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• How to Compare Outdoor and Indoor Mold Levels or Counts - what are the sources of error?
• Just how accurate and valid is air sampling for toxic mold testing?

How should we Compare Outdoor and Indoor Mold Levels or Counts - what are the sources of error? 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? 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.

Indoor Airborne Mold or Particle 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.

Section 2.4.2.2 of The ACGIH Bioaerosols: Assessment and Remediation offers:
Investigators should bear in mind that samples provide information about a site as it existed at the time tested. However, the findings may not represent conditions at a time in the past or future, even the relatively recent past or near future. Changes in the kinds, concentrations, and proportions of biological agents in the air can be rapid and substantial. -- thanks to S. Flappan for suggesting this citation.

OPINION: There are severe problems in the standard practice comparing indoor and outdoor spore counts to decide if a building has a mold problem.

1. Overall Outdoor Mold Spore Counts: Some mold testing laboratory reports give simply an "overall outdoor spore count" number which is compared with either a specific (genera/species) or an "overall" indoor spore count number of mold spores /M3 of air. This is a silly comparison since that data fails to identify the spore genera/species, thus masking any intelligence about the actual indoor spore risk. For example the outdoor spores at the time of measurement may be dominated by Cladosporium sp. or Basidiomycetes while the indoor spore level at the same number of spores/M3 may be Aspergillus versicolor - which could well indicate a problem but which would not be indicated as a problem by the lab approach I've described.

2. Outdoor Pen/Asp may be Different Species than Indoor: Even when outdoor spores are identified to the genera such as Aspergillus sp. few laboratories take the extra step to speciate indoor and outdoor airborne spore trap sample contents. In fact speciation of many species of airborne spores in a spore trap can be difficult or impossible by conventional means. So an outdoor "Penicillium/Aspergillus" spore count of 3600 spores/M3 of air may be compared with an indoor "Penicillium/Aspergillus" airborne spore level of 3500 spores/M3 of air and reach the completely mistaken conclusion that there is no evidence of an indoor air quality problem. Looking closely at the indoor spores might, however, have disclosed that the indoor "Penicillium/Aspergillus" was a completely different mold species than the outdoor species - making the indoor-outdoor comparison a meaningless "apples and oranges" comparison. Yet that comparison is the common one made by many field investigators. Worse, certain basidiomycetes are difficult to recognize in air samples and are counted by some laboratories as "Pen/Asp" when in fact they may not even be in those genera.

Air samples may miss important particles or may point to the "wrong" particles

High risk of false negative airborne mold test results: Indoor air samples are at high risk of giving a "false negative result" - indicating no problem when a problem is present, either completely missing the presence of the most problematic spores in a building or which indicating as "the problem" the wrong spores in a building simply because they were dominant at the time sampled. Outdoor or indoor "Pen/Asp" spore counts are often compared erroneously in cases where the indoor genera/species is quite different from the indoor genera/species. For example a "low" indoor count that is all Aspergillus niger may indicate a problem, even though it's lower than the outside "Pen/Asp" count if the outdoor count was actually Penicillium sp. or perhaps even basidiomycetes mistaken for Pen/Asp.

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

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