Photograph of Allergenco Mark III Impaction Air SamplerHow to Assess the Level of Mold & Debris Contamination in HVAC Ducts & Air Handlers

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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.

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?

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How to Test Air Ducts for Mold Contamination

Photograph of wet moldy air conditioner fiberglass insulation

Question: After dealing with a moldy washing machine and even a mold remediation we are focusing on our ductwork: 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.

Normal debris collecting in fiberglass lined HVAC ducts (C) Daniel FriedmanBecause 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).

Normal debris collecting in fiberglass lined HVAC ducts (C) Daniel FriedmanWe 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

Photograph of Allergenco Mark III Impaction Air SamplerWhat 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 at TEST KITS for DUST, MOLD, PARTICLE TESTS - you can use any qualified mold test lab (Please don't send us your mold sample. 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.
  • 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. As we note at BLOWER LEAKS, RUST & MOLD:

    While Cladosporium sp. is the most common mold found on earth and while it's just about everywhere, for some people this is an allergenic mold. We don't want it being blown around by our air conditioning system nor its growth improved by mis-handling of HVAC condensate. See MOLD CONTAMINATION LEVELS and Mold Atlas & Particles 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 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. (MOLD TESTING METHOD VALIDITY) .]
  • 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. It should be possible to properly launder sheets and towels to leave them smelling clean and fresh. But if a mattress or bed pad also got smelly from mold volatile organic compounds (MVOCs, that's what you are probably smelling), then laundering the sheets might make them ok at first (until they are placed on the bed) but the smell could still be in the bed due to the mattress or mattress pad. Details are at MOLD GROWTH on SURFACES, TABLE OF

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. [ has no financial nor business relationship with products or services discussed at our website

Variation in Airborne Particle Levels in Heating and Air Conditioning Ducts

Photograph of air trace HVAC passive Photograph of air trace HVAC aggressive

How much variation in airborne mold or dust level do we see inside heating and air conditioning ducts and air handlers?: The left hand photograph shows a one liter airborne particle trace collected inside of a heating furnace return air plenum using a Burkard Personal Air Sampler.

The right hand photograph 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 sampling process.

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 system. 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 see BLOWER LEAKS, RUST & MOLD and PARTICLE & MOLD LEVELS in DUCTWORK where we describe how to test HVAC systems and ductwork for mold.

Mold in HVAC ductwork is also discussed at WHY DOES MOLD GROW in INSULATION?. See SLAB DUCTWORK for the role of in-slab placement of air ducts in the formation of mold contamination in HVAC systems. These critical mold testing accuracy 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 or select a topic from the More Reading links shown below.

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