HEPA filters are composed of a randomly arranged fiber mat. The key issues affecting their function are fiber density and diameter, and filter thickness. The air space between HEPA filter fibers is much greater than 0.3 µm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibers can not pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fiber) by one of the following three mechanisms:

Advanced close pleated media technology enables these filters to be manufactured without separators allowing them to handle a much higher than normal air volume at much lower resistances, leading to a greatly increased filter life. Close pleat type filters available in panel form for use in laminar flow benches and fixed ceiling housings or in hooded form as totally disposable units with integral diffusers and spigots for circular duct connections to fit direct into grid ceilings.