Photograph of Allergenco Mark III Impaction Air Sampler Windows & Doors as Sources of Error in Indoor Mold Tests

  • PARTICLE COUNTS vs WINDOWS, DOORS - CONTENTS: How do open or shut windows or doors affect airborne mold level counts when air sampling for toxic mold? What are the sources of variation in the mold level that air tests can detect?
  • POST a QUESTION or READ FAQs about the errors caused in airborne mold or other particle counts depending on the open/shut position of both windows, exterior doors, and interior doors

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This article explains the impact of open or shut windows & Doors as 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.

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Variation in Airborne Particle Levels as Windows or Doors are Open or Closed

Photograph of mold growth on a window (C) Daniel Friedman


This photograph of an open window in a New York City high rise office building displayed an enormous convection current - notice our low-tech demonstration? The tissue taped to the underside of the window sash is blowing outside in this photo.

When this window was opened the level of indoor airborne particulate debris of all types was increased significantly, and further, any problem particles from lower floors were at risk of being drawn up into and through this office even though there was no problem reservoir in the office itself.



Photograph of air trace with windows shut Photograph of air trace with a window open

We would expect that opening the windows in a building to "air it out" will result in an indoor airborne particle level which is qualitatively and quantitatively similar to outdoor air. In some cases this is quite true if ventilation is long enough and if there is not an usual indoor mold reservoir in the room where testing is performed.

But opening the windows on the upper floor of a residence and certainly opening the windows in a tall building will usually cause a considerable increase in upwards moving air by convection currents in the building. Warm air rises from lower to higher areas, exiting at the open window on the upper floor.

The two photographs shown here were taken by stereomicroscope examining two sequential airborne mold and other particle traces in air samples collected just minutes apart. The second, very dense particle sample shows what happened when we opened the window in this high rise building in New York City.


Continue reading at MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD or select a topic from the More Reading links shown below.

Or see MOLD LEVEL IN AIR, VALIDITY, and for a more in-depth critique of popular mold testing methods than this tutorial


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