Photo of water stains, rust, and possible insulation in air conditioner air handler fiberglass insulation (C) Daniel Fr4iedmanClean Mold-contaminated Fiberglass HVAC Ducts ?
Cleaning Advice for Moldy HVAC Ducts & Fiberglass Duct Insulation

  • DUCT CLEANING ADVICE - CONTENTS: can moldy air ducts be cleaned? Yes and no - it depends on the type of ductwork. Should HVAC ducts contaminated with asbestos, mold, or other contaminants be cleaned at all? Sometimes. HVAC duct cleaning advice.
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Advice for cleaning contaminated HVAC ducts:

This article gives advice on the cleaning of mold-contaminated air ducts in buildings. Some types of ductwork can and should be cleaned while other types of HVAC duct cannot be effectively cleaned and are at risk of being destroyed by aggressive cleaning efforts. If those ducts are badly contaminated they should be replaced.

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. 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.

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Advice for Suspected or Known Mold-Contamination in HVAC Ductwork (Air ducts, heating ducts, air conditioning ducts)

White deposits on fiberglass HVAC Duct interior (C) JCHere is a photograph of white or light gray mold growth on the interior surface of a fiberglass-lined air duct.

[Click to enlarge any image]

While more investigation was required, we speculated that the Atlanta Georgia home where this duct mold was found had either suffered water or high moisture in the ductwork or there had been another source of high levels of Aspergillus, Penicillium, and perhaps other molds in the building's indoor air.

High exposure levels, or repeated exposure to high levels of this type of airborne mold can cause serious illness such as Aspergillosis.
See ASPERGILLOSIS for details.

Details of this air duct mold contamination study are found at FIBERGLASS AIR DUCT MOLD TEST.

Should You Clean Flooded, Contaminated, or Moldy Air Ducts?

If visual inspection, possibly supported by testing (not usually required) confirms that the interior of an air conditioning or heating duct system is badly soiled or contaminated with high levels or large areas of asbestos, mold, rodent feces or urine, insect fragments such as cockroach parts, or other allergens or pathogens then the movement of building air through the duct system is likely to distribute those particles into air breathed by building occupants.

The actual hazard level of such ductwork can be difficult to determine. Air testing in the building or in the ductwork might confirm high levels of hazardous particles but if it doesn't that is not an assurance that the ductwork is safe, since the level of airborne particles varies enormously depending on when and how a test is performed and on variations in building conditions.

If HVAC ducts have been flooded (photo below) or exposed to sewage leaks they should be cleaned and sanitized, or replaced completely if the ductwork is a type that cannot be cleaned. at WATER & ICE IN DUCT WORK you will find more advice for cleaning ductwork subjected to flooding

Only Metal Ductwork May be Able to Be Cleaned

Flooded HVAC ducts (C) Daniel Friedman

Metal HVAC ducts like the ductwork shown above, whose insulation is on the duct exterior and that present a simple metal surface on the duct interior can usually be successfully cleaned by a duct cleaning professional.

Fiberglass Lined HVAC Ducts & Flex-Duct Cannot be Safely Cleaned by Mechanical Means

Photograph of damaged duct fiberglass lining Fiberglass-lined HVAC ducts badly damaged by mechanical duct cleaning (C)

Watch out: Fiberglass lined ductwork may be seriously damaged by mechanical cleaning, increasing the subsequent release of irritating airborne fiberglass particles into building air and actually reducing the resistance of such ductwork to future debris and moisture and even mold accumulation. Both of the photos above show mechanically-damaged fiberglass on the inside of air ducts. More about mechanically-damaged HVAC air ducts is at DUCT DAMAGE, MECHANICAL.

Other badly contaminated ductwork such as flex-duct, moldy fiberglass-lined or fiberglass-panel ductwork should be replaced. And in any case you should address the cause of mold growth and correct that as well or the problem will simply repeat itself.

HVAC Duct Cleaning Advice from the US EPA - supplemented by additional expert opinion

If you think the heating or air conditioning (HVAC) system may be contaminated with mold, read the EPA's guide SHOULD YOU HAVE the AIR DUCTS in YOUR HOME CLEANED? [PDF] before taking further action. Retrieved 2016/11/12, original source:, or call the US EPA at (800) 438- 4318 for a free copy.

Turn off Air Conditioning or Heating Systems That are Mold-Contaminated

The U.S. Environmental Protection Agency recommends that you do not run contaminated air conditioning or heating systems (that use forced air and air ducts).

Watch out: in addition to the EPA advice given here, you should not run HVAC systems in buildings known to be contaminated with anything that is likely to be airborne, as doing so virtually assures that the interior of the HVAC system is going to become also contaminated if it wasn't already in that condition.

Here is the US EPA advice - excerpted:

Do not run the HVAC system if you know or suspect that it is contaminated with mold - it could spread mold throughout the building”. [1]

Buildings in areas where high humidity and high use of air conditioning such as Florida and Louisiana in the U.S. experience very high levels of condensate produced by the air conditioning system, so high that condensate sometimes blows into the ductwork itself rather than all draining successfully into the condensate drain system. According to the Florida Department of Health:

Unfortunately, it is thought that most, if not all, heating and air conditioning systems in Florida will support mold growth at some point. Stopping the use of an air conditioning system due to suspected mold growth would make most Florida buildings very uncomfortable during hot and humid weather.

Should you turn off an air conditioner if a mold problem in the system is found? Ideally, yes. The system should be shut down while cleaning or mold removal is performed. If the water and/or mold damage was caused by sewage or other contaminated water, then call a professional who has experience cleaning and fixing buildings damaged by contaminated water. [2]

Watch out: the U.S. EPA and other sources recommend: [among other details found in the articles listed in our REFERENCES] that

Have your air ducts cleaned if they are visibly contaminated with substantial mold growth, pests or vermin, or are clogged with substantial deposits of dust or debris.

EPA Advice on Using Biocides, Sanitizers & Ozone to Treat Contaminated HVAC Ducts

Worker spraying  a biocide on mold-contaminated carpeting (C) Daniel FriedmanWatch out: we do not recommend relying on ozone treatments to "clean" or "disinfect" HVAC ducts, and there is question about the effectiveness of both ozone and sanitizers or chemical biocides used in ductwork. Here are excerpts from the EPA document cited above:

Air duct cleaning service providers may tell you that they need to apply a chemical biocide to the inside of your ducts to kill bacteria (germs), and fungi (mold) and prevent future biological growth. Some duct cleaning service providers may propose to introduce ozone to kill biological contaminants.

Ozone is a highly reactive gas that is regulated in the outside air as a lung irritant. However, there remains considerable controversy over the necessity and wisdom of introducing chemical biocides or ozone into the duct work.

Among the possible problems with biocide and ozone application in air ducts:

Little research has been conducted to demonstrate the effectiveness of most biocides and ozone when used inside ducts. Simply spraying or otherwise introducing these materials into the operating duct system may cause much of the material to be transported through the system and released into other areas of your home.

Some people may react negatively to the biocide or ozone, causing adverse health reactions. 


See OZONE MOLD / ODOR TREATMENT WARNINGS for a summary of the problems that ensue, particularly from over-treatment using ozone in buildings.


Continue reading at PARTICLE & MOLD LEVELS in DUCTWORK 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.

Or see WATER & ICE IN DUCT WORK - advice for cleaning ductwork subjected to flooding



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