Water Damage Repair | Fort Myers & Naples, Florida

Water activity (Aw) is defined as the partial pressure of water relative to the vapor pressure of pure water at the same temperature or a measurement of the water that is available for biological and chemical reactions. The Aw scale starts from 0 (dry) and goes to 1.0 (pure water). Microbial growth can start as low as 0.6 Aw. However, environmental molds have genera/species specific variability regarding how much water is required for their growth.

Fungi that require high amounts of available water are called hydrophilic and grow at water activities above 0.90. Stachybotrys sp., Chaetomium sp., Trichoderma sp., Memnoniella sp., Acremonium sp., and Fusarium sp. are examples of hydrophilic molds. They colonize only in moisture rich, chronically moist environments. If building materials do not contain >90% (Aw >0.9) moisture content then their growth is impaired. The largest group of fungi falls in the mesophilic range.

Mesophilic fungi include common indoor molds such as Cladosporium sp. and Alternaria sp. These fungi typically grow on continuously damp building materials with water activities between 0.80 and 0.90. Xerotolerant molds include Aspergillus sydowii, A. versicolor and some species of Penicillium are able to grow at water activities below 0.80 but grow optimally above this value. They are common on water-damaged materials.

The final group of fungi, known as xerophilic, actually grow best at water activity ratios below 0.80.

A common xerophilic fungus is Aspergillus restrictus. Building materials such as drywall and wood contain cellulose-based materials chocked full of nutrients that serve as suitable food sources for mold growth. However, unless water is present at levels presented above, mold growth will not occur. Water impact into a structure then provides the enriched moisture environment fungi need to grow. Once a building product is soaked with water without immediate drying, mold spores present on the water-laden building material, germinate and grow.

With some molds, the building product must remain nearly saturated for 8-12 days (e.g., Stachybotrys chartarum) before growth appears. Once they grow and water activity falls below their requisite Aw level, the fungi either become dormant or die - but they don't disappear. They remain on the building material as a forensic clue or signature documenting the historic water impact of the building. This is why the aforementioned molds are referred to as "Water Impact" or "Signature Molds."

Their presence, either in air or on a surface, is a like a fingerprint...a distinct and measurable surrogate indicator that the building material in question sustained an anomalous and significant water impact. Their absence indicates either that there was no water impact, or that water was not retained for an adequate period of time for these organisms to germinate and colonize. Building materials may have water damage without the presence of water impact molds.