Water Activity Requirements For Mold Growth

1 Flares 1 Flares ×

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.

Mold growth due to high water activity

Mold growth due to high 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

Note:

  1. 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.
  2. 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
Alternaria alternataPhialophora sp
Aspergillus fumigatusFusarium sp.
Chaetomium sppUlocladium sp.
Trichoderma (some species)Yeasts (Rhodotorula spp.)
Exophiala sp.Memnoniella echinata
Stachybotrys chartarum
(synonym S. atra)
1 Flares Twitter 0 Facebook 0 Google+ 0 LinkedIn 1 1 Flares ×
The following two tabs change content below.
Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at www.drjacksonkungu.com.
Dr Jackson Kung'u

Latest posts by Dr Jackson Kung'u (see all)

Join Us to Get Free News and Tips

Mold and bacteria testing tips, news and insights that we only share with our private newsletter subscribers.

WANT THE BEST OF MOLDBACTERIA.COM IN YOUR INBOX?

GET THE LATEST NEWS

ON MOLD & BACTERIA TESTING & ANALYSIS

No thanks, I don't need today's most important news.
1 Flares Twitter 0 Facebook 0 Google+ 0 LinkedIn 1 1 Flares ×

Free Report Reveals Simple Ways to Improve Your Lab Results

This free 16-page report was written to provide you with the information we've learned after decades of analysis and interpretation.

  • 3 simple ways to improve laboratory reports
  • Understanding terminology used in laboratory reports
  • How to interpret laboratory results for airborne fungal samples
  • And much more...
Close this popup

Enter your email address below to get instant access