Airborne fungal spores exposure limits

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airborne fungal spores

Airborne fungal spores

A common question often asked is what are the airborne fungal spores exposure limits in both indoor and outdoor environments?

In other words, what are the maximum spore concentrations that a healthy person can inhale without getting sick?

While there are no universally acceptable airborne fungal spores exposure limits, existing evidence suggests such limits would probably be hundreds of times higher than what the majority of people are exposed to daily.

Airborne fungal spores concentrations

Outdoor fungal spore concentrations vary between regions and seasons. Indoor levels are usually influenced by the outdoor levels. Outdoor fungal spore levels are highest in warm regions and in the summer and fall in temperate regions, and lowest in the winter. In summer outdoor fungal spores may range from 1,000 to 50,000 spores/m3 with Cladosporium as the most dominant fungal spore type.

Extremely high spore concentrations can be encountered in certain environments. For example, concentrations of 2 × 106 spores/m3 in buildings with water damage and visible mold growth have been reported. In another study, a gram (dry weight) of moldy hay gave 102 million fungal spores and 1200 million actinomycetes spores. Spore concentrations of up to 1600 million spores per m3 of air were recorded in a farm buildings while hay associated with Farmer’s Lung was being shaken for animal feed.

Composting sites can also generate very high concentrations of fungal spores. Aspergillus fumigatus spore concentrations in the range 104 to 107 colony forming units (CFU) per cubic meter of air are released when the compost is disturbed through activities such as shredding and turning.

Therefore handling of large quantities of compost by industrial workers exposes them to very high concentrations of A. fumigatus on a daily basis, which typically weakens their immunesystems leading to respiratory diseases.

Airborne fungal spores exposure limits

Fungal spores

Fungal spores

As mentioned earlier there are currently no guidelines on acceptable levels of airborne fungal spores exposure. However, existing evidence suggests that for non-sensitized persons, very high concentrations of spores would be required for them to experience any health effects.

In his book, The Fifth Kingdom (third edition), Dr. Bryce Kendrick states that about 20% of the human population is atopic, and easily sensitized by normal spore concentrations of up to 1,000,000 spores/m3. The other 80% would require higher spore concentrations (106-109 spores/m3) to be affected.

In a study involving individuals with sick building syndrome, it was found that inhalation of a single spore dose at levels of 4 × 103 Trichoderma harzianum spores/m3 and 8 × 103 Penicillium chrysogenum spores/m3 had not observed effects.

Similarly, in asthmatic patients allergic to Penicillium sp. or Alternaria alternata, lowest observed effect levels (LOELs) for reduced airway conductance were 1 × 104 and 2 × 104 spores/m3, respectively. Epidemiological studies of highly exposed working populations showed that lung function decline, respiratory symptoms and airway inflammation appeared at exposure levels of 105 spores/m3. The LOELs for nose irritation was approximately 3 × 106 spores/m3 whereas that of eye irritation was approximately 1 × 105 spores/m3 for an 8-hour exposure duration.

These studies support LOELs of approximately 105 spores/m3 for diverse fungal species in non-sensitized populations. However, it’s still difficult to set acceptable airborne fungal spores exposure limits due to the large number of fungal species and strains in indoor and outdoor environments and the large inter-individual variability in human reaction to fungal spores exposure.

If you want the expertise of our team to assist with your mold or bacteria testing, contact us by email or telephone to further discuss your project.


  1. O’Gorman CM (2011). Airborne Aspergillus fumigatus conidia: a risk factor for aspergillosis. Fungal Biology Reviews 25:151-157.
  2. Eduard W (2009). Fungal spores: A critical review of the toxicological and epidemiological evidence as a basis for occupational exposure limit setting. Crit Rev Toxicol. 39(10):799-864.
  3. Lacey, M. and West J(2006). The Air Spora: A Manual for Catching and Identifying Airborne Biological Particles. Chapter 2. Springer.
  4. Baxter DM, Perkins JL, McGhee CR, Seltzer JM (2005). A regional comparison of mold spore concentrations outdoors and inside “clean” and “mold contaminated” Southern California buildings. J Occup Environ Hyg 2:8–18.
  5. Kendrick, Bryce (2000). The Fifth Kingdom, Third Edition. Focus Pub.
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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

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