Water

Water is critical to life-forms, even fungi.  Without water, growth and reproduction simply will not occur.  

The amount of water for optimum growth varies by species and is also influenced by environmental factors such as temperature and the types of nutrients available.  

Some species can grow at relatively low moisture levels, levels that would not be considered 'wet' in ordinary terms (think of mold growing on bread).  Other types of fungi require significant amounts of moisture for optimum growth.

Substrate Moisture
Many materials are porous and will absorb moisture in attempt to reach equilibrium with the source.  It is the moisture contained in these materials that allows fungi to survive and grow.  

Dealing with water in liquid form is relatively straight forward.  Most everyone recognizes that liquid water moves by forces of gravity causing 'water to run downhill'.  But there are other forces which warrant consideration including hydrostatic pressures, capillary actions, and differences in the partial pressures of water vapor.  In simple terms, water will move from 'wet towards dry' in an attempt to reach equilibrium between these forces.

Note that water does not have to be liquid in order to saturate a substrate.  Water vapor, a frequent cause of mold growth, is naturally absorbed by most porous materials.  Moisture in air (humidity), or water vapor migrating through substrates is often slow developing and may be more difficult to assess.  Note that most building materials including concrete, brick, mortar, grout, drywall, wood, etc. are porous and do allow water vapor to pass.

Water Activity
For fungi, the amount of water required is termed the water activity level (A_w); a measure of water in the substrate that an organism can use to support growth. 

Examples of water activity ranges are shown below.  For the types of mold compatible with indoor environments, few are considered capable of growing below A_w = 0.65. 

(Source - ACGIH; Bioaerosols: Assessment and Control; 1999)

Water Activity

Scientists establish the water activity values by allowing a growth medium to reach equilibrium at a given relative humidity and observing growth performance for that organism.  Therefore, A_w values are equivalent to relative humidity:

A_w = %RH/100

Range

0.65 - 0.80

0.80 - 0.90

>0.90

Classification

Xerophilic/Xerotolerant

Mesophilic

Hydrophilic

Examples

Some Aspergillus and Penicillium

Alternaria, Epicoccum, Cladosporium, Aspergillus, etc.

Fusarium, Rhizopus, Stachybotrys

Quick response in drying all materials impacted by plumbing breaks, rain intrusion, etc. and controlling humidity in areas with organic substrates is of paramount importance.

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