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• 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

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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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, I am doubtful that even a fiberglass-based filter is a meaningful contributor to the total load of 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.

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

Technical Reviewers

• 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.
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of 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.

More Information on Building Diagnostic Inspections and Repairs

For more information about fiberglass as an indoor air quality concern see:

• Fiberglass in Indoor Air, HVAC Ducts, and Building Insulation
• Fiberglass Particle Identification in the Fiberglass Test Laboratory
• Goodman Gray Flex Duct Deterioration and Failures
• Mold in Fiberglass Insulation
• Optimum Indoor Air Filters
• Owens Corning Flex Duct Deterioration and Failures
• World Trade Center Dust Particle Identification

For more information about asbestos as an indoor air quality concern with focus on easy ways to identify asbestos materials in buildings, see:

• Asbestos Identification in Buildings: How to find & identify asbestos-containing materials
• Asbestos in Good Condition
• Asbestos HVAC Ducts - Asbestos Materials On or In Heating and Cooling Duct Work
• Asbestos paper duct wrap
• Asbestos Transite Chimneys, Flues, & Pipes in Buildings
• Asbestos under the microscope
• Asbestos Vibration Dampers & Asbestos in the Air Handler
• Ceiling tiles: How to recognize ceiling tiles that may contain asbestos
• Examples of unusual uses of asbestos in buildings

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