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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:
spore counts valid? These critical questions are discussed in this paper.
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.
How to Compare 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.
We 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 18.104.22.168 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.
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.
You might as well say we found 100 oranges outdoors and 100 apples indoors. What the heck does that mean?
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.
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(Mar 5, 2014) Jane said:
On a single family home, why would air-o-cell lab results show numbers for the interior, but show zero count for the exterior for the following spores.. aureobasidium, pithomyces, pestalotia, tetraploa and spegazzinia?
First, it is entirely possible that particular species of mold may grow and flourish indoors at a time when outdoors the airborne concentrations of those same species are so low as to be not detected in a brief air sample, even though as Dr. John Haines says, "all mold is everywhere all the time".
Second, some of the indoor mold genera you name may be at such a low concentration that we really would not worry about them. A few Tetraploa spores that happened to find their way indoors and get stuck there don't present a health risk nor do they indicate a flooded, mold-damaged building.
A "mold test" that only produces some "counts" and does not inspect the building is not very diagnostic.
In fact even when (some) mold reports give outdoor levels and indoor levels of what looks like the very same mold genera/species, such as Penicillium sp., the truth is that the outdoor and indoor individual species of Penicillium could be completely different, making the comparison rather misleading.
In sum, we don't ever expect to find the same mold levels indoors and outdoors but they may be close if the building's windows have been opened for some time, a good breeze is blowing, and if there is no indoor mold reservoir that changes what we are seeing inside.
While there are about 1.5 million different mold genera/species, and while we like to say that "all molds are everywhere all the time" - which is true in a sense - one must also point out that not all molds are at the same *concentration level* all the time at a given location. So there is no reason to expect indoor mold counts to ever exactly match outdoor counts and vice versa.
A more useful question is "what does my mold test report mean?"
Without some interpretation by an expert who has investigated the building and who understands building science, moisture movement, the building leak history, occupant complaints, and the exact protocols under which an air test or other mold test was conducted, and especially considering that you don't give any actual spore level or count data, the answer is, the information doesn't mean a darn thing. It is close to useless.
(Apr 22, 2015) Joe said:
My lab results reflect the following:
Penicillium/Aspergillus inside aire sample near return as. raw ct. 16 & per m3 850.
While the outside air @ front door to be Raw ct. 7 & per m3 370.
Would I be correct in determining the mold count was not very high to begin with?
Yes, Joe but of course we cannot conclude much about the actual mold risk in the building from an air test alone.
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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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Kansas State University, department of plant pathology, extension plant pathology web page on wheat rust fungus: see http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Leaf%20Rust.asp
"A Brief Guide to Mold, Moisture, and Your Home",
U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
US EPA - Mold Remediation in Schools and Commercial Building [Copy on file at /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
US EPA - Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol
Allergies, Allergens, Allergy Testing in Buildings - References & Products
Allergen Tests in Buildings advice about how to test, what to look for, in evaluating the level of dog, cat, or other animal allergens in a building
"IgG Food Allergy Testing by ELISA/EIA, What do they really tell us?" Sheryl B. Miller, MT (ASCP), PhD, Clinical Laboratory Director, Bastyr University Natural Health Clinic - ELISA testing accuracy: Here is an example of Miller's critique of ELISA
http://www.betterhealthusa.com/public/282.cfm - Townsend Letter for Doctors and Patients
The critique included in that article raises compelling questions about IgG testing assays, which prompts our interest in actually screening for the presence of high levels of particles that could carry allergens - dog dander or cat dander in the case at hand.
http://www.tldp.com/issue/174/IgG%20Food%20Allergy.html contains similar criticism in another venue but interestingly by the same author, Sheryl Miller. Sheryl Miller, MT (ASCP), PhD, is an Immunologist and Associate Professor of Basic and Medical Sciences at Bastyr University in Bothell, Washington. She is also the Laboratory Director of the Bastyr Natural Health Clinic Laboratory.
Allergens: Testing for the level of exposure to animal allergens is discussed at http://www.animalhealthchannel.com/animalallergy/diagnosis.shtml (lab animal exposure study is interesting because it involves a higher exposure level in some cases
Allergens: WebMD discusses allergy tests for humans at webmd.com/allergies/allergy-tests
Animal Allergens: Dog, Cat, and Other Animal Dander - Cleanup & Prevention Information for Asthmatics and regarding Indoor Air Quality.
Recognizing Allergens: What various indoor allergens look like - identification photos to help identify pollen, dust mites, animal dander, toxic or allergenic mold - Common Mold and other Allergens, Irritants, Remedies & Advice
Rodent control issues, including dander, fecal, and urine contamination of Buildings and Building insulation are discussed at our
Associations: Sick House, Sick Building, SBS - Air Quality, Government, Private Associations and Information Resources
Atlas of Clinical Fungi, 2nd Ed., GS deHoog, J Guarro, J Gene, & MJ Figueras, Centraalbureau voor Schimmelcultures, Universitat Rovira I Virgili, 2000, ISBN 90-70351-43-9 (you can buy this book at Amazon) - The Atlas of Clinical Fungi is also available on CD ROM
"A Brief Guide to Mold, Moisture, and Your Home", U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
"Disease Prevention in Home Vegetable Gardens,"
Department of Plant Microbiology and Pathology,
Department of Horticulture, University of Missouri Extension - extension.missouri.edu/publications/DisplayPub.aspx?P=G6202
Fifth Kingdom, Bryce Kendrick, ISBN13: 9781585100224, is available from the InspectAPedia online bookstore - we recommend the CD-ROM version of this book. This 3rd/edition is a compact but comprehensive encyclopedia of all things mycological. Every aspect of the fungi, from aflatoxin to zppspores, with an accessible blend of verve and wit. The 24 chapters are filled with up-to-date information of classification, yeast, lichens, spore dispersal, allergies, ecology, genetics, plant pathology, predatory fungi, biological control, mutualistic symbioses with animals and plants, fungi as food, food spoilage and mycotoxins.
Ozone Warnings - Use of Ozone as a "mold" remedy is ineffective and may be dangerous.
Rot concerns in buildings-some building mold such as Meruliporia incrassata "Poria" risks serious rot and hidden structural damage
US EPA: Una Breva Guia a Moho - Hongo [Copy on file as /sickhouse/EPA_Moho_Guia_sp.pdf - en Espanol
OTHER IAQ ISSUES: How To Find and Address Other Indoor Air or Indoor Environment Contaminants Besides Mold
Mold or allergens may not be the only or even the main indoor environmental contaminant. Don't let media attention to mold
cause so much enviro-scare fear that other, possibly more urgent hazards go un-addressed.
Rodents, Mice, Squirrel Control - I find high levels of mouse and rodent dander, fecal dust, and urine-contaminated dust in some buildings,
and high levels of these materials in building insulation in those locations. If you have a mouse problem, particularly if mice and their waste (fecals or urine) are contaminating
the building HVAC or building insulation, may need both steps to clean up or remove infected materials and steps to stop an ongoing
rodent problem. If squirrels are a problem, the cleanup needs to include closing off entry openings into the building. Get some
help from a licensed pest control expert.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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