Often, people wonder if it was possible to tell how old mould growing in a house was and/or whether the mould was actively growing and what the source of the mould was. The answers to these questions would help in situations where say somebody buys a house and some months later they discover mould. If they would tell the mould was there before buying the property, they would follow it up with the seller for not disclosing the mould problem. Unfortunately there are no simple tests that can tell us the age of mould growing in a house, whether it is actively growing and the source. We can only speculate on a range of number of days since the current growth appeared and whether it was likely to be actively growing. It would be difficult to tell the source of the mould and how long it has been there before the growth was seen.
How moulds interact in nature
Mould propagules are ubiquitous in indoor environments and are well adapted to inhabit this ecological niche if just sufficient water is available. Based on the water activity (or equilibrium relative humidity) requirements, moulds can be divided into 3 broad groups:
- Primary colonizers
Primary colonizers (also referred to as xerophiles) are the first to appear. They are capable of growing in relatively dry conditions at water activities of between 0.6-0.8. These kinds of conditions can be achieved through slow leaking pipes, condensation or other sources of moisture. At this low water activity viable propagules (spores and mycelial fragments) of species of Wallemia, Penicillium, Aspergillus and Eurotium would germinate and form growth.
- Secondary colonizers.
As the water activity of the building material increases to between 0.8 and 0.9, the secondary colonizers would proliferate. Secondary colonizers include species of Cladosporium, Phoma, Ulocladium, Alternaria and also some species of Aspergillus such as A. flavus and A. versicolor. If nutrients are still available on the substrate and the water activity remains unchanged the secondary colonizers would gradually become the dominant moulds.
- Tertiary colonizers.
At water activity of 0.9 and above, the water loving moulds (also called hydrophiles) would dominate as the tertiary colonizers. These levels of water activity are generally only met by incoming water as under flooding conditions and not just high humidity or condensation on indoor surfaces. Immediately after the flooding or serious water leakage, for example, all types of moulds would colonize the wet building material but with those that require higher water activity dominating. Presence of tertiary colonizers in a building is an indication of a serious water problem. Tertiary colonizers include moulds such as Stachybotrys, Chaetomium, Ulocladium, Trichoderma, Aureobasidium as well as actinomycetes and other bacteria.
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