Photograph of Allergenco Mark III Impaction Air Sampler False Negative Mold Test Results & What They Mean

  • FALSE NEGATIVE MOLD TEST RESULTS - CONTENTS: What causes False Negative or ok toxic mold test results - and why are these a Source of Error in Indoor Mold Tests. What are the sources of variation in the mold level that air tests can detect?
  • POST a QUESTION or READ FAQs about false negative mold test results: thinking that there is no mold contamination when in fact it is present
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Mold test errors: false negative results. When does a mold test miss important mold contamination in a building?

Here we define and explain the causes of False Negative or OK toxic mold test results 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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High Risk of False Negative Results in Mold Tests

Mold spore counts are questionable in the first place, given the high variability that we discuss in this article. But "low" or "negative" mold spore counts, especially if not accompanied by a thorough visual inspection and collection of the building's leak history and occupant complaint history, are particularly suspect.

Individual samples of particles in air show tremendous variation from minute to minute, making "ok" airborne mold spore test results a thing to view with skepticism.

Examples of factors which can cause an exponential difference in particle levels in indoor residential air over short time intervals include: mechanical disturbance (walking across a carpet or moving a moldy cardboard box), operation of hot air heating system or central air conditioning system, operation of other building fans, particularly ceiling fans and vacuum cleaners, turning lights on and off, and opening or closing windows and doors. In situations of particular risk, additional or periodic testing should be considered.

Below we list these sources of mold test variation and give photos and mold test examples for specific variation sources.

Our experience is that the variation in the level of small (mold spore sized) indoor airborne particles is so significant that any indoor airborne particle count is likely to be an inaccurate guess at the actual occupant exposure level where indoor problematic molds are present. This means that visual inspection, building history, and occupant complaints or occupant health fragility need far more attention than they receive if the "mold investigator" simply performs a cursory visual inspection or worse yet, simply collects a few air samples in a building.

At Conditions that Cause High Variation in Indoor Airborne Particle Levels this article list some of the chief sources in the short term variation in the level of airborne particle levels. Even if we used a long term sampling method, say 24 hours, or interval sampling over days, these same variations will occur, and worse, without careful study of the actual sources of airborne particle level variation in a specific building, and without a documentation of the typical conditions in the building during the time that it is occupied we cannot describe the occupant exposure level with any accuracy whatsoever.

Risk of false negative mold tests: Experienced mold contamination investigators should have little confidence that a one-time low or "ok" "toxic mold test spore count" is an assurance that problematic particles are not present.

The presence of certain spores or fungal particles, even at low levels in air can indicate or confirm an indoor reservoir and amplifier of mold spores in the building, particularly if a careful microscopic examination of the "low level" sample contents shows evidence of nearby active fungal growth such as Aspergillus sp. or Penicillium sp (or certain other genera/species) spore chains. The presence of extensive off-season pollen can also indicate a pollen reservoir in the building - a potential concern for people with pollen allergies or asthma.


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and for a more in-depth critique of popular mold testing methods than this tutorial see MOLD TESTING METHOD VALIDITY

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