Mold growth colonies of several genera/species on the interior surface of fiberglass-lined HVAC ducts in a Georgia home (C) Daniel Friedman JCFiberglass HVAC Duct Mold Test Results

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How to inspect & test air ducts for mold contamination:

Visual inspection followed by a simple adhesive tape test confirmed mold growth inside the fiberglass-lined HVAC duct shown here.

This article describes how to test the surface of fiberglass-lined or fiberglass-panel air ducts for the presence of problem mold growth. While some debris and low levels of mold, pollen and other particles is normal inside HVAC systems, large areas of harmful mold genera such as Aspergillus or Penicillium may be hazardous to building occupants.

This article series explains the cause, detection, and hazards of mold growth in fiberglass insulation in residential and light-commercial building and gives advice about dealing with moldy building insulation or ductwork. We describe the types (genera/species) of mold most often found in HVAC ducts and the relationship between mold in ductwork and indoor air quality complaints by building occupants. We include authoritative citations for key research on mold contamination in HVAC ductwork.

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Water Leaks & Mold Contamination in Fiberglass HVAC Duct Interiors

Leak stains may point to water entry into duct work, a cause of duct mold (C) JCHere we discuss and illustrate inspection of air ducts for mold contamination following signs of leaks and suspected mold.

Mold may grow at extensive or problematic levels in some building insulation materials used in walls, floors, ceilings as well as in HVAC air duct systems.

[Click to enlarge any image]

Above: leak stains may point to sources of water entry into a building's HVAC duct system that in turn can trigger problematic or harmful mold growth and contamination of the duct system.

These stains combined with visible mold inside a fiberglass-panel air duct and plenum (below) lead a reader to collect surface samples of suspected mold contamination for analysis in our laboratory.

Below we describe the test samples and the lab results finding large areas of several genera of mold contaminants.

Photo below: white, gray, light green round-like deposits on the interior of fiberglass HVAC ducts look like a substantial area of mold contamination. Mold test tape sample locations were chosen specifically to attempt to capture mold-suspect material in these locations on the ductwork fiberglass liner surface.

Mold contamination visible in HVAC duct interior (C) JC

In the laboratory we found that the white, gray, and light-green fuzzy round spots on the HVAC duct interior in this photograph were indeed several genera/species of mold.

Really? Well yes, it's mold. Just how much of a health hazard is posed by the mold in the photograph is questionable. But the samples and building case history support a couple of warnings:

  1. High airborne levels of the mold found in these tests would be hazardous to building occupants and might occur if there is in fact a larger mold reservoir in the HVAC system or elsewhere in the building.
  2. Wet or moldy conditions that produced the mold found and tested here might indeed also have invited a problem mold reservoir elsewhere in the building.
  3. Further investigation is warranted. Details are below.

Photo below: white, gray, light green round-like deposits on the interior of fiberglass HVAC ducts. The shape of these spots and color suggests they might be mold contamination but other possibilities such as drywall debris were also under consideration. The dark, regularly-spaced gray "spots" are natural deposits of house dust (and possibly contaminants" on the factory-surface of the fiberglass duct liner.

Mold growth colonies of several genera/species on the interior surface of fiberglass-lined HVAC ducts in a Georgia home (C) Daniel Friedman JC

Question: are these white splotches on the interior of my fiberglass lined ductwork mold or drywall dust or something else?

I recently leased an apartment a few miles northwest of Atlanta, GA, and have been trying to track down the source of the combination of odors permeating the apartment. The apartment manager is not being very helpful. As I inspected the ductwork, I came across a substance which looks like it could be mold. Pictures are attached. I thought at the very least, you might be interested in using the pictures on your website, whether on the "white mold" page or the "not mold" page.

If you have the time, I would also appreciate your opinion on whether this warrants calling in a mold expert or if it just looks like dirt or debris to you (perhaps drywall dust?). The zoomed out picture shows the view as I insert the camera about 18 inches into the bedroom register towards the plenum (visible on the left) and the main trunk (extending towards the right). Two other pictures [not displayed here - Ed] show what I suspect to be mold (one on the floor of the main trunk, the other on the wall where the bedroom branch hits the main trunk (just outside the zoomed-out picture).

The apartment is 20-odd years old, though according to maintenance the HVAC system was replaced about 10 years ago. For all I know the ductwork could be original.

The sampling area where these mold photographs and samples were taken is described thus:

- 2016/11/05 Anonymous by private email

Reply: Procedure for detection & identification of Aspergillus sp. & Penicillium sp. as well as significant amounts of Cladosporium sp. as well as low levels of other molds and particles on these fiberglass duct interior surfaces

The white splotches in your photos look very much like small whitish mold colonies. On magnification one can see the characteristic center color and mounding and growth outwards from that point. We see such round-ish mold colonies when a single spore lands on a surface whereon it finds hospitable growing conditions. In HVAC ducts I imagine that the mold growth would be due to a combination of organic debris in dust stuck to the fiberglass surface (house dust contains skin cells and other organics) and moisture.

Regrettably I'm not optimistic about disinfection nor cleaning of mold contaminated fiberglass hvac ducts. If the total area were just a few inches I'd probably wipe gently or spray with a household disinfectant/cleaner and blot gently and then concentrate on addressing the controllable causes: filtration of the return air and control of moisture.

For large areas - many square feet or more, and for homes where occupants are at extra risk: elderly, immune compromise, asthmatic, infant, etc., I'd probably replace the ductwork.

While fiberglass is an excellent insulating and noise control material, because of these cleaning difficulties I prefer metal ductwork with insulation on its exterior. IF conditions are bad enough that you decide to replace some duct sections I'd consider that change.

At TEST KITS for DUST, MOLD, PARTICLE TESTS you can see how to use clear adhesive tape and a freezer-type ziplok bag to collect suspected mold samples.

When this test procedure is being used to screen for mold, the location selected for testing is very important: it should be representative of the largest or most-likely dominant mold or suspect-particle reservoir, chosen by location, size, and other visual properties.

Other help for detecting & testing mold contamination in HVAC ductwork

Also see TAPE & BULK SAMPLING & TESTS for MOLD for alternative details on how to use tape to sample settled dust or suspected mold growth.

Also see MOLD APPEARANCE - WHAT MOLD LOOKS LIKE for help in recognizing what mold looks like in buildings.

And for more photos of what white or light colored mold looks like on building surfaces see WHITE MOLD PHOTOS.

Tests for airborne mold particles inside air handlers, blower compartments or inside HVAC ducts are described separately at PARTICLE & MOLD LEVELS in DUCTWORK.

Also see MOLD APPEARANCE on VARIOUS SURFACES for a photo guide to what mold looks like growing on more than 100 different building surfaces, including this photo Mold on/in Air Ducts

Also see MOLD APPEARANCE - STUFF THAT IS NOT MOLD for examples of stuff sometimes mistaken for mold.

Duct Surface Test Lab Analysis Results: observations of tape samples of suspected mold inside fiberglass HVAC ductwork

White deposits on fiberglass HVAC Duct interior (C) JC

[Click to enlarge any image] Above: sample 1 mold test location on the floor of the HVAC duct. Click to enlarge to see the position of the adhesive tape used to collect this mold test sample.

Fiberglass-lined HVAC Duct Sample #1 sample information

Fiberglass-lined HVAC Duct Mold Test Sample Results

Fiberglass duct  surface mold test results (C) Daniel Friedman Fiberglass duct  surface mold test results (C) Daniel Friedman

Above first photo: sample 1, Cladosporium sp. fungal spores and hyphal fragments. These conditions confirm active fungal growth. This is not simply an air deposit of mold spores.

Above second photo, sample 1, hyaline (colorless) spores and spore chains, generically "Amerospores" but more likely Pen/Asp spores in this sample. Other unidentified mold spores. Hyphal fragments. Typical indoor dust debris. Pen/Asp spores in chains confirm nearby fungal growth of one or both of these mold genera.

Fiberglass-lined HVAC Duct Sample #2 sample information

White deposits on fiberglass HVAC Duct interior (C) JC

Above: sample 2 mold test location on the upper surface of the interior of the HVAC duct.

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Fiberglass-lined HVAC Duct Mold Test Sample Result Conclusions

In the two mold samples on adhesive tape we found evidence of the following molds:

  1. Penicillium/Aspergillus spores in long spore chains consistent with active nearby fungal growth.
  2. Aspergillus sp. conidiophores and mold spores
  3. Cladosporium sp. - very common in building air handlers and ductwork, commonly allergenic. Occasionally pathogenic.
  4. Penicillium sp. conidiophores and mold spores
  5. Trichoderma-like fungal spore clusters.
  6. Wallemia-like fungal spore chains.

Lab photos of these duct mold test findings and observations are provided in this article.

Warnings: When Aspergillus or Penicillium spores are present in chains in indoor mold samples we can be confident that there is active fungal growth of these mold genera. When the conidiophores (mold producing growth structures) are present it is possible to separate the identification these two genera.

In these samples we found both types of conidiophores. At high levels in building air either of these two mold genera are potentially serious health hazards. Without a more extensive inspection of the building and its HVAC systems and ductwork to determine the total extent of mold growth by area and type, we cannot conclude the level of actual health hazard to building occupants. I recommend that further inspection of the building and its HVAC system be performed.

Fiberglass duct  surface mold test results (C) Daniel Friedman Fiberglass duct  surface mold test results (C) Daniel Friedman

[Click to enlarge any image]

Above first photo, sample 1, long hyaline spore chains, Wallemia serbi-like but possibly desiccated Pen/Asp spores. Other typical indoor air debris.

Above second photo sample 1, dense profusion of Cladosporium sp. fungal spores and some hyphal fragments.

Fiberglass duct  surface mold test results (C) Daniel Friedman Fiberglass duct  surface mold test results (C) Daniel Friedman

Above first photo, sample 1, typical fiberglass fibers with dark debris, normal in indoor fiberglass duct surfaces.

Above second photo sample 1, Cladosporium sp. fungal spores and some hyphal fragments.

Other photographs illustrating the appearance of white or light colored mold in buildings are at WHITE MOLD PHOTOS

Fiberglass duct  surface mold test results (C) Daniel Friedman Fiberglass duct  surface mold test results (C) Daniel Friedman

Above first photo, sample 1, Aspergillus-like conidiophore and hyphae at upper left, Penicillium-like conidiophore at lower right.

Above second photo sample 1, Penicillium sp. conidiophore.

Fiberglass duct  surface mold test results (C) Daniel Friedman Fiberglass duct  surface mold test results (C) Daniel Friedman

Above first photo, sample 2, Aspergillus sp. conidiophore (photo center) hyphae (photo upper left).

Above second photo sample 2, Cladosporium sp. in dense fungal growth on the surface.

What to do about mold contamination in ductwork

Watch out: The reader's photos showing the duct interior looks very moldy over more than just a trivial area; it's likely that this is itself a significant mold reservoir and of course there may be hidden mold on the cavity side of drywall that got wet from the leaks you show.

Large reservoirs of Aspergillus sp., possibly Penicillium sp. and possibly Cladosporium sp. (depending on the species in this case) are likely to be health hazards to building occupants, can even healthy non-allergic people to become sensitized to mold, and for people who are asthmatic, allergic, immune impaired, elderly, infant, or otherwise at risk for respiratory illness, such mold contamiantion can be a serious hazard.

The type of ductwork in these photos, fiberglass-panel constructed ducts (as well as any fiberglass lined ducts) cannot be effectively cleaned by conventional duct cleaning methods - most likely the ductwork needs to be replaced. But before risking contaminating new ductwork with airborne moldy dust, the building merits further investigation.

Please see DUCT CLEANING ADVICE for guidance on cleaning mold-contaminated HVAC ductwork.


Continue reading at WHY DOES MOLD GROW in INSULATION? or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BLOWER LEAKS, RUST & MOLD for Readers concerned with mold contamination in heating and air conditioning air handlers and ductwork




Or see PARTICLE & MOLD LEVELS in DUCTWORK where we describe how to test HVAC systems and ductwork for mold.





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FIBERGLASS AIR DUCT MOLD TEST at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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