According to the NAFA Guide to Air Filtration (1993), the Institute of Environmental Sciences (IES) defines a HEPA filter as "a throw-away extended-media dry-type filter in a rigid frame, having minimum particle-collection efficiency of 99.97% for 0.3 micrometer (micron) thermally-generated dioctyl phthalate (DOP) particles or specified alternative aerosol, and a maximum clean-filter pressure drop of ... 1.0 in w.g. when tested at rated air flow capacity." The NAFA Guide offers the following general description of HEPA filters,
- Media is the filtering material. It is a paper that can be pleated back and forth to form a compact filter element. Close-pleating is necessary to fit all the required media into the desired space because the paper has a high resistance to air flow and the media velocity is usually in the range of 6 fpm (.03 m/s).
- Separators are devices which support the media and provide channels through which the air can flow to reach the media and then, after passing through the media, flow out of the filter.
- Filter Pack is the combination of media and separators.
- Filter Frame (Cell Side) is the rigid box into which the pack fits.
- Sealant is the adhesive or other material intended to create a leak-proof seal between the filter pack and the filter frame.
- Edge sealant is intended to prevent any bypass of unfiltered air around the filter. In many instances, it is a gasket attached to the filter frame.