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HEPA Construction:

Although HEPA filter construction can vary considerably, most HEPA filters consist of a rigid frame into which a filter pack, constructed by folding a continuous sheet of media into closely spaced pleats, is sealed. Conventional HEPA media is an all-glass paper which is composed of an extremely large number of randomly oriented micro-fibers which utilizes some fairly complex mechanical principles to achieve its effectiveness on sub-micron particles (see principles of filtration). Because of the density of the media, air flow is restricted and the pleating is used to increase the amount of media (surface area) allowing for a greater ratio of media to air flow. Over the years, filter manufacturers have developed different methods and materials for maintaining the spacing of the pleats all in an effort to optimize media surface area and reduce air flow resistance.

In addition to sealing the filter pack in the frame, the filter must be sealed when it is installed in equipment in order to prevent air flow and the sub-micron particles it contains from by-passing the HEPA filter. Like the formation of pleats, filter manufacturers have developed different methods and materials to accomplish this. The use of closed cell neoprene gaskets is perhaps the most common method of sealing the HEPA filter installation.

Many different HEPA filter specifications and standards have been written for the myriad applications in which HEPA filters have been put to use. From UL-586 with environmental testing to ASME AG-1 with seismic qualification for nuclear use, all have been developed to ensure the suitability of a filter for a particular use.


The most comprehensive guidelines for HEPA filter construction is set forth in the Institute of Environmental Sciences and Technology Recommended Practice IEST-RP-CC001.3. The IEST Recommended Practice specifies many of the materials used for HEPA filter construction and describes the various types of filter packs as differentiated by the method used to maintain the spacing of pleats. IEST-RP-CC001.3 also defines six levels of performance and six grades of construction that distinguish between the types of service the filter units may experience. For example, Type "A" filter performance uses the Mil-Std-282 penetration test with a minimum efficiency rating of 99.97% on 0.3 microns whereas Type "F" filter performance is for ULPA filters (ultra low penetration air) with a minimum efficiency of 99.999% at 0.1 to 0.2 microns. Construction Grades range from Grade 1 which meets severe environmental and handling tests to Grade 6 for noncritical or nonsafety-related applications.

IEST recommended practices are available for purchase from the Institute of Environmental Sciences and Technology at

Other Standards:

As was mentioned earlier, there are many HEPA filter specifications and standards. Some have been developed by individual companies for their own use, some are for specific applications or industries. It is also common to find some specifications or standards misapplied to a HEPA filter or HEPA filtration, in general. Following is a list of specifications or standards often found associated with HEPA filters used in vacuum cleaners and a brief description of its relevance.

ANSI Z9.2 - Fundamentals Governing the Design and Operation of Local Exhaust Systems. Although this standard is often applied to a vacuum cleaner as an exhaust system it does not specify any HEPA filtration efficiency, construction or test requirements. C-vac replacement HEPA filters will maintain the proper compliance of any vacuum operating within the parameters of this standard.

ASME N509 - Nuclear Power Plant Air-Cleaning Units and Components.

ASME N510 - Testing of Nuclear Air Treatment Systems.

ASME NQA-1 - Quality Assurance Requirements for Nuclear Facility Applications.

ASME AG-1 - Code on Nuclear Air and Gas Treatment. Written for nuclear power plants and nulear fuel cycle facilities, this code includes standards for design, fabrication, inspection, and testing of air cleaning equipment and components. Although special filters can be manufactured to this code, standard HEPA filters used in most vacuum cleaners do not meet this code and the particular configuration of the filter may present other specific limitations.

Mil-F-51068 - Filters, Particulate (High-Efficiency Fire Resistant). A military specification approved for use by all Departments and Agencies of the Department of Defense, this specification covers eight sizes and types of HEPA filters which must first be qualified to be furnished under this specification. Although some HEPA filters used in vacuum cleaners can be manufactured with the materials and testing required by this specification, there are no cylindrical filters included in its scope and most standard HEPA filters used in vacuum cleaners do not comply.

Mil-F-51079 - Filter, Medium, Fire-Resistant, High-Efficiency. A military specification covering one grade of high-efficiency, fire-resistant, filter medium normally used for compliance with Mil-F-51068.

UL-586 - Standard for High-Efficiency, Particulate Air Filter Units. An Underwriters Laboratories standard for HEPA filters including construction and environmental test requirements where samples are tested for minimum performace characteristics. Filters employing different materials or forms of construction from those detailed may be determined to be in compliance with the standard if substantially equivalent. Filters listed by UL under this standard may carry a UL-586 label to identify compliance.

UL-900 - Test Performance of Air Filter Units. Underwriters Laboratories classifies air filters submitted for testing as to flammability only. This standard investigates the combustibility and smoke generation of a filter, not the ability of a filter to remove airborne particles.

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