How Moldy is too Moldy?

By Michael Buettner: Back to Environmental Articles Page

Home buyers concerned about mold contamination often ask the question, “What level of indoor mold contamination is considered a “red flag” by mold inspection professionals?”  The quick answer is ten times the outdoor level.  The second part of that question is usually, “Wow, ten times is a pretty high indoor count, what gives?”

Indoor levels of suspect contamination ten times higher than outdoor levels(or a non-suspect baseline), would seem extreme and obvious for an individual conducting an Indoor Air Quality (IAQ) Investigation, but not so obvious for a homebuyer with no background in IAQ.

As indoor air quality awareness grows, the need for sampling when there have been no inhabitant complaints has grown with it.  The need for an IAQ screen has become a property transfer issue.  Many times buyers will ask the question, “How do I know that this house is not infested with mold?”  This is a very valid question given all of the recent media attention.  When faced with this question, the real estate agent typically turns to the inspector.  The buyer asking about the indoor air quality of a potential home purchase has a right to be answered…scientifically and economically.

Most sellers are selling you their “perfect home” and will not disclose any problems they may have had unless required by law to do so.  Currently there is no disclosure for “unexplained illness experienced by inhabitants” during a property transaction, and there probably never will be.  Buyers have the responsibility to perform due diligence during the inspection period.  Agents have the responsibility to raise the awareness of a homebuyer in regards to indoor air quality; and inspectors have the responsibility to point out factors that may contribute to poor indoor air quality, such as lead, radon, asbestos or mold.

Performing a limited screen for indoor contaminants during a property transaction is of immeasurable value to a potential homebuyer.  In most cases the homebuyer has little knowledge of the potential of indoor contaminants.  By having a limited screen performed utilizing accepted sampling standards; a homebuyer has the ability to cost effectively identify potential indoor contaminants that would otherwise be over-looked.

Okay, the reason why an individual would want to have at least an IAQ screen performed makes sense even if no inhabitants complain.  But where is the reference for ten times the levels in an indoor (suspect environment) in comparison to an outdoor or non-suspect environment?

To quote the American Conference of Governmental Industrial Hygienists Bioaerosols: Assessment and Control Book, pages 14-6 and 14-7.

14.2.2 “Many replicate samples are required to document small concentration differences with statistical significance.  This essential means that unless investigators have collected a great deal of data, they cannot say that there is a quantitative difference between 100 and 200 or between 500 and 1000. Rather, differences that can be detected with manageable samples sizes are likely to be in 10-fold multiplicative steps (e.g. 100 versus 1000 or 500 versus 5000)."

14.2.3.1 "Data from individual sampling episodes is often interpreted with respect to baseline data from other environments or the same environment under anticipated low-exposure conditions." 

Unless you take a high number of redundant samples and compare them to each other (a procedure typically required for a full investigation), you have no way of knowing if an indoor spore count that is twice, three times or even five times higher than an outside spore count is significant.  Rather, differences that can be detected with manageable (few) samples sizes are likely to be in 10-fold multiplicative steps.  If you have an indoor spore count that is ten times higher than outdoor spore counts then is becomes likely that an indoor condition exists that favors mold growth.

So if I have a count that is eight times higher I don’t have a problem?  Or if I have an indoor count that is less than outdoor counts then I have no mold problems?  These are tough questions, but questions that a homeowner will potentially ask.

Michael Buettner, CRMI, CIE, CIAQM
office: 800.283.9512
fax: 480.275.3237
email: Mike@IAQUS.com
website: www.IAQUS.com
All rights reserved ®, do not reprint without express permission from Michael Buettner

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