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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
First, 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 / ENVIRONMENTAL EXPERT, HIRE ? also explains 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.
Mold Count Precision is NotMold Exposure Accuracy
Counting 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.
"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:
Swab sampling - what works or does not work about swab sampling 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
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
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.
See AIR MOVEMENT in BUILDINGS
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
"A Brief Guide to Mold, Moisture, and Your Home",
U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
US EPA - Mold Remediation in Schools and Commercial Building [Copy on file at /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
US EPA - Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol
Atlas of Clinical Fungi, 2nd Ed., GS deHoog, J Guarro, J Gene, & MJ Figueras, Centraalbureau voor Schimmelcultures, Universitat Rovira I Virgili, 2000, ISBN 90-70351-43-9 (you can buy this book at Amazon) - The Atlas of Clinical Fungi is also available on CD ROM
Atlas of Indoor Mold, Online Clinical Mold Atlas, Toxins, Pathogens, Allergens and Other Indoor Particles - Medical Health Effects of Mold (separate online document)
Fifth Kingdom, Bryce Kendrick, ISBN13: 9781585100224, is available from the InspectAPedia online bookstore - we recommend the CD-ROM version of this book. This 3rd/edition is a compact but comprehensive encyclopedia of all things mycological. Every aspect of the fungi, from aflatoxin to zppspores, with an accessible blend of verve and wit. The 24 chapters are filled with up-to-date information of classification, yeast, lichens, spore dispersal, allergies, ecology, genetics, plant pathology, predatory fungi, biological control, mutualistic symbioses with animals and plants, fungi as food, food spoilage and mycotoxins.
"Disease Prevention in Home Vegetable Gardens,"
Department of Plant Microbiology and Pathology,
Department of Horticulture, University of Missouri Extension - extension.missouri.edu/publications/DisplayPub.aspx?P=G6202
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