HEPA is an acronym for “high efficiency particulate arrestor” and ULPA is an acronym for ULTRA LOW PARTICULATE ARRESTOR.  An ULPA is a higher efficient grade of filtration with five 9’s of efficiency.   An even higher grade is called a SULPA, a Super ULPA with eight 9’s of efficiency. 

HEPA filters are They are used as air filters to remove particles such as dust, pollen, mold, and bacteria. HEPA filters were originally developed for the Manhattan Project by the Atomic Energy Commission to trap radioactive particles.  

HEPA filters are also employed to filter out highly hazardous aerosols such as those that are radioactive, biohazardous and highly toxic (eg. carcinogins). In the event of a nuclear, biological or chemical outbreak, HEPA filters are the last line of defense between the contamination and the those who could be exposed to it. They are now used in operating rooms and biomedical applications to prevent the spread of airborne bacteria and viral organisms and in cleanrooms in the development of computer chips.  

True HEPA filters are regulated by the United States Department of Energy (DOE) to be 99.97% efficient on 3 tenths of a micron, allowing no more than 3 particles in 10,000 to penetrate the filter.  A micron, short for micrometer, is a unit of measurement equal to one millionth of a meter. A micron is actually 0.000039 of an inch.  To give you an idea of what we are talking about, the period at the end of this sentence is about 397 microns. The eye of a needle is usually elongated, but is 749 microns wide. A human hair is 40 to 120 microns in diameter and the smallest particle that can be seen by the unaided human eye is approximately 10 microns.  These particle can only be seen with reflected light.  

The chart below shows the relative size of various categories of particles.

Partical Size Chart


HEPA filters are composed of high density mats composed of randomly arranged fibers enclosed in a specifically designed frame..  The HEPA mat is designed to “seek out” smaller particles and pollutants and have them stick to the fibers in three separate ways.

  1. Interception, where particles following a line of flow in the air stream, come within one radius of a fiber and are trapped by the fiber.
  2. Impaction, where larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly; this increases with diminishing fiber separation and higher air flow velocity.
  3. Diffusion, an enhancing mechanism is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 micron in diameter, which are thereby impeded and delayed in their path through the filter allowing the interception process to capture the particle.

Due to the denseness of the filter mat, HEPA filters have a high resistance to air flow.  Because of this, the fan power to overcome this resistance is much greater that a normally designed HVAC fan unit.  Consequently, HEPA filters cannot be added to a home air conditioning system.  Another important fact to know about HEPA filters is that they will not remove chemicals (VOC’c) and odors.  Filters that perform the removal of these contaminants are referred to gas phase filters.  This issue is addressed on this web site under the Molecular Filtration tab.