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ENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY
AIR CLEANER PURIFIER TYPES
AIR FILTER EFFICIENCY
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER / BLOWER UNITS
AIR LEAK DETECTION TOOLS
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR QUALITY IMPROVEMENT STRATEGIES
AIR SEALING STRATEGIES
ALLERGEN TESTS for BUILDINGS
ALLERGY TESTS for PEOPLE
ANIMAL ALLERGENS / PET DANDER
ANIMAL or URINE ODOR SOURCE DETECTION
ASBESTOS-FREE INSULATION MATERIALS
ASBESTOS IDENTIFICATION IN BUILDINGS
ATTORNEYS and EXPERT WITNESSES
BIBLIOGAPHY ENVIRONMENTAL HEALTH, MOLD, IAQ
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - ENVIRONMENTAL
CAR MOLD CONTAMINATION
CARPET DUST IDENTIFICATION
CARPET MOLD CONTAMINATION
CARPETING & INDOOR AIR QUALITY
CAT DANDER in BUILDINGS
CHIMNEY INSPECTION DIAGNOSIS REPAIR
COMBUSTION GASES & PARTICLE HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
CPSC Indoor Air Pollution Book Online Copy
DUCT SYSTEM & DUCT DEFECTS
EMF RF FIELD & FREQUENCY DEFINITIONS
EMERGENCY RESPONSE, IAQ, GAS, MOLD
EMF ELECTROMAGNETIC FIELDS & HUMAN EXPOSURE
FIBERBOARD INSULATION SHEATHING MOLD
FIBERGLASS DUCT, RIGID CONSTRUCTION
FIBERGLASS INSULATION IDENTIFICATION
FIREPLACES & WOODSTOVES CONTAMINANTS
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FLOODS in BUILDINGS, MOLD PREVENTION
FLOORING MATERIALS, Age, Types
FORMALDEHYDE GAS HAZARD REDUCTION
GAS DETECTION INSTRUMENTS
GAS EXPOSURE EFFECTS, TOXIC
HOME HEATING SAFETY
HUMIDITY CONTROL & TARGETS INDOORS
HOUSE DUST ANALYSIS
HOUSE DUST COMPONENTS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
ASBESTOS INSULATION on PIPES
INSULATION AIR & HEAT LEAKS
INSULATION FACT SHEET- DOE
INSULATION IDENTIFICATION GUIDE
INSULATION INSPECTION & IMPROVEMENT
INSULATION MOLD TEST
INSULATION MOLD RESISTANCE of FOAM
INSULATION, UFFI UREA FORMALDEHYDE FOAM
LIGHT, GUIDE to FORENSIC USE
LEED Building Designation & IAQ
MILDEW REMOVAL & PREVENTION
MOISTURE CONTROL in BUILDINGS
MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD
MOLD APPEARANCE - WHAT MOLD LOOKS LIKE
MOLD APPEARANCE - STUFF THAT IS NOT MOLD
MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD
MOLD or INDOOR AIR EMERGENCY RESPONSE
MOLD EXPERT, WHEN TO HIRE
MOLD GROWTH on SURFACES, TABLE OF
MOLD GROWTH in/on BUILDING INSULATION
MOLD TESTING METHOD VALIDITY
MOLD TESTING SERVICES
MSDS MATERIAL SAFETY DATA SHEETS
MVOCs & MOLDY MUSTY ODORS
MYCOPHOBIA, STAINS MISTAKEN for MOLD
MYCOTOXIN EFFECTS of MOLD EXPOSURE
Museum Artifact Preservation
NOISE / SOUND DIAGNOSIS & CURE
ODORS GASES SMELLS, DIAGNOSIS & CURE
OIL TANKS INSPECT LEAK TEST ABANDON REGS
OZONE MOLD / ODOR TREATMENT WARNINGS
PARTICLE SIZES & IAQ
Particulates & Allergens Indoors
RADON HAZARD TESTS & MITIGATION
SAFETY HAZARDS GUIDE
SICK HOUSE IAQ QUESTIONNAIRE
SMELL PATCH TEST to Track Down Odors
SOUND CONTROL in BUILDINGS
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
THERMAL TRACKING & HEAT LOSS
UFFI UREA FORMALDEHYDE FOAM INSULATION
URETHANE FOAM Deterioration, Outgassing
VAPOR BARRIERS & AIR SEALING at BAND JOISTS
VAPOR BARRIERS & CONDENSATION in BUILDINGS
VAPOR BARRIERS & HOUSEWRAP
VAPOR CONDENSATION & BUILDING SHEATHING
VENTILATION in BUILDINGS
VINYL SIDING or WINDOW PLASTIC ODORS
VOCs VOLATILE ORGANIC COMPOUNDS
WATER ODORS, CAUSE CURE
WATER TESTS, CONTAMINANTS, TREATMENT
WORLD TRADE CENTER 9-11 DUST PHOTOS
Causes of mold growth in HVAC ducts: This article expalins 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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Mold growth & Mold Contamination are Common in HVAC Systems
This website discusses health hazards associated with moldy fiberglass in buildings, with focus on fiberglass insulation, fiberglass fragments, fiberglass in heating and air conditioning duct work, and invisible but toxic mold growth in fiberglass which has been wet, exposed to high humidity, or exposed to other moldy conditions.
[Click to enlarge any image]
Mold is often found in basement fiberglass insulation, crawl space fiberglass insulation, fiberglass wall insulation, heating or cooling duct fiberglass insulation, and attic or roof insulation in buildings which have either been wet or have been exposed to high levels of mold from other sources.
It is common for a careful inspection of air handlers and HVAC ductwork to find mold contamination on the duct interior as well as in the air handler on the blower assembly squirrel cage fan blades and other components.
In our experience as both field and lab investigators, several genera/species of mold are quite common in these environments (as well as in fiberglass and possibly some other building insulation products).
Our photo of a moldy air handler interior in a Florida home (left) is provided by home inspector & educator Mark Cramer.
Where conditions support mold growth within the HVAC system air handler and ductwork, we also may find significant mold colonization of the surfaces of air supply registers or the ceilings around them. (Next photo, below).
HVAC Studies for Mold Contamation - Methodological Error Risks
Tests for mold in HVAC systems are vulnerable to methodology errors, particularly in the selection of sampling sites.
Variations in moisture and uneven distribution of organic debris and dust through the system are very important effects on what mold is found where.
For example, sampling immediately downstream from the blower assembly we are more likely to find more water-tolerant fungi (Cladosporium spp.) associated with condensate blow-off spun into that area by the blower fan.
Sampling further into the duct system may discover Aspergillus spp. that prefers somewhat drier surfaces. Ducts exposed to special conditions (flooding, greases or organics from cooking, etc) may support additional or different-dominant mold genera/species.
Fungal growth on fiberglass appears to be supported by the normal organic debris (skin cells that dominate house and other building dust), combined with moisture or even condensation and water found in the HVAC system.
But we also find mold contamination even in some clean-looking fiberglass building insulation, possibly supported by a combination of moisture and some organic resins or binders, or at times, simply having been absorbed by insulation that is installed in a very moldy environment whose mold is from another source reservoir. Details are at INSPECTION of INSULATION for MOLD.
And see SLAB DUCTWORK for the role of in-slab placement of air ducts in the formation of mold contamination in HVAC systems.
Common Mold Genera/Species Found in HVAC Systems & Ductwork & Building IAQ Complaints
Experts studying both mold contamination in HVAC ductwork and related building indoor air complaints have confirmed our own lab experience that identifies Cladosporium spp. or C. herbarum, (most common), Aspergillus versicolor (common) A. flavus (common), and A. fumigatis. 
Those studies also point out that even when apparently modest levels of mold contamination traced to HVAC systems are removed (usually by removing the contaminated or "mold colonized" duct insulation or if that isn't possible, by replacing the ductwork) building IAQ complaints decline significantly.
Dirty HVAC Ducts That Cannot be Cleaned
The left photo shows how fragile is the fiberglass insulation in some HVAC ducts. The rough surface attracts and collects organic and other particulate debris moving through the duct system (unless good filtration is installed at the return air inlets).
The surface of an HVAC duct lined with fiberglass cannot be mechanically cleaned - you can see what happens when someone tries to brush or vacuum it by looking at this photo. Once disturbed by improper "cleaning' efforts, the release of airborne asbestos in the building will certainly increase. If this insulation is wet by leaks or improper condensate handling, or if the building is exposed to high levels of airborne mold from another source, ducts that look like this are likely to become a problem mold reservoir and will need to be replaced.
The second photo at above right shows typical debris, usually skin cells and fabric fibers, which collects on the rough surface of exposed fiberglass inside ductwork. A return opening filter would have helped keep this duct clean and thus extend its life.
Water or condensate leaks into an HVAC duct system such as those shown by the above photographs of stains on the interior of this rooftop mounted commercial HVAC duct (left) and indoor residential air handler unit (right), are an invitation to mold or bacterial contamination in the system.
DF-OPINION: it is more (or less) likely that problematic mold will be found growing in or present in building insulation at a level sufficient to be a potential problem for building occupants in these conditions:
Advice for Suspected or Known Mold-Contamination in HVAC Ductwork (Air ducts, heating ducts, air conditioning ducts)
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?" before taking further action. Visit www.epa.gov/iaq/pubs/airduct.html, or call (800) 438- 4318 for a free copy.
The U.S. Environmental Protection Agency recommends:
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:
Watch out: the U.S. EPA and other sources recommend: [among other details found in the articles listed in our REFERENCES] that
But 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. In our opinion mold-contaminated ductwork that is metal lined can be cleaned successfully.
Other 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.
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?.
Also see DUCT DAMAGE, MECHANICAL where we discuss other types of damage to HVAC ducts including the partial or complete loss of HVAC duct insulation.
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Technical Reviewers & References
Fiberglass in buildings: hazards, testing, cleanup, prevention: references & products
For more information about fiberglass as an indoor air quality concern see: