungi, if not all, are capable of causing allergic responses in susceptible individuals. This happens when one inhales excessive airborne spores.

Apart from the allergenic property of fungal spores, some fungi produce mycotoxins, which can be present within the spores. Mycotoxins are toxic compounds but are only produced under certain environmental conditions. Some mycotoxins are carcinogenic and others can damage the immune system. Though there has been a great deal of interest in the past few years on mycotoxins in indoor air little is known about their negative health effects.

While it is generally known that indoor fungi are potentially a health hazard, there is no agreed upon level of airborne fungal spores that signifies contamination and no health-based guidelines exist. In absence of guidelines it is recommended that any indoor fungal growth be removed and the moisture problem be fixed so as to control fungal growth.

Fungal Spores Trapping and Identification

As mentioned earlier, airborne fungal spores can pose major health and economic risks to humans, animal populations and even plants.  Therefore, there is need to be aware of the composition and concentration of airborne fungal spores to better control them. Fungal spore trapping is used to predict plant disease outbreak and can also be used to determine if occupants of a moldy building are exposed to excessive levels of spores in the air. Fungal trapping can be performed by using various air sampling methods, each of which has its particular advantages and disadvantages. Many types of samplers and analytical methods have been used over the years.

Generally there are two sampling methods for airborne fungal spores. Both methods have their advantages and disadvantages. The first method is the non-culturable sampling that provides an indication of the composition and concentration of total (viable and nonviable) airborne fungal spores present in the environment. Direct microscopy is used to analyze the samples, providing both a qualitative and quantitative assessment of spores in the air in a short time frame. Non-culturable sampling method is used to determine whether the composition of airborne fungi in a building is normal and typical or indicative of moisture problems.

The report for direct microscopy provides both raw spore count for individual spore types and total concentration expressed as spores per cubic meter (spores/m³) of air. The proportions of each spore type are also calculated and important groupings of fungal types are summarized to facilitate interpretation of results. The most common sampling devices for non-culturable air samples are the Allergenco and Air-O-Cell cassettes. These air sampling cassettes are designed for the rapid collection and analysis of a wide range of airborne aerosols. These include fungal spores, pollen, insect parts, skin cell fragments, fibers, and inorganic particulates.

The second method is the culture-based air sampling method. This is a selective method for only those fungal spores that are viable and can grow on selected agar media for growth. Culture-based air sampling is the method of choice if one is interested in determining the specific fungal species contaminating the air. A number of samplers for culturable fungi and for total spore counts are commercially available. These samplers are widely used by allergists, industrial hygienists, and environmental professionals. Sampling for culturable fungi includes settling method and volumetric samplers. Settling method is not recommended

Acceptable Levels of Airborne Fungal Spores

Acceptable levels of airborne fungal spores have not been established and the sampling methods and analytical techniques employed to assess airborne bio-contaminants are varied and non-standardized. There is no one ideal sampling or analytical method. Selection of sampling and analytical methods depends on the nature of the data that is sought. This in turn depends on the question sampling was intend to answer. Combinations of sampling and analytical methods can provide a wide range of data that can be effectively adopted to different environmental settings.

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Dr Jackson Kung'u

Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at www.drjacksonkungu.com.

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