How to Compare Indoor to Outdoor Mold Counts InspectAPedia® -
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?
Why outdoor mold levels may have little or nothing to do with indoor mold counts
Why comparing outdoor mold counts to indoor mold levels may be misleading
Accuracy of mold level comparisons
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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: Are
spore counts valid? These critical questions are discussed in this paper.
Readers should also see MOLD LEVEL IN AIR, VALIDITY, and
for a more in-depth critique of popular mold testing methods than this
tutorial see MOLD TESTING METHOD VALIDITY
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 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.
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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
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
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
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)
Atlas of Indoor Mold, Online Clinical Mold Atlas, Toxins, Pathogens, Allergens and Other Indoor Particles - Medical Health Effects of Mold (separate online document)
Black Mold that is Harmless Photos of recognizable, usually harmless black mold on wood, bluestain, ceratocystis, ophistoma
Building Floods: quick steps after a building flood or plumbing leak can prevent costly mold contamination
Classes of Mold: what types of cosmetic, allergenic, or toxic mold are a problem? Can mold be cleaned-up successfully?
"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 Program for Certain Vegetable Crops," David B. Langston, Jr., Extension Plant Pathologist - Vegetables, University of Georgia (PDF document) original source: www.reeis.usda.gov/web/crisprojectpages/209797.html
"Disease Prevention in Home Vegetable Gardens,"
Patricia Donald,
Department of Plant Microbiology and Pathology,
Lewis Jett
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.
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.
Ozone Warnings - Use of Ozone as a "mold"
remedy is ineffective and may be dangerous.
Pet control - if you can't say goodbye to your bird, cat, dog, guinea pig, hamster, tropical fish, then limit the
areas they occupy and limit the airflow from that area to sleeping or other areas of the building, use allergenic
bedding, eliminate wall-to-wall carpeting, improve housecleaning including use of a HEPA-rated vacuum cleaner. For more details
see our article Dog, Cat, and Other Animal Dander - Information for Asthmatics and Indoor Air Quality
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.
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