Microbial growth occurs at the surface level of a material.
Microbiologists have learned that the moisture at the surface level must be at a certain minimum level for microbial growth to begin and continue.
Different materials require different levels of surface wetness to sustain microbial growth. The measurement that best describes the amount of water at the surface is water activity. Water activity (abbreviated as aw) is the amount of water at the material’s surface from which microbial life can draw sufficient water for germination and growth.
Water activity can be measured by using a thermo-hygrometer. Place a thermo-hygrometer under a small sheet of plastic wrap. Tape the edges shut, trapping the meter inside the plastic envelope. The meter is now locked inside an envelope of air — one side is the affected material and the other side is the plastic bag (non-permeable).
The air in this envelope will reach what is known as equilibrium, meaning that the moisture in the material will evaporate into the air space of the envelope, and as this air becomes more humid, the moisture will be absorbed back into the material. Thus the air in the envelope space and the moisture at the surface of the material will have reached equilibrium.
The relative humidity reading inside the envelope space indicates what is known as Equilibrium Relative Humidity (ERH). The reading provides the relative humidity of the air at the surface of the material which is available to support microbial growth. The time required to measure this ERH varies with the material, porosity, surface coating and amount of moisture in the material.
The ERH reading is easily converted to water activity:
ERH / 100 = aw
An ERH reading (inside the space of the envelope) of 60% would equate to an aw reading of 0.60.
The minimum aw required for microbial organisms to germinate and grow is 0.66, which is equal to an ERH of 66%. Virtually no microbial growth can occur if the aw is below 0.65. Students in water damage restoration classes are taught to lower indoor relative humidity (RH) below 60% as quickly as possible. If the indoor humidity stayed above 66% RH for an extended period of time, the wood could reach 0.66 aw — which would support microbial growth.
Both moisture content (MC) and water activity are measurements that tell us about the amount of water in a material and on the surface of a material. Moisture content reflects the amount of moisture at a specific location on a material. Water activity reflects the amount of moisture at the surface of a material for sustaining microbial growth.
Brandon Burton is the technical education manager for the Restoration Sciences Academy (RSA), a part of Legend Brands. He teaches IICRC-approved classes in the categories of Applied Structural Drying (ASD) and Water Damage Restoration (WRT). Burton has served the restoration community for more than 15 years as an IICRC-approved instructor, ANSI/IICRC S500 chapter chair, RIA restoration council member and in many other industry roles. You can contact him at BrandonB@RSA-HQ.com. (Burton worked with various restoration experts at the RSA to produce this material, excerpted from the RSA’s comprehensive new textbook, The Complete Guide to Cleaning and Restoration.)