Stachybotrys spores (left) and structure (right) Mold Tests: Spore Count Validity

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Mold spore count validity: this document discusses a serious question about the currently-popular "spore counts" obtained by industrial hygienists, home inspectors, and "mold investigators" (and the mold testing laboratories they use). Airborne or other mold counts are used to estimate the toxic or allergenic mold exposure level of building occupants in buildings where mold may be present.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Accuracy & Validity of Indoor Fungal Spore Counts & Cultures

Very moldy building interior (C) Daniel FriedmanFirst, should we be testing for mold at all? If you see mold on indoor surfaces, NO mold testing is needed just to confirm that mold is present in a this building and that cleanup is needed. The clean-up procedures for mold contamination do not depend on the mold genera/species, with the sole exception of cosmetic-molds on some framing lumber. (BLACK MOLD, HARMLESS )

Only an idiot, or perhaps someone out to prey on mold fear (MYCOPHOBIA, STAINS MISTAKEN for MOLD) would require a mold test to determine if the home at left needs professional mold remediation.

But there may be other reasons to test to identify the dominant mold genera/species in a building.

For example, if a large remediation project is planned, tests may be needed for project control - to be able to prove later that other building mold contamination or moldy dust did or did not come from an improperly-handled mold remediation job. We also may include tests for airborne mold as a part of a more thorough building investigation and screening for hidden mold contamination, but we would not rely on an air test alone in that case.

Finally, we may want to identify the dominant mold genera/species in a building as an aid to medical diagnosis and treatment. Details about reasons to test for mold and warnings about mistaking a "mold test" for a useful building inspection and diagnosis to find hidden mold or to determine how to prevent future mold contamination are found at

Mold Count Precision is Not Mold Exposure Accuracy

Air test for mold, particle trace variation (C) Daniel FriedmanCounting indoor mold spore levels per cubic meter of air or "liter" produces numbers which may be very precise (many digits or decimal places) but which are generally highly inaccurate (wrong by one to three orders of magnitude).

Enormous variations in the level of airborne particles in buildings occur from even the simplest changes such as walking through a room or turning on a furnace blower.

While many laboratories, including our own, participate in programs to calibrate and standardize their in-laboratory particle counting, slide preparation, and microscopy procedures, no amount of precision in lab counting can overcome the several orders of magnitude in variation of indoor particle levels that actually occurs in a building over intervals as short as a few seconds and as long as days or months.

While there is a useful place for every environmental investigation tool, inadequacies in field procedure, field condition reporting, and visual inspection that would permit an interpretation of lab results limit the usefulness of "bare lab reports" which simply give a number. The number may be impressively precise, but highly inaccurate.

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

Spore Counts Obtained by Airborne Mold Spore Traps are of Questionable Accuracy

Warning: interpret all quantitative data, particularly counts of particles in indoor air, with great caution. Individual samples of particles in air show tremendous variation from minute to minute, making "ok" test results a thing to view with skepticism.

Examples of factors which can cause an exponential difference in particle levels in indoor residential air over short time intervals include: mechanical disturbance (walking across a carpet or moving a moldy cardboard box), operation of hot air heating system or central air conditioning system, operation of other building fans, particularly ceiling fans and vacuum cleaners, turning lights on and off, and opening or closing windows and doors. In situations of particular risk, additional or periodic testing should be considered.

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.

Indoor counts vs. outdoor counts

The University of Minnesota fungal experts observe that an outdoor-baseline comparison to indoor air is not valid when the outdoor sample was taken during or immediately after precipitation (spore counts plummet outdoors in the rain and might soar right after it), and the comparison is probably not valid in winter when outdoor counts tend to be below indoors. We agree and add other constraints: snow cover practically eliminates spores from outdoor air.

Even in warm weather spore counts vary during the day as weather conditions (humidity, temperature, period after rainfall) affect sporulation and spore movement.

Air sampling by culture plate or surface testing by swab are questionable

Similarly, tests which rely on culture to identify particles are at severe risk of giving a "false negative" result, missing a serious problem, or of giving a "misleading positive" result by asserting that a particular spore which grew on the culture is the problem in the building. Fungal spores grow at different rates on different culture media.

Spore "A" may "overgrow" spore "B" in a particular test, obscuring the presence of spore "B" which might be the real problem in the building. Some fungal spores won't grow at all in culture media (non-viable spores and many Ascospores) but may still be present at toxic levels in a building.

More about mold testing and the validity of air sampling and home test kits for mold:

As a collector of studies, papers, books on this topic, and as someone conducting our own studies, we have seen a very wide range of opinion among experts in the field. Spore allergenicity or toxicity varies widely among fungal genera/species. So does the sensitivity of humans and other animals to fungal spores.

Mold Spore Toxicity Variations by Species, Genera, particle size, even growth substrate

So no single number will be absolutely correct. Just as spore toxicity varies by species, so does the physical size of individual spores. The effect of breathing air contaminated by 5000 Penicillium sp. spores per cubic meter is unlikely to be identical to the effect of breathing 5000 Stachybotrys chartarum spores per cubic meter of air.

Not only does their chemistry and toxicity vary, but a typical Pen/Asp spore is about 2 microns in diameter (1/25th the width of a typical human hair) while a typical Stachybotrys chartarum spore might be 8 x 12 microns -- much larger and thus providing more potentially harmful material per individual spore.

Spore Toxicity Variation Precludes Credible Single Number Mold Exposure Standards

You can see that writing federal or state standards for permissible fungal spore exposure by "count" or "levels" is difficult. Not only are there many variables to consider, but using currently popular air sampling or culture methods, even a low or "OK" test result cannot guarantee that there is no problem in the building.

Fortunately one can become reasonably confident about the level of mold or allergen risk in a building through competent visual inspection, judicious use of various sampling tools and methods, and competent laboratory determination work. Because this expertise is costly and the work time consuming, it should not be ordered without reasonable justification.

Readers should see Shortcomings of air sampling and also see Extent of Variation of Airborne Particle Counts and Mold Testing: Air samples and their interpretation - a quick tutorial.

For a more in-depth critique of popular mold testing methods than this tutorial see Mold Sampling Methods in the Indoor Environment or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Watch out: changes in air movement velocity or direction can make any indoor air quality test including for airborne mold spores very inaccurate.

Mold Test Accuracy Articles


Continue reading at CAUSES of VARIATION in AIRBORNE PARTICLE LEVELS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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