Alternaria mold spores and hyphae from an indoor surface (C) Daniel FriedmanA Guide to Good Practices in Mold "Testing"
Examination of the Validity of Current Indoor Mold Sampling Techniques

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Validity of various mold & environmental testing methods used in buildings.

This article series discusses the validity of nearly all of the popular mold testing methods currently in use, pointing out the strengths and weakness of each approach to mold sampling in the indoor environment, beginning with air sampling for airborne mold levels indoors.

The mold testing methods comparison study reported here presents a summary and critique of some popular methods used to examine indoor air quality to test for presence or absence of problematic levels of toxic or allergenic mold or other bioaerosols.

We describe and critique specific "testing" or "sampling" methods used to "test" buildings for mold in the course of a building investigation. The appropriateness of testing at all is discussed on this and other pages at this website.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Introduction to Mold Testing and Sampling Validity

Mold culture plate (C) Daniel Friedman15th Annual North Carolina/South Carolina
Environmental Information Association Technical Conference
Myrtle Beach, SC
Daniel Friedman 23 September 2005, Updated 2/8/2012 & 2017/08/01

The contents of this series of article originally appeared in a more abbreviated form at the 15th Annual North Carolina/South Carolina Environmental Information Association Technical Conference, Myrtle Beach, SC, by Daniel Friedman, 23 September 2005. The bottom of each article page shows the latest revision date.

We will discuss: Tape sampling for mold - sampling location is critical. Vacuum samples - useful for some materials & surfaces & for insulation, not valid for building cavities. Cultures to "Test for Mold" - unreliable. Swab sampling - damages samples. PCR methods for Mold Identification - accurate if you already know what you're looking for.

Article Contents

An illustrated power-point version of this document is at - that copy involves larger files of images and data - if you are viewing this document from a low-speed line you should stick with this text version.

First, should we be testing for mold at all? If you see mold on indoor surfaces, NO mold testing is needed to confirm that mold is present in a this building and that cleanup is needed. But if a large remediation project is planned, tests may be needed for project control.

See MOLD / ENVIRONMENTAL EXPERT, HIRE ? for a discussion of when it is or is not appropriate, justified, and ethical to hire a mold consultant to inspect, diagnose, and advise about mold contamination in a building.

Black mold (C) Daniel FriedmanAn IAQ or "mold investigator" cannot reliably identify a mold genera or species growing on a building surface by the naked eye. Despite the recent public fear created by media attention, including a now-refuted "Toxic black mold" Stachybotrys chartarum study involving infant illness and death, it is simply not the case that "black mold" in buildings is necessarily a danger to occupants.

In fact, having investigated many buildings quite carefully, we assert that in most cases the "black mold" that you see in buildings is most often an "indicator organism" telling you that conditions are right for mold formation. Our photo (left) shows Craig Balchunas during an environmental inspection.

We have often found more problematic molds in such properties, such as species of Aspergillus sp. or Penicillium sp. which are airborne, travel in the building, and are more likely to be bothering the occupants than a large, sticky, non-easily-airborne spore like S. chartarum.

These harder to see, often light-colored molds, are quite often missed entirely by inexpert owners and amateur "mold inspectors" who go on to collect, over-report the occurrence of, and react to the "black stuff."

One result has been unnecessary expense for "mold testing" and in some cases unnecessary mold remediation. Some of our articles, as well as writing by others who study this field, address these problems and offer help in deciding what level of trouble and expense are appropriate.

In any case, mold in a building should be cleaned-up, and large amounts might require professional inspection and advice. Before doing anything drastic or costly, get some competent advice.

Opinions Expressed in This Article

This is a working document and our study of indoor air quality, mold testing, and other sampling methods is ongoing. These views are undergoing and are subject to peer review and revision. (Critique from any reader is welcome).

Trained in building science, aerobiology, and forensic microscopy, we perform on-site building investigations to evaluate the need for and extent of mold remediation in buildings in order to give mold remediation and prevention advice.

Also trained in mold and other particle identification (forensic microscopy), we operate our own aerobiology laboratory where samples, collected during an investigation, are examined.

In the course of our field surveys we often collect multiple samples even at the same surface or building area, in order to study the relative effectiveness and consistency of the various methods, just as we test and compare other tools used to find hidden moisture and similar problems. (These extra samples, collected for purposes of scientific study, are not billed to our client.)

Our opinions about the usefulness of various methods derive from both study by other experts and by our own ongoing field and laboratory testing.

Does everyone need to hire an "expert" like me to examine their home? Of course not. This article offers help in choosing mold sampling and testing methods and devices.

Other articles at our website offer help in determining when it's appropriate to hire an expert.

The Minnesota Department of Health has useful advice [ ] about hiring an investigator too.

Consultant certification - who's an "expert" and do you even need one?

Where the extent of mold damage and/or the risk to building occupants justifies a professional survey, the inspection, sampling and testing performed, the laboratory determination of sample contents, and mold cleanup must be performed competently, lest the building owners/occupants face an unexpected health risk (someone makes a mess of problematic mold) or cost (a poor remediation plan or bad clearance testing leaves a mold problem and the work has to be done all over again).

Whoever performs sampling or testing needs to use effective methods for inspection and testing.

For professional inspections, education and experience are required for this work as substantial costs and the health of present and future building occupants can be at risk. Consumers should review experience and education of whomever they hire to survey a building or to clean up an environmental problem.

Regrettably various mail-order "certifications" are available which are confusing to consumers, and even within organized professions such as industrial hygiene, environmental inspection firms, engineering firms, and home inspection companies, individual training, experience, and depth of knowledge vary enormously.

Consumers hiring an inspector or remediation company should consider the education, experience, and track record of people they may employ and should not rely only on a professed "certification."

(Years ago our dog Katie became a "certified inspector," and received a full diploma from a national "certification" agency.

Try going to the "certifying" agency's website to see if, for simply mailing in a fee, or perhaps taking a weekend course, you, too, could become "certified" as a "mold remediator" or "environmental inspector.")

The CIH designation is a real credential for industrial health investigations (in industry) as is P.E. "real" for certain specialties such as engineering design or forensic engineering.

But some CIH's and P.E.'s are not familiar with building failures and leak tracing, and may match other "mold investigators" who are not familiar with mycology, aerobiology, air and particle movement indoors, microscopy, chemistry, proper ways to use equipment, how to evaluate the expertise of their "mold lab," or even what mold looks like.

The Pan American Aerobiology Association [ ] has recently completed and turned over to an independent testing agent [ ] a national examination specifically to test the competence of people identifying fungal spores, a test which was offered beginning in 2004.

Look for this and other pertinent professional associations or certifications provided they are from "real" professions and professional associations. And look at the background and experience of the investigator. AIHA is also interested in further investigator and lab certifications.

A Description & Critique of Contemporary Indoor Mold Sampling/Collection/Testing Methods

Mold "Testing" - A Guide to Good Practices in Mold "Testing": Examining the Validity of Current Indoor Mold Sampling Techniques

A Guide to Using Air Samples to Test Buildings for Harmful or Toxic Mold

Spore traps (C) Daniel FriedmanA collection device, slide, cassette, or tape are used with a calibrated air pump to collect airborne particles. We am not referring to culture plates which rely on passive collection or settlement of particles as these have even worse inaccuracies and errors than the reservations we describe about active air sampling described here.

Some common examples of air samplers include:

  1. Impaction samplers (Burkard™, Allergenco™) collect airborne particles through a venturi -slit onto a prepared microscope slide using a equipment incorporating a calibrated air pump and a static or moveable slide permitting single-use or time-lapse sampling.

    Photos of these two sampling devices are shown just below.
  2. Cassette samplers (Air-o-Cell™) also often called spore traps, use a patented 37mm cassette connected to a calibrated pump (typically at 15 lpm).

    The cassette is used to collect airborne particles through a venturi -slit onto a proprietary media on a cover-slip which is in turn mounted onto a microscope slide in the laboratory. (Photo above)
  3. Cassette samplers using an MCE filter cassette connected to a calibrated pump.

    The filter can be cleared and examined microscopically (qualitatively, counting is tough except with the BI -Air™), or used to prepare a culture, or both.
  4. Cassette-like samplers use a calibrated pump connected to a patented stainless steel venturI -device which accepts a microscope slide prepared with a collection media.
  5. Anderson-type pollen, mold spore & airborne particle samplers which collect particles directly onto a culture dish - for "viable sampling" to "test" for mold.

    Culture samples may be useful in some cases to better identify or to speciate mold which has already been found in a large reservoir. Otherwise it has limitations we discuss elsewhere in this document.
  6. Anderson-type impaction samplers which collect particles on tape wrapped around on a drum or Rotorod samplers which collect particles on glass rods are used for outdoor pollen and spore counting and reporting in the IAAA allergy network.

    They are not well-suited for indoor building assessment though some folks use the high-error rotorod sampler indoors.


Shortcomings of air sampling for mold - common test conditions cause very wide variation in mold test results

Burkard air sampler (C) Daniel Friedman

Airborne particle levels vary enormously from minute to minute in a building, a fact that is not considered in airborne indoor mold spore counts. The sources of this variation or even the observation of their condition (windows open or shut, fans on or off) are rarely recorded during indoor air tests for mold.

As a "mold test" for the presence or absence of a mold problem in a building, a negative air sample result is of little confidence.

As a measure of "mold exposure" quantitative analysis (spores/cubic meter) is invalid unless long-term time-weighted average measurements are made under all varying building conditions. While this is an important diagnostic tool, and one which we apply regularly, it cannot be relied alone on in the way some investigators would use it: dashing into a building and grabbing an air sample.

Air sampling as a "pass/fail" or "present/absent" test for mold in buildings is unreliable as a "mold test." Many variables cause a wide range of particle levels in air over short intervals.

However this method has a place in a thoughtful building investigation: air sampling can be useful as a background check for the possibility that there is a large reservoir source which has not yet been found in a building.

But a negative result is of absolutely no confidence, and quantitative work (spores/cubic meter) is invalid unless long-term time-weighted average measurements are made under all varying building conditions.

Allergenco impaction sampler (C) Daniel Friedman

Our own time-lapse sampling (such as using the Allergenco Mark II time lapse impaction sampler shown at left) proves that indoor airborne mold and other particle levels easily vary by a factor of 10,000 due to common variations in building conditions such as fan on off, human activity, mechanical disturbance of moldy stuff, etc.

While this is an important tool it cannot be relied on in the way most "inspectors" would use it, dashing into a building and grabbing an air sample.

See MOLD in AIR: Quantitative Analysis below for an elaboration

or see ACCURACY OF VARIOUS MOLD TEST METHODS for more details about the accuracy of air tests for mold.

Photograph of variation in airborne mold levels

Summarizing these mold test shortcomings,

  1. Airborne mold spores and other particle levels vary significantly indoors from minute to minute as well as from area to area depending on human activities as well as use of fans, vents, windows, doors, and even lighting equipment.

    You can see in our photo at left that in a single building location small variations in sampling conditions (like waving a notebook in the air) can produce enormous differences in the number of particles collected.

    More detail is at MOLD LEVEL IN AIR, VALIDITY.
  2. Spores are collected but not spore producing conidiophores, hyphae and other components are collected, making identification incomplete and more difficult than with tape or cultured samples.
  3. Spores are desiccated and may be damaged by the collection method, interfering with culture efforts if this follow-up step is needed.

In sum, air sampling is useful as a background screen for the possibility that there is a large reservoir source which has not yet been found in a building, and may be useful to compare relative particle levels between a problem area and a control area, or in an area before and after cleaning.

But a negative result (no problem mold found) when performing air sampling is one in which you cannot have high confidence unless it is accompanied by a number of other critical building visual inspections and other test methods.

Independent from spore counts, a qualitative analysis of an air sample by an expert technician can provide compelling evidence of a nearby problem mold reservoir in certain cases, for example if the indoor sample contains long chains of Penicillium or Aspergillus spores.

These spore chains are so fragile that they break apart quickly while traveling through the air. So spore chains found in (a pre-remediation) indoor sample, even if the total count is low, deserve some further thinking and investigation.

Measuring Airborne Mold - Mold in Air: Quantitative Analysis - "Spore counts"

Examining particles on a slide to count Mold spores/Meter3 of air, Mold Spores/M2 of surface area, or Mold CFU's - colony-forming units is a common practice in building investigations. These measures can be used to describe the results of some sampling or "mold testing" methods in building.

While there is no fixed (nor fix-able) standard of absolute allowable "spore counts" in air (we'd need a standard for every genera and species by toxicity, weight, mobility, etc.), investigators have now seen so many moldy and not-moldy buildings, and complaint and non-complaint areas that it's reasonable to have some general guidelines about what levels are probably a problem and what levels may not be a problem.

Acceptable Mold Levels: While there is no well-established quantitative standard for fungal spores on surfaces or in air, mold contamination is considered present in a building when the total mold spore concentration per cubic meter of air is above 10,000. (Baxter, ETS).

Acceptable levels for individual species vary since species toxicity varies widely as does spore size, weight, and other features which affect risk to building occupants.


E.g. Aspergillus/Penicillium in a "clean" residential building study was at a mean of 230, in buildings known to have a moisture or flooding problem it was at 2235 and in mold contaminated buildings the figure was 36,037.

Surface samples of mold: the presence of toxic or allergenic mold as a dominant particle in any sample (surface or air) is usually a cause for further investigation or remediation.

The presence of incidental occurrences of toxic or allergenic material in surface samples requires interpretation in light of other building conditions, type of particle (spore chains), and other factors. For example, in a pre-remediation building where occupants have building related complaints and mold contamination is suspected, even a modest indoor level of 500 spores/M3 of air might suggest a problem if those spores formed a high percentage of the total indoor spore count and only a low percentage of the corresponding outdoor spore count taken as a control.

Our own field experience is similar to the counts suggested by Baxter. Since indoor environments in the U.S. and many other parts of the world are similar in conditions and often in building materials, it is likely that these levels are not very dependent on region of the country.

Warning: interpret all quantitative data with great caution. Individual samples of particles in air show tremendous variation from minute to minute, making "ok" test results a thing to view with care. In situations of particular risk additional or periodic testing should be considered.

However, because indoor conditions of mechanical disturbance, ventilation, occupancy, and use vary widely over short intervals of just a few minutes, an indoor "mold spore count" is at high risk being inaccurate, incoherent, and confounding.

Recipients of such reports are may not realize the distinction between highly precise results (lots of decimal places) and results which are fundamentally accurate or inaccurate, depending on the design of the sampling plan and the variation in building conditions during the sampling interval.

Unless there is good experimental design, quantitative results are precise (lots of decimal places in the answer) but almost certainly wrong (off by a factor of thousands) as a characterization of a building.

Therefore quantitative studies are most useful as part of a more broad qualitative approach, indicating either relative differences in mold levels between a control area and a problem area or the relative change in building conditions before and after cleaning.

They are much less reliable as an absolute pass/fail criteria. Other substantial quantitative report issues other than accuracy include wide variation among labs in counting and skill levels, and more interestingly, the lack of and virtual impossibility of establishment of valid quantitative standards for mold exposure.

For example, two different particles have different toxicity, different air movement; two of the same particle can vary in toxicity depending on supporting growth surface.

Shortcomings of Quantitative Analysis of Mold Spores in Air

  1. Quantitative Analysis producing low results is unreliable: Building conditions vary enormously, causing huge variations in what particles may be found in the air at any given time. Our own time-lapse sampling suggests that particle levels vary by a factor of at least several orders of magnitude due to common variations in building conditions such as fan on off, human activity, mechanical disturbance of moldy stuff, etc.

  2. Quantitative results which are superficially similar between indoors and outdoors may be misleading: An outdoor "Pen/Asp" count of 500 spores/M3 may have captured a genera or species which is completely different from a corresponding indoor "Pen/Asp" count which also found 400 spores/M3 .

  3. Quantitative results which seem "low" and qualify as a "clean building" may be misleading: a lab reporting an outdoor "Pen/Asp" count of 700 and an indoor "Pen/Asp" count of 450 may look like an acceptably "clean" building - the indoor count is "below the outdoor count."

    But suppose in fact the outdoor "Pen/Asp" is actually an unidentified amerospore, or a basidiomycete mis-classified, or is a common outdoor species of Penicillium, while the indoor "Pen./Asp" is actually representing Aspergillus niger?
  4. This is an argument for doing some subsequent culture work or better lab microscopy (Aspergillus niger can be identified by light microscope alone.) In this case the "low" level of 450 spores may in fact be a weak indicator of a large problem mold reservoir in the building.


Continue reading at MOLD TEST PROCEDURES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see INDOOR AIR QUALITY METHODS COMPARED for a more comprehensive collection information about mold test methods

Or see these

Mold Test Accuracy Articles

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