Causes of Variation in Indoor Mold or Airborne Particle Levels / Concentrations
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What are the sources of variation in the mold level that air tests can detect?
Just how accurate and valid is air sampling for toxic mold testing?
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?
How should an environment be inspected and tested to get a reliable estimate of toxic mold levels or mold exposure levels?
Questions & Answers about the causes of variation in airborne particle or mold levels & how that affects building indoor environmental test accuracy
References

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Conditions that Cause High Variation in Indoor Airborne Particle Levels: this document is a brief tutorial which provides information about tests for the level of allergenic and toxic mold in residential buildings: Are spore counts valid? Are cultures and swab tests valid? These critical questions are discussed in this paper. Readers should see AIR TEST SAMPLING CASSETTE STUDY and also see MOLD LEVEL IN AIR, VALIDITY, and for a more in-depth critique of popular mold testing methods than this tutorial see MOLD TESTING METHOD VALIDITY

Conditions that Cause High Variation in Indoor Airborne Particle Levels

Some of the site conditions that cause this wide variation in the level of airborne particles in a building are listed below. After reviewing indoor air quality reports produced by many investigators, we conclude that almost never does the IAQ investigator or "mold test expert" document much less control these building conditions. When we have no documentation of building conditions we cannot easily interpret the meaning of spore counts or airborne particle level counts in a building. The sources of variation in the level of indoor airborne mold spores or other particles are arranged very roughly in order of increasing effect on mold test results.

Empty rooms where no windows are open, no fans are operating, and no occupants are present will tend to produce lower airborne particle counts than when the same rooms are in normal use.
Culture plates or air samples using settlement plates are inaccurate both by species (only a small percent of molds will grow on culture media so we miss up to 90% of genera/species with this method) and also by quantity (large spores and other particles settle out of air faster than small spores or other particles. For more detail see Air sampling by culture plate, mold swab, or surface testing by swab are questionable
Outdoor mold counts are tricky to compare with indoor mold spore counts because the same genera counted by a lab, say Pen/Asp may both be the same genera (say Aspergillus but the mold species may be completely different, making comparison of these two numbers meaningless. Yet many mold labs cannot or do not speciate to this level of detail, and in fact in many air samples produced by spore traps and other air sampling equipment do not contain sufficient data to make the distinction among certain similar spores (amerospores).

Further, even a low indoor airborne Pen/Asp mold spore count might be significant if the mold spores occurred in the sample in spore chains rather than individual spores - indicating that a growing mold source reservoir is nearby. For more detail see Indoor airborne particle counts vs. outdoor counts and also review Concentration Bursts of Mold Spores
A room with one individual sitting at a desk quietly reading will produce higher particle counts than an empty room but lower than the conditions listed next.
The genera/species or "type" of mold contamination makes a big difference in what is detected in indoor air: for example, Stachybotrys chartarum is a large (so not easily airborne) mold spore which is sticky (not easily airborne) which evolved to be spread by sticking to a cow's foot as she walked through moldy straw. These spores are not normally found in indoor air even if a large mold reservoir is present, unless the Stachybotrys chartarum is being disturbed, say by incompetent demolition and cleaning, or by carrying moldy cardboard boxes out of a wet moldy basement. By contrast, Aspergillus sp. are usually much smaller mold spores, are often easily made airborne by simple air currents (varying depending on growth stage of the mold and indoor humidity and temperature levels). Air tests tend to find particles that are naturally more easily airborne.
A carpeted room with someone walking across the carpeting will produce a higher particle count than an un-carpeted room, other conditions being equal.
An air sample collected at floor height may vary considerably from a sample collected on a table depending on where mold is growing there could be surprises. We obtained airborne mold spore counts that varied by at least one order of magnitude across each of three samples collected in the same room: on the floor under a moldy pool table, on top of the pool table, and in still air versus waving a notebook once at the moldy pool table. For more detail see Variation in Airborne Particle Levels due to Placement Height of Air Sampling Device
The aggressiveness of air sampling methods for mold makes an enormous difference in the level of airborne particles. Waving a notebook at local tabletops, rapping on a hot air duct, and similar acts produce much higher counts than very gentle passive (and unrealistic) mold sampling methods. For more detail see Variation in Airborne Particle Levels in Heating and Air Conditioning Ducts
A room occupied by several people walking about or a bedroom with a child bouncing on a bed will produce higher airborne particle counts than the conditions listed above.
A room with in-use portable heaters, air cleaners, or other devices which stir indoor air may produce higher particle levels than the conditions listed above.
A room located close to a large airborne-type problem mold reservoir will usually show higher spore counts than if the reservoir is more distant. However the simple act of opening or closing building interior doors can completely change this picture. In a mold investigation of a home with a large airborne-type mold reservoir in its basement and crawl space, we found very low indoor airborne mold spore counts in the building's first floor over these mold reservoirs when the basement door was kept shut. But within minutes of opening the basement door we observed a significant rise in the level of indoor mold spores at the same test location. For more detail see Variation in Airborne Particle Levels as Windows or Doors are Open or Closed
A room with windows open in a tall building will have considerably more air movement (due to convection currents in the building) and may have considerably higher airborne particle counts than when the window is shut. For more detail see Variation in Airborne Particle Levels as Windows or Doors are Open or Closed
A room where central air conditioning or hot air heat is operating will produce substantially higher airborne particle levels than the same room when that equipment or its blowers are off.
A room where a ceiling fan or other fans are in use will produce among the highest levels of airborne particle counts, other conditions being equal.
Building air change versus indoor mold spore trap data: any comparison of indoor airborne mold spore counts to outdoor mold spore counts is further confounded by the observation that indoor air always contains a different particle mix than outdoor air, confirming that by no means does normal indoor air become "equalized" with outdoor air for any particular particle. Experienced microscopists are usually able to determine whether or not an air sample slide was collected indoors or outdoors by a quick notation of the types and mix of particles that appear in the sample.
Also see ACCURACY vs PRECISION of MEASUREMENTS where we argue that measurements should be reported to include their perentage of error or a +/- figure to give a realistic understanding of the actual reliability of the data. There we include a catalog of sources of Errors When Measuring or Counting Airborne Particle or Mold Spore Levels

Effects of Overall Dust or Mold Level on Mold or Dust Particle Count Variation

What is remarkable about all of these observations is that a large amount of variation in airborne particle level will occur as these conations vary regardless of whether the room is relatively clean or relatively dusty. The absolute numbers will of course be higher in a dusty environment, but the relative numbers will still vary by several orders of magnitude even in a less dusty area.

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Technical Reviewers & References

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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

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Rodents, Mice, Squirrel Control - I find high levels of mouse and rodent dander, fecal dust, and urine-contaminated dust in some buildings, and high levels of these materials in building insulation in those locations. If you have a mouse problem, particularly if mice and their waste (fecals or urine) are contaminating the building HVAC or building insulation, may need both steps to clean up or remove infected materials and steps to stop an ongoing rodent problem. If squirrels are a problem, the cleanup needs to include closing off entry openings into the building. Get some help from a licensed pest control expert.
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