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Written by Robert Sheriff, MS, CIH, CSP, and Henry P. Shotwell, Ph.D., CIH, Vice President
March 18, 2020
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.
Suspected Mold Growth, False Negative
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 only condition is a moldy musty odor. 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 that come back showing essentially no difference between the indoor and outdoor samples. This is an example of a False Negative.
Suspected Mold Growth, False Positive
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 is 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.
Essential Factors To Know
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.
Interpreting Results of Open Settling Plates
Often in order to save money and time, individuals buy settling plates (also called Petri dishes) from your local home improvement store and set them out in-home or offices and then send them into laboratory analysis and interpretation. The results from the laboratories will usually scare you to death – all the terrible things the identified molds can do to you.
These tests have no real value – I strongly recommend not purchasing these settling plates – If you do and get the results – the results regardless of what they say are of no value.
The tests are not quantitative and thus have no value as to whether you have a little problem, a moderate problem, a great problem, OR NO PROBLEM AT ALL.
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. Also, the type of symptoms experienced by the occupants can be valuable in interpreting the test results.
The team at Atlantic Environmental can help you with your Mold Testing – Interpreting Air Sample Results. Give us a call or contact us using our online form. We are here to help.
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