Photograph of Allergenco Mark III Impaction Air SamplerTest for 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.

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

Photograph of wet moldy air conditioner fiberglass insulation

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.

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

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

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

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 sampling process.

Photograph of air trace HVAC aggressive

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


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


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


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 closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Mold on Fiberglass HVAC Duct Interior Surfaces: Tape Sampling

White deposits on fiberglass HVAC Duct interior (C) JCWhile 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 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



Continue reading at PARTICLE SIZES & IAQ or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see PARTICLE & MOLD LEVELS in DUCTWORK FAQs - questions and answers posted originally on this page

Or see BLOWER LEAKS, RUST & MOLD explains causes of leaks & mold in HVAC ducts

Or see DIRTY A/C BLOWERS includes leaks, rodents, & mold in air handler units




See TAPE & BULK SAMPLING & TESTS for MOLD for details of how to use adhesive tape for surface sampling of dust, particles, or suspected mold growth on building surfaces.




Or see WHY DOES MOLD GROW in INSULATION? that includes photos and test results examining suspected mold on the surface of fiberglass-lined HVAC ductwork.

Suggested citation for this web page

PARTICLE & MOLD LEVELS in DUCTWORK at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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