Mold Testing – Interpreting Air Sample Results

If you need mold testing/sampling assistance discussed in this article call us at 1-800-344-4414 or email us at for details and a free estimate.


Written by Robert Sheriff , MS, CIH, CSP and Henry P. Shotwell, PhD, CIH, Vice President

November 5, 2018


Budget considerations often limit the number of air, surface or bulk samples that can be taken in an IAQ investigation. Once the laboratory has returned the test data, then someone has to decide how to interpret it. It’s not enough to simply compare indoor data to the data collected in the outdoor or control sample. Indoor air samples are subject to two kinds of error: False Positives and False Negatives.

Let’s take the case of a building where mold growth is suspected, but not visible. Let’s further assume that there is a small leak in a water pipe behind a wall, but there are no wet spots, discolorations or signs of visible mold growth. The IAQ investigator arrives are 11:00 A.M. to sample, but windows and doors have been held open most of the morning to allow for furniture delivery. Everything is closed up again shortly before the investigator arrives and blithely takes samples which come back showing essentially no difference between the indoor and outdoor samples. This is an example of a False Negative.

On the other hand, let’s assume mold growth is suspected in a room that has no mold infestation. Just before the arrival of the investigator, someone notices a moldy orange in the back of the refrigerator and casually tosses it in the garbage. This causes the release of thousands, if not tens of thousands of mold spores into the air, which are then dutifully collected and sent to the lab. The lab then reports levels of mold spores indoors that are grossly higher than those seen outdoors at the same time. This is an example of a False Positive result.

This is why recording a careful history is essential. Is the building a residence or an office? Has there been a history of roof or window casement leaks? Is the basement or structural level of the building which is in contact with the ground, subject to flooding? In a residence or the kitchen area of an office, has very recent food preparation included the handling of mushrooms? What is the weather like? Is it dry, warm and sunny or is overcast, damp and cool? The mold level outdoors is a critical factor in determining if mold is from outdoors or growing indoors.  Also the type of outdoor molds as well.

It’s essential to know what these conditions were at the time of sampling/testing in order to improve the chances of correctly interpreting the lab data. Questioning the residents or employees of an office about their recent activities, and about any water intrusions or releases that may have occurred recently is a very effective way to get the additional information needed to correctly interpret the test results.

Our primary service areas for Mold Testing/Sampling are: NJ, NY, NYC, PA, CT, DE, (Boston) MA, RI, Wash DC, WI, MD, MI, (Chicago) IL, VA, IN, (Atlanta) GA, AL, NC, SC, TN, (Dallas, Ft Worth) TX, OK, DC, AR, we can service most other areas of the U.S. but with some added travel charges.