Indoor mould colonizations vs. normal background mould exposures
Anyone reading about mould-related problems in homes will soon encounter the phrase ‘mould is ubiquitous,’ meaning ‘mould is everywhere.’ Many moulds produce spores that are airborne. Thus, in fungal studies, at least some mould can be grown from any air sample that hasn’t been strongly filtered. It is deceptive, though, to use this fact to suggest that mould growing indoors is therefore not a problem. Moulds are not all the same in terms of their possible effects on people.
Many of the differences among moulds arise from whether they are adapted to produce materials that irritate and repel animals, including humans. Most of the moulds found in normal outdoor air are not generally irritating. They are produced on the surfaces of living leaves and plant stems. In this habitat, there is no advantage in making anti-animal toxins and irritants. Also, our species has evolved with these leaf-surface fungi (“phylloplane fungi”) as part of our normal breathing environment, and our system usually does not react to them as a microbial threat. Just a small fraction of people become allergic to some leaf-surface fungi that are common in outdoor air, like Cladosporium and Alternaria. Similarly, most fungi that colonize soil in temperate regions, like Canada, do not produce significantly irritating airborne materials.
On the other hand, some fungi have evolved to colonize decaying plant material – material that some animals could otherwise eat. Competition against animals for these materials has led to evolution of toxic secretions, off-putting odours, and chemistry that triggers strong immune reactions.
The so-called ‘black toxic mould,’ Stachybotrys, in nature, colonizes decaying grass and straw, as might be found in overly wet hay. It has evolved toxins that cause symptoms suggestive of burns on the mouth parts of grazing animals. These toxins also inhibit a basic process (protein synthesis) that is part of normal functioning inside animal and human cells. Finally, spores, cellular fragments and toxins from this fungal group are recognized by cells of the immune system, defending our lungs, as a “danger signal.” (Kankkunen et al., 2009). Cells in our immune system may react by sending out chemical signals to other immune cells, leading to a strong defense reaction that can feel like a cold or a chest infection.
Some fungi are more likely to colonize plant seeds or fruits than stems or leaves. Such fruits and seeds are often used by humans as food. They are rich in nutritional value, and this energizes fungi growing on them to produce large amounts of toxic material in order to chemically ‘claim’ the material. Aspergillus flavus, which colonizes materials such as peanuts, corn kernels, and dried red peppers, produces one of the strongest natural poisons ever found, aflatoxin.[1] This toxin is surpassed only by radiation as a cause of mutations that might lead to cancer. A closely related fungus, Aspergillus versicolor, produces toxins that are related, though not quite as potent. It is among the most common species growing in homes with serious moisture problems.
If studies are done of airborne mould spores in houses where there is no moisture problem, the results usually show the same species that are found in outdoor air, but at lower count levels per cubic metre of air. The largely wind-and-draft-free atmosphere inside buildings allows the normal spore levels to settle out slowly into dust, clearing the air. In Ontario, these outdoor moulds tend to form a highly recognizable pattern: Cladosporium and Alternaria are most common year-round, joined during warmer parts of the year by Epicoccum, some Penicillium types, basidiomycetes (wood-decay fungi and mushrooms), Fusarium species that grow on grain heads, and a few others. Yeast colonies also commonly grow in these samples, especially if there has been recent rain to knock spores off the leaf surfaces where they grow.
Extensive indoor mould growth, in studies that trap identifiable mould spores from the air, tends to be signalled by levels of Aspergillus and Penicillium far beyond the level in outdoor air; these types are sometimes accompanied by Stachybotrys, Chaetomium, or greatly elevated levels of Cladosporium or Alternaria. Such findings, as well as seeing extensive visible mould on walls and other household surfaces, or seeing evidence of uncontrolled leaks, wall sweating or any other heavy moisture source damaging wallboard or wood, can indicate the likelihood of health problems developing among occupants (from Summerbell & Hart, 2021; ultimate source Hung et al., 2020). If that happens, and you’re a renter or recent home buyer, don’t let your landlord or vendor tell you, “mould is everywhere.”
D/M = dampness or mould
[1] Note however that grain, corn, and products like peanuts, etc. are screened by the Canadian Food Inspection Agency for these mycotoxins to ensure they are not on the market in Canada.
References
Hung LL, Caulfield SM, Miller JD. Recognition, Evaluation, and Control of Indoor Mold, 2nd edition (AIHA Green Book). American Industrial Hygiene Association, Falls Church VA, 2020.
Kankkunen P, Rintahaka J, Aalto A, Leino M, Majuri ML, Alenius H, Wolff H, Matikainen S. Trichothecene mycotoxins activate inflammatory response in human macrophages. J Immunol. 2009 May 15;182(10):6418-25.
Summerbell RC, Hart R. Health impacts of indoor dampness and mould and effective remediation and prevention strategies. RentSafe and Canadian Environmental Law Association (CELA), Dec. 7, 2021. https://rentsafecanada.files.wordpress.com/2022/02/mould-expert-report_health-impactsremediation.pdf
NOTA BENE: This blog piece is mostly an excerpt from Summerbell and Hart (2021) with a few insertions of extra material by R. Summerbell, the author of this blog.
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