Definition of Water Activity
Water activity (aw) refers to “free” water available for microbial growth. For microbial growth to occur, moisture must be freely available. The water activity of a material determines which types of mold would grow on that material.
Technically, the water activity is defined as the ratio of the vapour pressure exerted by the water in the material to the vapour pressure of pure water at the same temperature and pressure. Check out our Mold Inspection, Identification and Control course to learn more about Water Activity.
Types of Mold based on Water Activity
Indoor molds vary in their water activity requirements and this ranges from 0.7 to >0.9. Higher aw materials tend to support the growth of more microorganisms. Unlike molds, bacteria usually require a water activity of at least 0.91 aw. Molds can be grouped according to their moisture requirements as follows:
- Extremely to moderately xerophilic. These molds are the primary colonisers and they are capable of growth below 0.8 aw. Examples include Aspergillus and Penicillium
- Slightly xerophilic. These molds are the secondary or intermediate colonisers. They are capable of growth between 0.8 and 0.9 aw. Examples of molds in this category include Altemaria and Cladosporium
- Hydrophilic. Molds in this category are tertiary colonisers and require at least 0.9 aw for growth. Such high water activity can only be achieved through water intrusion and to a lesser extent high humidity and condensation. Examples of hydrophilic molds include Stachybotrys and Chaetomium
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- Xerophilic means “dry loving.” Thus, xerophilic molds are those that can or prefer to grow in “dry” environments. “Dry” is used here in relative sense since no mold can grow in a completely dry environment.
- Hydrophilic means “water loving.” Thus, hydrophilic molds are those molds that require very high levels of moisture to grow.
Indicators of Water Damage
Water damage may occur over many months, mainly through roof leakage, but also via rising damp and defective plumbing, which result in mold growth. Indoor molds as well as bacteria are usually saprophytic, meaning that they obtain nutrients from dead organic matter. The nutrients are from the breakdown of simple to complex sugars such as starches, cellulose and pectin. The materials most susceptible to mold growth are organic materials containing cellulose (i.e. jute, wallpaper, cardboard and wooden materials). The tertiary colonisers are used as the indicators of moisture damage. The table below shows some of the most common indicators of water damage in buildings.
Indicators of excessive moisture or chronic condensation
|Molds indicative of moisture or chronic condensation in a building
|Trichoderma (some species)
|Yeasts (Rhodotorula spp.)
(synonym S. atra)
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