More
I recently observed some posts on the social networking site LinkedIn that have to do with the question, "What moisture content is required for microbial growth?"
It was a valid question, which generated many comments and different responses, ranging from 16 to 19 percent moisture content.
The problem with the question is that moisture content is not the measurement used to determine if there is adequate moisture for microbial growth to start or to be sustained.
The measurement that is used for this determination is water activity(1).
In the water damage restoration industry, we use the term moisture content (MC) every day to describe the amount of moisture (water) that is in a material.
For the purposes of this article, we will assume we are talking about framing lumber made from pine trees. We will expand this definition later in the article.
Moisture content defined
Moisture content (MC) is defined as: The mass of moisture in a material, measured as the mass of water as a percentage of the dry mass of the material(2). As an example, if a piece of wood weighs 120 pounds, and when totally dry weighs 100 pounds, its moisture content is determined by: (120 pounds – 100 pounds) / 100 pounds = 20 percent.
This measurement is usually obtained by using a moisture meter, most accurately a penetrating type meter, and the reading observed is an approximate percentage (%).
However, this moisture is not spread uniformly throughout the material. By taking numerous measurements, we might find readings such as 16, 20, 26 percent, etc., indicating some locations are wetter or drier than other locations. Thus the MC is a reading of the moisture content for the location at which the reading is taken, and does not describe the wetness of the entire board.
When looking at a two foot by four inch piece of lumber, we may think the wood is uniform, as it has been cut into a specific shape, is fairly smooth and looks uniform. However, on a microscopic level, the cell structure inside the board is very much non-uniform, which helps explain why different locations have different levels of moisture. Given sufficient time, the entire material will reach equilibrium moisture content.
Microbial growth and water activity
Microbial growth occurs at the surface level of the material being "attacked"(3). From research, microbiologists have learned that for microbial growth to occur, the moisture at the surface level has to be at a certain minimum level for microbial growth to begin and continue.
The term that is used to describe and measure the amount of water at the surface is water activity. Water activity (Aw) is the amount of water that is at the surface of a material from which microbial life can draw water, which will allow it to germinate and grow.
As will be seen later, different materials have to be at certain levels of surface wetness, such as water activity, for microbial growth to occur.
The easiest way to measure water activity (Aw) is by using a thermo-hygrometer. Place a thermo-hygrometer under a small sheet of plastic wrap (see photograph on page 30). Tape the edges shut, trapping the meter inside the envelope. The meter is now trapped 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.
Now, taking a relative humidity reading inside the envelope space will give us what is known as Equilibrium Relative Humidity (ERH). The reading is the relative humidity of the air and the surface of the material which is available to support microbial growth. The time required for this ERH to be obtained varies with the material, porosity, surface coating and amount of moisture in the material.
The conversion that is used to change from ERH to Aw:
ERH / 100 = Aw
Minimal Aw requirement
An ERH reading (inside the space of the envelope) of 60 percent would equate to an Aw of 0.60. This is the amount of moisture that is at the very surface of the material upon which microbial growth could draw water to germinate and begin to grow.
From research, we have learned that the minimum Aw that is required to allow microbial organisms to germinate and grow is 0.66, which is equal to an ERH of 66 percent. Virtually no microbial growth can occur if the Aw is below 0.65(4).
This is the reason why in water damage restoration classes, students are taught to try to get the indoor relative humidity (RH) below 60 percent as quickly as possible. If the indoor relative humidity stayed above 66 percent for an extended period of time, the wood could reach an Aw of .66, which would support microbial growth.
Many of us were told that when wood has a moisture content above 16 percent the wood is wet enough to support microbial growth. This is marginally correct.
As discussed, it is water activity that is the amount of moisture available to support microbial growth. Here is a table(5) that shows different materials, with the same Aw of 0.80, and the corresponding MC:
| Material | Moisture Content |
| Brick 0.1% to 0.9% | Gypsum Board 0.7% |
| Wall Paper 11.3% | Soft Wood 17% |
It needs to be understood that the traditional moisture meters used on water damage and mold remediation work are calibrated for wood. The previous MC readings were taken by laboratory equipment.
The same Aw can mean different MC, in different materials. The two (MC and Aw) are not directly related.
This then raises the question, "How does water activity relate back to moisture content?" Well, it does not directly relate and that is the problem with answering the initial question, "What moisture content is required for microbial growth?"
There are no direct measurement techniques to correlate MC and Aw. MC and Aw can be directly related, but only on a material by material basis(6). Again, from research, many different building materials have been investigated, and the relationship between MC and Aw (ERH) is shown in graphical form, called sorption/desorption isotherms.
The graph below depicts southern pine(7). There will be different sorption/desorption isotherms with different values for every species of wood. There are literally hundreds of sorption/desorption isotherms, one for each type of material.
Many sorption/desorption isotherms are used extensively in food production to ensure microbial growth is prevented. The isotherm shows two lines, a sorption line and a desorption line. The lower curve represents the sorption (adsorption) and the upper curve represents desorption. The sorption curve is when a material is getting wetter (gaining moisture) and the desorption curve is when the material is getting drier (losing moisture).
The moisture content is the "Y" axis and the RH is the "X" axis. In this graph the RH is really ERH. There is no direct correlation between moisture content and water activity, as it varies with every material (type of wood).
This isotherm illustrates that for the minimum level of water activity that will support microbial growth, which is 0.66, (ERH of 66 percent), the corresponding moisture content would be about 14 percent. Most molds require an Aw of 0.80 or higher to sustain growth, from which the isotherm above would be about 16 percent MC.
As we have discussed, moisture content and water activity are measurements that tell us about the amount of water, both in a material and on the surface of a material.
The value of 16 percent moisture content is generally accepted as the dry level that will prevent microbial growth.
But MC is not the correct measurement used in the microbial world; water activity is the correct measurement.
References:
(1) American Conference of Governmental Industrial Hygienists (ACGIH): Prevention and Control of Microbial Contamination. In Bioaerosols: Assessment and Control, J. Macher (ed.). Cincinnati, OH: ACGIH, 1999
(2) American Conference of Governmental Industrial Hygienists (ACGIH): Prevention And Control Of Microbial Contamination. In Bioaerosols: Assessment and Control, J. Macher (ed.). Cincinnati, OH: ACGIH, 1999
(3) American Industrial Hygiene Association (AIHA): Indoor Mold. In Recognition, Evaluation, and Control of Indoor Mold, B. Prezant, D. M. Weekes, J. D. Millers (editors). Fairfax, VA: AIHA 2008.
(4) American Industrial Hygiene Association (AIHA): Indoor Mold. In Recognition, Evaluation, and Control of Indoor Mold, B. Prezant, D. M. Weekes, J. D. Millers (editors). Fairfax, VA: AIHA 2008.
(5) American Industrial Hygiene Association (AIHA): Indoor Mold. In Recognition, Evaluation, and Control of Indoor Mold, B. Prezant, D. M. Weekes, J. D. Millers (editors). Fairfax, VA: AIHA 2008.
(6) Bailey, Hollace S. 2005. Fungal Contamination: A Manual for Investigation, Remediation and Control. Building Environmental Consultants Inc., Jupiter, FL.
(7) Zelinka, Samuel L; Glass, Samuel V. 2010. Water vapor sorption isotherms for southern pine treated with several waterborne preservatives. Journal of Testing and Evaluation. Vol. 38, no. 4 (2010): p.1-5. Paper ID JTE102696.
Richard Driscoll has a B.S. degree in mechanical engineering from Clarkson College of Technology, an MBA from the University of Dayton and is currently working on his doctorate. He is a professor at Webster University, where he provides graduate and under-graduate level lectures on marketing, international business management and business metrics. He is an Institute of Inspection, Cleaning and Restoration Certification (IICRC) Certified Master Restorer and an approved instructor. Driscoll has been consulted by state governments on legislation related to the cleaning and restoration industry. He also is the author and instructor for Restoration Sciences Academy''s MR-110 and MR-210 microbial remediation classes. He can be reached at Richard@mayhemmishaps.com.
