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• Levels of Mold in Heating or Cooling Ducts as Sources of Error in Indoor Mold Tests - what causes variation when testing ducts for mold contamination?
• What are the sources of variation in the mold level that air tests can detect?

Here we discuss how Levels of Mold in Heating or Cooling Ducts and the aggressiveness of mold testing (do we agitate the ducts) form Sources of Error in Indoor Mold Tests This document is a brief tutorial which provides information about the accuracy of and sources of errors in 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 also see Mold Spore Counts: are indoor fungal spore counts valid?, and for a more in-depth critique of popular mold testing methods than this tutorial see Mold Sampling Methods in the Indoor Environment © Copyright 2008 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.

Variation in Airborne Particle Levels in Heating and Air Conditioning Ducts

How much variation in airborne mold or dust level do we see inside heating and air conditioning ducts and air handlers?: The left hand photograph shows a one liter airborne particle trace collected inside of a heating furnace return air plenum using a Burkard Personal Air Sampler. The right hand photograph shows a second particle trace collected in the same location, with the same volume of air, with one difference: we tapped lightly on the side of the air plenum during the sampling process. Even before counting the number airborne particles of any type per liter of air it is obvious that even modestly aggressive sampling (rapping on the plenum to stir up local dust) can make a large difference in the level of particles seen in the sample result.

We conducted this test during a post mold remediation clearance inspection of a previously mold-contaminated air conditioning system. The remediation contractor vehemently disagreed with the procedure of tapping on the ductwork during testing, informing us that "... his hygienist never did such a thing". We agree that consistency in test methods is important in order to be able to compare one mold test with another. However if we're looking for the presence or absence of a significant mold or allergenic dust reservoir that should have been removed, a little aggressiveness in sampling can be useful and in fact more accurate as well.

In sum, do not rely on the accuracy of airborne particle counts: Since air samples do not capture a representative picture of the indoor environment an indoor air quality investigator should not rely simply on conventional airborne particle quantitative analysis (particle counts per cubic meter).

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 VARIOUS MOLD TEST METHODS
  Causes of Variation in Airborne Particle Levels
  Indoor vs. Outdoor Spore Counts
  Extent of Variation of Airborne Particle Counts
  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

Technical Reviewers & References

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material.

• 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.
• Daniel Friedman - principal author/editor of the InspectAPedia ^TM Website
• Technical reviewers are invited to comment or ask questions - contact us

More expert information on this topic

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• 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
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