HEPA is an acronym for "high efficiency particulate absorbing" or "high efficiency particulate air."  A HEPA filter (theoretically) can remove from the air at least 99.97% of dust, pollen, mold, bacteria and any airborne particles with a size of 0.3 micrometers.  The 99.97% specification is for particles exactly 0.3 micrometers in diameter because that is what is known as the MPPS or most penetrating particle size.  Particles that are smaller or larger are trapped with even higher efficiency.  In other words, 0.3 micrometers is the worst case particle size and 99.97% is the worst case efficiency.

HEPA filters are composed of a mat of fibers.  The air space between fibers is much greater than 0.3 micrometers.  The common assumption is that a HEPA filter acts like a sieve where particles smaller than the hole size get through is entirely incorrect (membrane filters do work that way).  In a HEPA filter, particles are trapped (they stick to a fiber) by one of three mechanisms; diffusion, interception or impaction.

The original HEPA filter was designed in the 1940's to be used in the Manhattan Project to prevent the spread of airborne radioactive contaminants.  It was commercialized in the 1950s and the original term became a registered trademark and a generic term for highly efficient filters.  Over the decades, filters have evolved to satisfy the higher and higher demands for air quality in various high technology industries, such as aerospace, pharmaceutical processing and electronic microcircuitry (computer chips).  Today, a HEPA filter can be any highly efficient air filter that can attain the same filter efficiency performance standards of at least 99.97% and 0.3 micrometers of larger.