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Mold Spore Counts: are indoor fungal spore counts valid?
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• Mold Test Procedures - are spore counts valid?
• Causes of variation in indoor air mold tests and airborne particle counts
• Degree of variation in the level of indoor air particles over short time intervals

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.

Accuracy & Validity of Indoor Fungal Spore Counts & Cultures

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.

Spore Counts are questionable

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.

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. I 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:

• More about Validity of culture methods for toxic mold inspections and "home test kits"
• Validity of air samples for mold and their interpretation
• Validity of Bulk Surface or Tape Sampling
• Mold inspecting, testing, or sampling methods in the indoor environment - description and critique

As a collector of studies, papers, books on this topic, and as someone conducting my own studies, I 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. 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. 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.

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More expert information on this topic

More Information on Building Diagnostic Inspections and Repairs

• Mold Information Website Home Page
• Mold Classes: what types of cosmetic, allergenic, or toxic mold are a problem? Can mold be cleaned-up successfully?
• Toxic Mold, Allergenic Mold: How Much is Bad?
• Validity of Bulk Surface or Tape Sampling
• Validity of mold cultures and "home test kits"
• Validity of culture methods for toxic mold inspections
• What to look for in a toxic survey report or toxic mold test lab report
• Plant Flower Tree Pollen - a Photo ID Library for asthma and allergy sufferers

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