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Tests for mold in HVAC air ducts: this article explains how to assess the level of mold contamination in heating or cooling air ducts, and the aggressiveness of mold testing (do we agitate the ducts) that can form sources of error when testing HVAC systems for mold contamination.
How should I test my heating or cooling duct system for mold contamination?, Levels of Mold in Heating or Cooling Ducts as Sources of Error in Indoor Mold Tests - what causes variation when testing ducts for mold contamination?
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?
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 Test Air Ducts for Mold Contamination
Question: what is the best way to test for mold in the heating/AC system
Thanks for your website - it is truly a public service.
My husband and I are both very ill - my husband has recently been diagnosed with metastasizing melanoma in his eye, which has spread to his liver and spine - his prognosis is not good.
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I have been sick with sinusitis for over a year now and am known to have severe allergies to mold (causing recurrent bronchitis and sinusitis).
After many scans and tests, my doctors feel the most likely reason not responding to treatment (many courses of antibiotics and sinus irrigations) is because of an environmental allergy.
Because of my husband's illness, we decided we had to move into a more maintenance-free living situation and bought a townhouse a year ago.
Because of my known allergy, and because of a small area of what looked like black mold (and smelled unbelievably vile) discovered during the renovation, we had two different mold remediation companies as well as an air quality testing company come in.
They felt the black mold was removed completely (under negative pressure and other precautions) and then they did a fair amount of preventive work - since there were a few tiny areas of green mold in the basement and attic.
Air quality testing afterwards supposedly showed no mold anywhere.
We recently discovered our front-loading washing machine is one of the brands known to have major mold problems and there are now several class action suits against them.
We had several puzzling episodes of moldy sheets, which weren't recognized until our son came home to visit (my sense of smell has been wiped out by the sinusitis and my husband never had a good sense of smell).
We couldn't figure out how this was happening since we are very careful not to leave wet clothes/linens lying around or in the washer.
According to our son, the mold smell permeated the master bedroom. We then moved to the other bedroom and the same thing happened again, discovered when our son was visiting again, and again the smell permeated the room.
The washer is in the master bath.
I first got sick in our old house (which was a house that, up until then, had been a very healthy environment for me) after we bought the washing machine.
We had a couple of episodes of moldy-smells in our wash in the old house, but I was still able to smell and threw things out right away, but was puzzled as to why they were occurring.
We have replaced the washer, aired out the rooms, bought air filters, have an HEPA filter on the return heating duct, but I am not feeling better (after having rewashed all our clothes and bedding.
We're concerned that mold could be in the heating/AC duct system.
We've thrown out all our linens twice now (after our son discovered the mold) but before we learned about the washer.
We're also wondering if we need to throw out everything again.
My basic question is what is the best way to test for mold in the heating/AC system and is there a company/person you could recommend to do this work?
Thanks for your time and any advice you can give us. - K.J., New Jersey
Reply: A combination of Steps can Help Assess Mold Contamination in an HVAC System
What makes sense to me is to take the following mold inspection and testing steps if you have not already done so:
A visual inspection of the air handler,
return and supply plenums, visible areas of ductwork for evidence of leaks, dirt and debris, visible mold, insect debris, etc.
I worry that it's too easy to "decide" that we have a mold problem in one specific place without an adequately thorough inspection and taking of the case history.
The result is the unnecessary expense of having to keep bringing back cleaning or testing people to the property. If we can do so, it's less costly to do it right the first time.
A tape sample test of a clean newly installed air filter
after it has been in place for 2 weeks to a month in your HVAC system - a procedure is
If you cannot afford mold sample processing, CONTACT us and we'll try to help.) This presumes you have a centralized air filter in your duct system or air handler.
A tape sample of debris accumulated on the central air return register grille
and on a dusty supply register grille sample too.
A tape sample of suspected mold growth on the HVAC duct interior
If visible mold is observed inside the ductwork it may be useful to collect a tape sample of that as well, but beware that a modest amount of Cladosporium sp. (several species) is very common where condensate blows around in the duct system, and may be visible, but may by no means be the significant problem in the building.
see MOLD ATLAS & PARTICLS LIST for a description of the health effects and air quality complaints associated with various kinds of mold.
Don't let the discovery of a very common mold throw you off the track of a more careful and more broad inspection.
Make sure you are finding and focusing on the "right" mold problem.
Attention to "toxic black mold" may have been warranted, and media attention often focuses on "black mold", but be warned that there are plenty of harmful molds of other colors, or even "no color", some of which are smaller and more easily airborne (e.g. Aspergillus sp.) than some of the popular "toxic black molds" (e.g. Stachybotrys chartarum).
A tape sample of settled dust from a room
where you spend the most time
Air testing for high levels of mold
or other troublesome airborne particles is useful as part of a more broad investigation that includes the items above, but used alone it is very unreliable, in that a negative result could be just wrong, and even a "positive" result (mold was found at high levels) is not diagnostic - it doesn't tell us where to look nor where the problem - if one is detected - is located.
Dispose of moldy HVAC ducts that cannot be cleaned
While you shouldn't have to throw away smelly sheets or bed linens nor similar items that can be laundered or dry-cleaned any more than you need to throw away a ceramic dish that can be washed, significantly-contamiated fiberglass-lined HVAC ducts should probably be replaced.
Watch out: attempts to use mechanical cleaning of fiberglass-constructed or fiberglass-lined HVAC ducts is very likely to destroy them, exposing building occupants to still higher level of both the original contaminants in the duct system and also fiberglass particles.
Similarly, thin plastic-lined flex-duct may be destroyed by mechanical duct cleaning methods.
There are also processes using a spray-sealant on HVAC duct interiors as an effort to seal and salvage contaminated fiberglass-constructed or fiberglass-lined ductwork.
That process may be suitable for some situations but I would be worried about leaving a large mold reservoir inside of a duct sytstem that cannot be easily and frequently inspected after such treatment.
Metal HVAC ducts can usually be cleaned successfull.
We have sometimes found that thick absorbent furnishings such as carpets, upholstered couches and chairs, and mattresses that suffer prolonged exposure to a moldy environment may be smelly from MVOCs even if there was no observable actual mold spore or mold growth contamination on those items.
I'd like to know more about the clothes washer you had trouble with and what authoritative references you have on that matter.
At MOLD & ENVIRONMENTAL INSPECTORS we list some inspectors/testers specializing in mold and indoor contaminants. But there may be other well qualified people in your area. Talk with anyone you are considering hiring.
Don't tell them the "right answer" (besides they are likely to have other good ideas). But name your concerns and ask how they will approach the job - stay away from superficial experts who dash in, collect a mold sample, charge you, and leave.
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Variation in Airborne Particle Levels in Heating and Air Conditioning Ducts
How much variation in airborne mold or dust level do we see inside heating and air conditioning ducts and air handlers?
The airborne particle sample trace photograph above, prepared by the author, shows a one liter airborne particle trace collected inside of a heating furnace return air
plenum using a Burkard Personal Air Sampler.
Our second airborne particle level photograph (below) shows a second particle trace collected in the same
location, with the same volume of air, with one difference: we tapped lightly on the side of the air plenum during the
Even before counting the number airborne particles of any type per liter of air it is obvious that
even modestly aggressive sampling (rapping on the plenum to stir up local dust) can make a large difference in the
level of particles seen in the sample result.
We conducted this test during a post mold remediation clearance inspection of a previously mold-contaminated air conditioning
The remediation contractor vehemently disagreed with the procedure of tapping on the ductwork during testing, informing us
that "... his hygienist never did such a thing".
We agree that consistency in test methods is important in order to be able
to compare one mold test with another.
However if we're looking for the presence or absence of a significant mold or allergenic
dust reservoir that should have been removed, a little aggressiveness in sampling can be useful and in fact more accurate as well.
In sum, do not rely on the accuracy of airborne particle counts alone: Since air samples do not capture a representative picture of the indoor environment an indoor air quality investigator should not rely simply on conventional airborne particle quantitative analysis (particle
counts per cubic meter).
Readers concerned with mold contamination in heating and air conditioning air handlers and ductwork should
for a more in-depth critique of popular mold testing methods than this
see MOLD TESTING METHOD VALIDITY or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Mold on Fiberglass HVAC Duct Interior Surfaces: Tape Sampling
While it is normal to find incidental levels of mold spores, pollen grains, insect fragments, mite fecals, and of course higher levels of fabric fibers and skin cells on the interior of HVAC ductwork, where the duct interior has been wet or has been exposed to high levels of airborne building mold, we may find actual mold growth on the duct interior surface.
Here we show a normally-soiled fiberglass lined HVAC duct, except for those white areas that made the home owner raise a question of the presence of mold contamination.
We were not sure if these white spots in the ductwork were mold or something else until we examined tape samples of the duct surface in our InspectApedia.com lab.
Take a closer look at this photo and you'll see clear adhesive tape (outlined in yellow) applied to the duct interior surface to collect a sample of this white stuff for lab analysis.
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I took care to place the tape over what I thought was a representative area of the duct liner where it would collect both the dark debris and the white particles that I suspected might be mold.
You can see that had the tape been placed in the area shown in the left of the photo the sample would not have collected these white particles and it might have missed identifying them.
Our study found Cladosporium sp., Aspergillus sp. and Penicillium sp. in heavy growth in the ductwork.
Details of this air duct mold contamination study based on clear adhesive tape samples of the mold-suspect surface of the air duct are found
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
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
"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.
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
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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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