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How much variation do we see when using Air Sampling to test for Mold Exposure or Contamination Levels in Buildings? Here we demonstrate that indoor airborne mold or other particle counts vary enormously from minute to minute, providing highly inaccurate (though precise) numbers of spores per cubic meter of air.

This document is a brief tutorial which provides information about the accuracy of 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 series of articles. At Causes of Variation in Airborne Particle Levels we discuss the causes of this high level of airborne particle variability. Readers should 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

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Extent of Variation of Airborne Particle Counts

There is normally very high variation in the level of airborne particles in indoor air from moment to moment in buildings. Actual field data easily demonstrates that particle presence in indoor air varies by orders of magnitude from minute to minute. Sampling identical quantities of air indoors just a few minutes apart regularly shows up in our data as enormous differences in particle density from interval to interval, as you can see in this photograph of parallel traces of airborne particles captured by an air sampler which collected these samples just minutes apart in the same location in a building. We always see this phenomenon in buildings, since unless we are measuring airborne particles released at a fixed rate in a controlled test chamber there are quite a few site conditions that agitate airborne debris. A simple visual examination of the five traces of airborne particles captured on this microscope slide make clear that the variation in particle level is significant. Further examples of wide variation of airborne spore counts indoors are provided at Particle Levels vs Sampler Height where you can see microphotographs of traces such as the ones above.

What is the Difference Between Precision and Accuracy in Airborne Mold Spore Counts?

Some people are confused about the difference between precision and accuracy.

An airborne mold spore count obtained from a spore trap or similar air sampling device can be processed in an aerobiology lab or mold test lab by several methods of varying accuracy depending on not only the site variables (which dominate the airborne particle count, but also depending on the skill and experience of the lab technician, the quality of the microscope and slide preparation, and the percentage of the particle trace that is examined.

In the hands of a skilled microscopist who uses the best practices and who examines 100% of the particle trace, the resulting lab report can give mold spore count that is very precise - for example, citing 5,731 Aspergillus niger spores per M³ (cubic meter) of air.

But what the mold test field investigator and laboratory technician may not be able to report is that the variability from minute to minute in the airborne particle level was enormous, easily by a factor of 100.

If the mold test field investigator had collected her or his sample in the very next five minute interval using exactly the same equipment and methods and processing the sample in exactly the same lab with the same technician may produce a very precise count of 573,124 airborne mold spore count of Aspergillus niger spores per M³ of air.

The two counts are very precise (5,731 and 573,124), but the airborne mold spore count produced by this spore trap method is completely inaccurate: 573,124 mold spores per cubic meter of air is 100 times more mold spores per cubic meter of air than 5,731.

If your airborne mold spore count can easily vary by a factor of 100 from minute to minute it may be precise, but it is inaccurate, and therefore in many cases, especially in cases of low counts that appear to be below the threshold of concern in many mold standards, such counts are completely unreliable.

Just try waving a note book across a table before counting mold spores in air and check the difference in airborne spore levels.

Then review mold inspection and mold test reports to notice whether or not the field investigator has noticed and recorded building conditions that affect air current activity in a building: fans on or off, space occupied or unoccupied, heat on or off, air conditioning on or off, etc. Any one of these completely changes the airborne particle level indoors.

The realization that airborne mold spore counts or counts of any indoor airborne particle are inaccurate is a reason to perform a thoughtful visual inspection of and history taking of a building that is under investigation for air quality problems. But such investigations, properly performed, may be costly and often are unnecessary and should not be performed at all. See MOLD EXPERT, WHEN TO HIRE.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ACCURACY OF AIRBORNE MOLD SPORE COUNTS
  Causes of Variation in Airborne Particle Levels
  Indoor vs. Outdoor Spore Counts
  Extent of Variation of Airborne Particle Count
  Particle Levels vs Sampler Height
  Particle Levels vs Windows/Doors
  Particle Levels in Ducts
  Concentration Bursts of Mold Spores
  False Negative Results in Mold Tests
  Mold Culture Plate Test Errors
  Why Use Airborne Mold/Particle Sampling?
  Visual Inspection and History for Mold

• Thanks to Susan Flappan, Flappan Consulting, moldetect.com, Overland Park KS, 913-402-1131, for contributing comments and some suggested text from ACGIH Bioaerosols: Assessment and Remediation 12/2006.

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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