If a water problem in a building is not repaired and the water dried out within 48 hours, growth of mould and other microorganisms would naturally occur. The types of moulds predominant at anyone time would be determined by the level of moisture in the building material. This level of moisture is usually referred to as the water activity and it determines the order in which different categories of moulds appear. The first group of moulds to appear (at water activities less than 0.85) is referred to as the primary colonizers, the second group (at water activities of 0.85-0.90) is secondary colonizers and the third group (at water activities greater than 0.90) is the tertiary colonizers.

Primary colonizers
The primary colonizers are capable of growing at water activities below 0.85. This group may include Alternaria citri, Eurotium amstelodami, Aspergillus candidus, Aspergillus glaucus, Aspergillus niger, Aspergillus penicillioides, Aspergillus repens, Aspergillus restrictus, Aspergillus versicolor, Paecilomyces variotii, Penicillium aurantiogriseum, Penicillium brevicompactum, Penicillium chrysogenum, Penicillium commune, Penicillium expansum, Penicillium griseofulvum, and Wallemia sebi.

Secondary colonizers
Secondary colonizers grow best at water activities of 0.85 to 0.90. Secondary colonizers may include Aspergillus flavus, Cladosporium cladosporioides, Cladosporium herbarum, Cladosporium sphaerospermum, Mucor circinelloides, and Rhizopus oryzae.

Tertiary colonizers
At water activities greater than 0.90, tertiary colonizers appear. These may include Alternaria alternata, Aspergillus fumigatus, Epicoccum spp., Exophiala spp., Fusarium moniliforme, Mucor plumbeus, Phoma herbarum, Phialophora spp., Rhizopus spp., Stachybotrys chartarum, Trichoderma spp., Ulocladium spp., Rhodotorula spp., Sporobolomyces spp., and Actinomycetes.

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