The colloquial term for fungi is ‘mould’, while the North American spelling is ‘mold’. Fungi are a diverse group of microorganisms that are separate from the plant and animal kingdoms. They include fleshy fungi like mushrooms, moulds and yeasts. The Dictionary of Fungi is a standard reference text used by biologists and in 2008 there were approximately 99,000 known species of fungi. However, the use of genetic tools for fungal identification has revised this estimate to be as high as 5.1 million fungal species (Blackwell, 2011). Fungi have two basic growth forms as: moulds or yeasts. Moulds are composed of broadly cylindrical cells called hyphae, which are organized into a branching network called the mycelium, which often form radially symmetric colonies and are truly multicellular. Their fruiting bodies then allow them to reproduce and contain the spore forming structures that can become liberated and airborne. Yeasts by comparison consist of unicellular structures that do not form true hyphae and reproduce asexually by a method called budding. Fungi are sometimes considered broadly as lower or higher fungi; and this classification makes it straightforward to group fungi like mushrooms that form large fruiting bodies from lower fungi that remain with the mycelial growth form. The main fungi that are of interest in water damaged buildings are of the mould type that grow by repeated extension from the hyphal tips on which the reproductive spore structures are supported. Over time, both the mycelium extends large distances and the reproductive spores become available to be liberated. In turn, when local conditions provide sufficient water and a food source, then these spores will germinate and the fungal life-cycle continues.
Moulds are microscopic and easily grow on water damaged building materials. The relative humidity is the major determinant for the availability of water on and within porous indoor surfaces and objects. (van Laarhoven et al., 2016). Fungi require both nutrients and water to grow. The reasons for water damaged indoor environments include: flooding and disrepair reflecting the twin causes of water ingress to be either caused by natural disasters or dilapidation. During growth moulds can cause structural damage to host surfaces and objects mainly through excretion of enzymes that cause chemical decomposition. Inside buildings, common sites of mould growth include: wood, concrete, paint, wallpaper, plaster, glass fiber, chipboard, cardboard and carpet (Andersen et al., 2011).
When persons look up “mould” on the Internet, there are a plethora of documents relating to many different aspects of its cellular form. Fungi are useful for fermentation (wine, cheese, tofu), can be used to breakdown toxic chemicals (dyes, pesticides) but can also cause disease in humans, plants and animals. Some fungi also produce chemical metabolites called mycotoxins which are poisonous (Tuomi et al., 2000). When moulds grow, they also may liberate volatile organic compounds that are detected as smells which might be pungent like an alcohol or malodorous. When persons refer to “mould toxicity”, it could be in response to: the physical cell mass, cell wall or spore fragments, spores, odours, mycotoxins or just the fear potential for contamination. Adverse health effects to “mould” have been reported after inhalation, ingestion and dermal skin contact. There is not always a dose-dependent relationship and mould is found naturally in the air, on surfaces and on persons (the human mycobiome).
Andersen, B., Frisvad, J.C., Sondergaard, I., Rasmussen, I.S. and Larsen, L.S. (2011). Associations between fungal species and water-damaged building materials. Applied and Environmental Microbiology. 7712: 4180-4188.
Blackwell, M. (2011). The fungi: 1,2,3,…5.1 million species? American Journal of Botany. 98(3): 426-438.
Tuomi, T., Reijula, K., Johnsson, T., Hemminki, K., Hintikka, E-L., Lindroos, O., Kalso, S., Koukila-Kahkola, P., Mussalo-Rauhamaa, H. and Haahtela, T. (2000). Mycotoxins in crude building materials from water-damaged buildings. Applied and Environmental Microbiology. 66(5): 1899-1904.
Van Laarhoven, K.A., Huinink, H.P. and Adan, O.C.G. (2016). A microscopy study of hyphal growth of Penicillium rubens on gypsum under dynamic humidity conditions. Microbial Biotechnology. 9(3): 408-418.