Photograph of Allergenco Mark III Impaction Air Sampler Airborne Mold Tests
How Accurate are Air Samples for Mold Testing?

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Accuracy of air sample tests for mold contamination:

Is your "mold expert" using valid mold testing methods? Does your mold consultant also perform a thorough visual inspection and history-taking? Is your mold test kit worth the bother? How should an environment be inspected and tested to get a reliable estimate of toxic mold levels or mold exposure levels? Importance of visual inspection in mold testing.

This document is a brief tutorial which provides information about the accuracy of and sources of errors in tests for the level of allergenic and toxic mold in residential buildings: Are air tests for mold reliable at all? Are mold spore counts valid? Are cultures and swab tests valid? These critical questions are discussed in this paper.

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Accuracy & Validity of Viable and Non-Viable Indoor Fungal Spore Counts & Cultures

Air test for mold, particle trace variation (C) Daniel Friedman

Is it Necessary to Test for Mold?

Here we discuss the following:

Why Use Airborne Mold/Particle Sampling? Visual Inspection and History for Mold. Just how accurate and valid is air sampling for toxic mold testing? What are the sources of variation in the mold level that air tests can detect?

What is the range of variation in air test samples for mold and what does that say about the accuracy of air tests used to state mold exposure? Meaning of comparison of indoor and outdoor mold levels. Accuracy of mold tests using culture plates.

First, should we be testing for mold at all? If you see mold on indoor surfaces, NO mold testing is needed to confirm that mold is present in a this building and that cleanup is needed. But if a large remediation project is planned, tests may be needed for project control - see WHEN to IDENTIFY MOLD.

The photograph (above left) shows microscope slides prepared from four airborne mold trace samples collected inside the same hot air heating furnace plenum. We varied conditions from passive to more aggressive sampling methods during the brief one minute sampling interval.

Even before microscopic examination or counting the spores per cubic meter of air, it is visually obvious that there is very wide variation of particle collection level among these samples. We discuss variation in HVAC duct dust samples in more detail in this article.

Also see Causes of Variation in Airborne Particle Levels.

Field and laboratory data recording and even the most basic examination of the results demonstrate that very significant variability in field conditions and in particle behavior make airborne particle counts extremely inaccurate except under controlled conditions such as in a research chamber.

Similar technical shortcomings raise serious questions about the use of mold cultures, whether by settlement plate, swab, or Andersen sampler, to characterize an indoor mold level.

Thus airborne mold exposure levels based on single-time-interval use of these tools are unlikely to be accurate.

Finally, the almost total absence of recording of site conditions at the time of such measurements adds another almost overwhelming degree of inaccuracy to this approach to characterizing mold exposure risk in buildings.

For example, simply turning on a fan or walking through a room during an air sampling procedure can completely change the results of the measurement. While there is a useful place for these tools, their application as simple tools to make a statement regarding mold exposure levels in buildings appears to be highly questionable. The underlying reasons for this view are discussed in this paper.

How are Air Test for Mold Conducted?

Non-viable air samples are usually collected using a slit-impaction sampling device which employs a calibrated air pump to collect a measured volume of air in the environment being studied. Particles in the sampled volume of air are collected as they impact on to specially prepared microscope slides or inside of patented cassettes placed in the collection device's air path.

The collection device may be any of several air sampling machines (Burkard, Allergenco), or it may be a calibrated air pump, or a vacuum pump (Gilair) used with an Air-O-Cell cassette. Multiple period time-lapse sampling is available to diagnose difficult situations but is not normally employed. In special cases, we may use a separate pump and cassette system to sample surfaces and wall or ceiling cavities, although in general building cavity vacuum samples made through small openings have time and again proven grossly inaccurate.

How are Mold Test Air Samples Examined in the Laboratory?

Mold spore sample slides are prepared in the laboratory and then examined at magnifications from 100x to 1920x to identify the dominant or other indicative particles collected. Particle counting, such as spores/M3 of air, if provided, is performed at 400x, 480x, 720x, 1000x, up to 1920x.

Microphotographs of lab samples are at 400/480x or 1000/1200x up to 1920x. We also use polarized light and darkfield methods and a variety of slide preparation chemicals and stains. "Subjective particle density" photographs (at 100x) may be provided to offer a gross but instructive comparison of air samples from various areas when different sample quantities are identical. Sample culturing is available but is used only in special cases as it is famously unreliable for characterizing indoor air.

What Should Be the Investigation Approach of Your Mold Test or Environmental Test Consultant?

If your indoor building-related environmental question is whether or not there is a mold, chemical, gas, or similar environmental problem present in a building, a superficial "test" is unlikely to give a reliable answer. We recommend that you have a careful conversation with whomever you decide to hire for any environmental investigation. Make certain that your expert has specific experience in the area of your particular concern, and that their approach is thoughtful and thorough, including sufficient detail and scope of onsite inspection, history taking, and pertinent local research.

There is a tendency among some investigators, especially those whose experience has been primarily industrial, to substitute tests alone for a more thorough approach when working in a home or apartment where there is a mold or other environmental concern.

The reliance on tests alone in a residential (as opposed to industrial) setting can be a mistake, as conditions are so variable, levels of contaminants variable, and the actual target, if there is one, is unknown. This is a very different problem solving task than entering an industrial environment, following a known cookbook procedure to check the levels of a specific, known contaminant, particle, or chemical associated with an industrial process, and comparing the test results with a specific industry standard.

See MOLD / ENVIRONMENTAL EXPERT, HIRE ? for a discussion of when it is or is not appropriate, justified, and ethical to hire a mold consultant to inspect, diagnose, and advise about mold contamination in a building.

Also see ACCURACY vs PRECISION of MEASUREMENTS where we argue that measurements should be reported to include their percentage of error or a +/- figure to give a realistic understanding of the actual reliability of the data.

In that article we include a catalog of sources of Errors When Measuring or Counting Airborne Particle or Mold Spore Levels

Reader Question: if the indoor mold level is the same as outdoors, does that say there is no mold problem?

I just had an air quality control test done on my bathroom for mold. They took air samples from in the bathroom and from outside. I’m told that if the [airborne mold spore] count is the same inside as it is outside there should be no mold problem. My questions is to what degree difference is it a problem? In other words is there a rating number or cut off number that says it’s safe vs. not safe? Thanks for your reply. - J.C.

Reply: ... sparrows are flying around through the air outside at 1 sparrow per cubic foot, and bats are flying around inside your living room at 1 bat per cubic foot. On that data comparison we conclude that you don't have a bat problem inside your home. Does that make any sense at all?


While the opinion about counts you state above is not absolutely without any merit, it is basically a mistake. The risk is that we're comparing apples and oranges - you could have an indoor count, even a "low" count of a highly toxic easily airborne mold, say Aspergillus niger spores in spore-chains (indicating a nearby mold colonization), and an outdoor count of harmless basidiomycetes that happened at that moment of testing to be at similar levels.

But the significance of these two molds is completely different. It's potentially as ridiculous as saying leaves are blowing around through the air outside at 100 oak leaves per cubic foot, and rats are running around inside your home at 100 rats per cubic foot, so probably inside you don't have a rat problem. Does that make any sense at all?

Now it's true that very low levels of indoor airborne mold might, in general mean that the test did not find an indication of an indoor mold problem, but

So while a high indoor problematic mold spore count most likely means that there is a problem to be found and corrected, the opposite is not necessarily true: a low indoor mold spore count does not promise that there is no problem, and an indoor airborne mold spore count that simply compares with the outdoor mold spore count is a leaves and rats comparison that to me makes very little sense.

There are many general mold exposure standards (MOLD EXPOSURE STANDARDS) that try to talk about levels of airborne spores, but they are only very vague guidelines. The level of indoor airborne particles varies enormously, by orders of magnitude, from minute to minute and depending on what's going on indoors.

So these indoor and outdoor airborne mold count comparisons and "tests", without being accompanied by

  1. a thorough building inspection indoors and out (to identify likely sources of trouble or even visible mold),
  2. an occupant interview (of health risks, vulnerabilities, observations, complaints), and
  3. a building history (of leaks, sewer backups, construction snafus),

... without those, a "test" is simply unreliable at best and junk science at worst.

Watch out: by no means do I suggest that every mold worry should result in such a thorough building investigation. I can find some mold on surfaces or contents in just about any building anywhere - it is not necessarily a worry at all. If there is mold growth on caulk or grout joints in a tiled bathroom adding up to a few square feet of moldy surface, no report nor concern for leaks into the bathroom walls, floor, ceiling, no occupant complaints, then the right response is clean off the moldy surfaces.


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