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Guide to vacuuming building cavities to screen for particles, dust, or mold: this article explains the advantages and shortcomings of using vacuum cassettes or spore traps to collect mold test samples (or other dust or particle samples) from building wall and ceiling cavities. In this article series discuss the validity of nearly all of the popular mold testing methods currently in use, pointing out the strengths and weakness of each approach to mold sampling in the indoor environment, beginning with air sampling for airborne mold levels indoors. Because mold test validity and mold test accuracy are often confused, readers should also see ACCURACY OF VARIOUS MOLD TEST METHODS. People who need to conduct mold inspection and testing indoors should see MOLD TEST PROCEDURES and TECHNICAL & LAB PROCEDURES.
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Vacuuming building cavities is a popular screening practice to look for mold reservoirs. The investigator is trying to explore wall cavities while doing minimal or no visible damage.
A wall-check™ system has been marketed for this purpose and several manufacturers have coped the basic idea: a receiving Air-o-Cel™ or MCE filter cassette is attached at its inlet side to a tube which is inserted as a probe into a wall cavity, permitting creation of only a small hole. The outlet or pump side of the test device is connected to a pump and operated, typically at 15 lpm. Where we have tested this method we have experimented with both passive collection (what most investigators use) or aggressive collection (banging on the wall/ceiling at various points to attempt to dislodge and stir particles).
Short duration samples, 1-2 minutes using an Air-o-Cell ™ permit a comparatively large number of samples to be collected in a reasonably short interval. Long duration samples, perhaps for up to two hours, are collected using an MCE filter cassette.
Our MOLD INFORMATION CENTER includes more broad discussions of the overall approach to building investigation, as do many expert references cited at that web. For a more comprehensive collection information about mold test methods see INDOOR AIR QUALITY METHODS COMPARED. For more on "mold classes" (Cosmetic mold vs. allergenic mold vs. toxic or pathogenic mold) see MOLD CLASSES, HAZARD LEVELS
Wall Cavity Vacuuming - Found Ineffective
Working with Louis Relle, a Louisiana mold remediation expert on a New
Orleans Building which was to be demolished we collected wall-check samples from every suspect Building cavity.
Shortcomings of vacuuming building cavities through a tube but our testing strongly suggests that this method is very unreliable for characterizing wall contents. We do not believe that enough air movement is created in the wall cavity (sucking any lpm flow through a small diameter tube) to reliably collect what could be a severe mold reservoir that happens not to be right next to the probe. Further if the cavity is insulated there will be virtually no air or particle movement except from very close to the probe.
Wall cavity vacuum tests, depending on how and when they are performed, can be like searching for a needle in a haystack while looking through a straw: if you find evidence of a problem you're lucky. But if you don't find evidence, when using a very limited-scope method, that doesn't mean that a problem is not there.
Our photo, left, is not showing the wall-check vacuum method that relies on vacuuming into a cassette through a hole in the wall. Instead, here we have pulled loose paneling and have inserted our spore trap cassette into the cavity.
This position combined with aggressive sampling by banging on the wall paneling with a flashlight is more likely to collect problem particles if they are present close to the point of vacuum cassette insertion into the wall. But even this approach is not a reliable characterization of mold risk in the building. And it is not looking into the wall cavity itself - rather we are looking at the two surfaces behind the paneling.
Short vacuum pump duration for microscopic examination (to avoid sample overload, e.g. 2-=3 minutes on an Air-O-Cell) does not move enough air to reliably find what may be in the wall cavity. An experiment done with Louis Relle in New Orleans LA demonstrated that wall-vac tests found less than 10% of large problem mold cavities that could be discovered by cutting drywall openings in a building.
Our photo (left) shows a short-duration but larger surface area or wall opening vacuuming system invented and tested by the author - it did not provide useful results.
Longer vacuum pump duration samples for viable sampling (2 hours into an MCE cassette for culturing) still may not move enough air to sample through a cavity, particularly if the cavity is insulated. Further, two-hour samples means that most-likely very few sample points were collected, making the inspection scope extremely limited and thus overall confidence in the accuracy of the picture of the building lower.
Consider that the popularly-marketed version of this wall cavity vacuuming approach to test for hidden mold contamination in buildings relies on culture of the sample. Did you know that only about 10% of molds grow in any culture at all? You're 90% uncertain of the accuracy of the test at the outset. One can't be sure that the mold that grew in the culture represents the dominant problem mold or whether it's just a low-occurrence (in the building) spore that liked the media (in the culture). We like cultures for further genera/species identification of samples but we are nervous about relying on them to tell me if the building has a problem or not.
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