Fiberglass, Air Filters and Indoor Air Quality: Questions and Answers InspectAPedia® -
Fiberglass, Air Filters and Indoor Air Quality Questions and Answers
Do HVAC filters release fiberglass fibers or fragments to the building air or do they remove such particles
Characteristics of fiberglass fibers and the range of size of fiberglass insulation or other fiberglass product fragments found indoors
Variation in fiberglass test lab analysis and reporting may fail to identify very small fiberglass fragments
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This website answers almost any question you might ask about air filters for heating or air conditioning systems.
We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. We include photographs to assist readers in
recognizing cooling system defects. We continue to add to and update this text as new details are provided.
Readers should also see Air Filter Effectiveness and Real-World Effectiveness of Air Cleaners in our INDOOR AIR QUALITY IMPROVEMENT GUIDE article series.
Air Filters and Fiberglass Contamination in Indoor Air
Air filters and their contribution to fiberglass fragments found in building air and dust
In our experience, concern for filter shedding, say of fiberglass fragments, is not a significant issue. The contribution of an air conditioning or heating furnace air filter fiberglass to the overall level of airborne or dust-borne
fiberglass particles in a building is likely to be insignificant, probably below the limits of detection by other than the most rigorous means, and
certainly in most buildings will be insignificant compared with the contribution of other fiberglass sources such as building insulation in unprotected ceilings or walls.
If necessary we can usually identify the source of the dominant fiberglass fragments in building air and dust. Samples of settled dust and building air are compared with samples of fiberglass from common building sources
such as fiberglass building insulation and fiberglass HVAC duct insulation.
Fiberglass fiber metric consistency and the color of fiberglass binders are often sufficiently distinct to permit positive identification of
the source of fiberglass fragments in the building environment. (See our article on laboratory identification of fiberglass found at More Information.
In sum, it is very doubtful that a fiberglass-based air filter is a meaningful contributor to the total load of airborne fiberglass particles in a building. There are simply too many enormously larger sources that overwhelm the measurement. Any air filter, properly selected and installed and maintained,
will reduce the overall level of airborne particles, including fiberglass fragments which are contributed to the building air and dust from other building sources.
Do test labs see or even look for very small fiberglass insulation fragments?
What about fiberglass particles? As we discuss in more detail in our fiberglass and asbestos IAQ articles some of the research on the possible hazards of airborne fiberglass is confusing because it asserts that the probable hazard of "large" fiberglass particles is low.
What's tricky is that hygienists or others
who check indoor air or dust for the level of fiberglass contamination, and even the labs which process these samples may examine and report particles only in the larger size range.
That makes sense insofar as it's apparent that
large particles dominate fiberglass dust.
But let's consider just a tiny bit further.
Fiberglass, particularly mechanically-damaged fiberglass insulation, say in a fiberglass-lined air duct which was mechanically cleaned,
can break to release very small glass fragments, even in the 1u range and below.
These particles are very hard to detect in the laboratory unless the lab is specifically looking for them. In fact, unless the laboratory
uses a slide preparation media with a refractive index nowhere close to that of glass, they won't see the particles at all in the microscope, no matter how many of them are present! In other words, you don't see what you're not looking for.
A reasonable conclusion from these observations about the presence and difficulty in seeing small airborne particles below 10u and especially below 3u is that the better we filter the air the less we need to worry about them.
Variations in fiber size in air conditioning and heating air filters
One of our clients wrote that her fiberglass-based air conditioning and heating air filters were described by their manufacturer as using fibers of 20-27u (in fiber diameter).
The maker promises the filters do not shed fiberglass fragments and that the fibers are too big in micron size to become repairable even if they do shed.
Laboratory examination of a filter sample sent by the client to an independent microscopy laboratory found that the filter fibers were 31 microns in diameter, and had a blue coating on them [probably the binder]. The binder coating increases the effective fiber diameter.
These measurements of fiberglass fiber diameters were within a normal range of variation in product manufacture that we've seen in own forensic laboratory.
Furthermore, the diameter of a fiberglass air filter fiber is as an indicator of the health hazard should the filter shed fibers is unlikely to be as useful measure as the
number of particles released and their overall size. For example, a fiberglass fiber can break so as to release several sub-micron glass fragments.
The actual filtering efficiency of a filter should not be estimated simply on fiber diameter since more than diameter goes into the filter design, including fiber placement, randomness, average
remaining opening diameter, total filter thickness, electrostatic properties of the filter, and other variables such as whether or not the filter has a gasket or is bypassing lots of air around itself.
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Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Wikipedia provided background information about the definition of HEPA and airborne particle interception.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
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