Photograph of Allergenco Mark III Impaction Air Sampler Causes of Variation in Indoor Mold or Airborne Particle Levels / Concentrations

  • CAUSES of VARIATION in AIRBORNE PARTICLE LEVELS - CONTENTS: 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?
  • POST a QUESTION or READ FAQs about the causes of variation in airborne particle or mold levels & how that affects building indoor environmental test accuracy
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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.

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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.

  1. Empty / unoccupied rooms with little air movement: 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.
  2. 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
  3. 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
  4. 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.
  5. 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.
  6. A carpeted room with someone walking across the carpeting will produce a higher particle count than an un-carpeted room, other conditions being equal.
  7. 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
  8. 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
  9. 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.
  10. 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.
  11. 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 - live link given just below.
  12. 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.

  13. 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.
  14. 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.
  15. 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.
  16. 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.


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.

Reader Question: should we use fans to stir up air before an airborne mold test?

2016/02/15 charles said:

Before and air quality inspection within the home, should to interior air be moved around by a low volume blower to stir up the
spores for evaluation by the lab.

This question was originally posted at AIRBORNE MOLD COUNT NUMBER GUIDE

Reply: agent slaps mold test consultant who stirred up particles during test


I really like your question and have to give some different answers.

If you were trying to construct a "worst case" test of airborne particles in a home, turning on a fan or blower to stir up dust will help do that. You cannot assume that the airborne particle counts from a worst-case test are necessarily an accurate representation of the normal exposure of the building occupants unless those fans are often running. I find several orders of magnitude in indoor particle counts depending on how much disturbance is going on in the test area. Just walking across a carpet or waving a notebook in the air will change particle counts over the test interval.

If you are trying to measure the probable occupant exposure level to airborne particles, conduct the test under the usual building conditions of fans on or off. If you've stirred up indoor air when you didn't want to you'd need to leave air moving equipment off for 24 hours to eliminate most of that effect. Even then some ultra-small particles may remain airborne even longer.

If screening a building for problematic particles, as airborne levels vary so widely, I like to collect settled dust samples. A quantitative analysis of settled dust is nonsense unless the sample was created under controlled and known conditions. But settled dust will usually represent a longer time interval, possibly weeks or months of settling dust particles. Screening that sample for unusual particles or for dominant particles other than fabric fibers and skin cells can be informative.

During an aggressive test of an HVAC system for contaminants I placed an air sampler right inside the return air plenum and turned it on. An agent who was watching the operation became incensed when I used my flashlight to give a good RAP to the side of the return duct. "That's not 'normal'" she exclaimed. My reply: "Really? Do you think the kids playing in this basement never toss a ball or throw a toy that bangs into this exposed ductwork?" She slapped me. [Not really, that's just my Irish heritage making the story better without letting truth get in the way.]

More truthfully, I was really looking for contaminants in the system, not characterizing usual exposure.


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