HEPA is an acronym that stands for High Efficiency Particulate Air. They are used in HEPA air purifiers, HEPA vacuums, industrial and laboratory exhaust hoods, and respirator masks. HEPA filters are defined as one which removes least 99.97% of airborne particles measuring at least 0.3 micrometers (µm) in diameter.

HEPA filters are made up of mats of randomly oriented fibres, usually fibreglass. The fibres are between 0.5 and 2.0 micron in diameter, while air space between the fibres in the filter mat is much greater than 0.3 μm.

Unlike other types of filters, rather than acting like a sieve HEPA filters trap particles through either Impaction, Interception or Diffusion. In impaction, larger particles unable to avoid fibres by following the curving contours of the air stream are forced to embed in one of them directly. In interception, particles following a line of flow in the air stream come within one radius of a fibre and adhere to it. In diffusion, small particles, especially those less than 0.1 µm in diameter, which are impeded and delayed in their path through the filter as a result of the collision with gas molecules.

Particles Filtered

Particles in the home that could affect the quality of air and your health include dust, smoke, pollen, animal dander, tobacco smoke, particles from combustion appliances like cooking stoves, and particles associated with organisms such as dust mites, molds, bacteria, and viruses. Mechanical filters such as a HEPA filter can filter these out, although larger particles will settle to the floor before they have a chance to circulate through the filter.

HEPA filters will not address gaseous pollutants like vehicle exhaust, tobacco smoke and volatile fumes from building materials, furnishings, adhesives, paints, varnishes, cleaning products, and pesticides.

Filter Efficiency

When choosing an air filter, you should look at its efficiency rating for removal of airborne particles from the air stream that passes through it. Filter efficiency is established by the minimum efficiency reporting value (MERV) for air filters installed in the ductwork of HVAC systems. This measurement method was developed by The American Society of Heating, Refrigerating and Air-Conditioning Engineers, or ASHRAE. MERV ratings, which range from a low of 1 to a high of 20, allow comparison of air filters made by different companies.

Flat or panel air filters with a MERV of 1 to 4 are used mostly in furnaces and air conditioners and are intended to protect the HVAC equipment from build-up of foreign materials on surfaces like fan motors and heating or cooling coils, to keep the equipment functioning efficiently. They have lower efficiency on smaller airborne particles and medium efficiency on larger particles, as long as they remain airborne and pass through the filter. Some smaller particles found within a house include viruses, bacteria, some mold spores, cat and dog allergens, and someof dust mite allergens.

Pleated surface filters with a MERV between 7 and 13 are likely to be nearly as effective as true HEPA filters. Medium efficiency filters with MERV 5 to 13 are practically efficient at removing small to large airborne particles. Filters with a MERV between 7 and 13 are likely to be nearly as effective as true HEPA filters at controlling most airborne indoor particles. Medium efficiency air filters are generally less expensive than HEPA filters, and allow quieter fan operation and higher airflow rates than HEPA filters since they have less airflow resistance.

Higher efficiency filters with a MERV of 14 to 16, sometimes misidentified as HEPA filters, are similar in appearance to true HEPA filters, which have MERV values of 17 to 20. True HEPA filters are normally not installed in residential HVAC systems; installation of a HEPA filter in an existing HVAC system would probably require professional modification of the system.

A typical residential air handling unit and the associated ductwork would not be able to accommodate HEPA filters because of their physical dimensions and increase in airflow resistance. Such filters are usually used in stand-alone air purifier units.