November, 2012
Inspection News and Views from the American Society of Home Inspectors

To Test or not to Test: That is the Question


Fotolia_38988888_M.jpgI feel ambivalent about home inspectors doing air testing for mold during home inspections because air samples for mold spores can cause undue concern.

An air sample doesn't identify the location of the mold growth. Sometimes, the source can be something minor, such as a small patch of mold on a bathroom wall. The answer is to remove the mold and keep the room dryer, and yet the test results could suggest that the entire house might be mold-contaminated.

And, depending on the season, mold spores from outdoor mold growth flow into the house with the outdoor air. Then it's just a matter of household cleaning rather than mold remediation. Very often, it's the seller's possessions that have surface mold growth, particularly carpets or rugs in finished basements, or antiques that once were stored in a damp basement or garage. After these possessions are removed from the house, these sources of mold spores no longer are present.

And what if you take the air sample at the end of your inspection, after you've spent time in a moldy crawl space, basement or attic? The mold spores detected in the test could have come from your own clothing. (A homeowner once insisted that I look at something in the attic before I started taking air samples, and I found some fiberglass fibers in the living room air sample. These fibers could not possibly have come from any source other than my knee, from when I leaned on the top of a dusty duct in the attic. Ever since this experience, I sample all the "clean areas" of the house before even entering a basement, crawl space or attic.)

The results of air testing for mold spores also can be falsely reassuring. The dust trapped in HVAC systems or on baseboard convectors can be very contaminated with mold growth, which an air test may not detect if the system is not running or the dust on the convectors is not disturbed.

When only comparing total spore counts, someone with mold allergies might feel relieved if levels of mold spores outdoors are higher than levels of mold spores indoors.
But exposures to spores from mold growth outdoors are seasonal or weather-dependent, whereas exposures to indoor mold spores tend to be more constant and thus potentially have a greater impact on someone's health. In addition, in their counts, many labs do not differentiate between individual mold spores and chains of mold spores.
When Penicillium or Aspergillus mold is growing indoors, the spores often are in chains, whereas if these molds are growing outdoors, wind turbulence breaks up most of the chains into individual spores. A count of 12 individual spores outdoors is therefore not the equivalent of 12 Pen/Asp spores in a chain indoors. (The only lab that that I know of that reports this vital information about the presence of spores on chains versus individual spores is QLab.)

Lab results can be inaccurate, so basing a decision on whether or not to purchase a property on air-testing results can be risky. In a recent study, the authors (Robertson, L. and Brandys, R.) sent identical spore-trap samples to seven different labs. At each lab, the most experienced microscopist examined 100 percent of the samples (most analysts routinely look at only 25 percent of a sample). Spore counts for identical types differed in some cases by as much as a factor of 10 and only 50 percent of the labs successfully identified Pen/Asp (Penicillium and/or Aspergillus) spores, the most common mold growth associated with mold problems in damp buildings.

I have seen many serious identification errors even from some of the bigger labs, where inexperienced microscopists probably rushed through samples. (One lab identified spray-paint spheres as Aspergillus mold spores and another lab identified candle soot as "toxic black mold.") And, the way in which testing data is presented can be confusing. For all these reasons, the results of air testing should be greeted with a healthy bit of skepticism. The visual inspection almost always trumps mold-spore air-test results.

I know it's odd for me, an IAQ professional, to generally question the usefulness of mold-spore testing since I often do air and surface testing myself. But air testing for mold during a home inspection worries me. I am happy to investigate moisture problems for buyers and will investigate a potential mold problem if the home inspector expressed concern about that issue. If a buyer calls me, however, and wants me to conduct air testing for mold spores to "be sure there's no mold in the property," I explain that such testing is meaningless. Mold can grow within 24 to 48 hours if moisture (spills, leaks, water intrusion or relative humidity over 80 percent) and a food source (such as dust) are present. Depending on conditions, a basement that is mold-free one week can be full of mold growth a few weeks later — and certainly within the 6 to 8 weeks that usually pass between the home inspection and the closing.

I will, however, admit a narrow exception to my concerns. Taking a surface sample (a swab or tape sample) can be useful if you suspect the presence of mold growth at a stain. (Wear a NIOSH N95, two-strap mask while gathering the sample.) But air sampling for mold by itself should not be undertaken to confirm the absence of mold growth.

In the end, if you see moisture problems, it's always a good idea to mention the possibility of the presence of mold growth. And, if you see what you think is mold growth, you should suggest further investigation as a matter of course. It's probably best, however, if your client hires another professional to do this work.

This article is from the May Indoor Air Investigations Newsletter, April/May 2012. Reprinted with permission of Jeffery C. May. © 2012 Jeffrey C. May. Visit

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