High-Efficiency Particulate Air or HEPA is a type of air filter
Air filter
A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. A chemical air filter consists of an absorbent or catalyst for the removal of airborne molecular contaminants such as volatile organic compounds...

. Filters that are awarded the HEPA accolade are used in various locations, whether in medical facilities, automotive vehicles, airplanes, home filters, or wherever very pure air is sought. The filter must satisfy certain standards of efficiency such as those set by the United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 (DOE). To qualify as HEPA by government standards, an air filter must remove 99.97% of all particles greater than 0.3 micrometer from the air that passes through. A filter that is qualified as HEPA is superior to those unqualified, but is also subject to interior classifications. Due to this, not all HEPA filters are equal and care should be taken when replacing or purchasing a HEPA filter that the filter is of the specific grade required.


HEPA filters are composed of a mat of randomly arranged fibres. The fibres are typically composed of fiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

 and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting function are fibre diameter, filter thickness, and face velocity. The air space between HEPA filter fibres is much greater than 0.3 μm
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

. The common assumption that a HEPA filter acts like a sieve
A sieve, or sifter, separates wanted elements from unwanted material using a woven screen such as a mesh or net. However, in cooking, especially with flour, a sifter is used to aerate the substance, among other things. A strainer is a type of sieve typically used to separate a solid from a liquid...

 where particles smaller than the largest opening can pass through is incorrect. Unlike membrane filter
Membrane technology
The membrane technology covers all process engineering measures for the transport of substances between two fractions with the help of permeable membranes...

s at this pore size, where particles as wide as the largest opening or distance between fibres cannot pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles. These particles are trapped (they stick to a fibre) through a combination of the following three mechanisms:
  1. Interception, where particles following a line of flow in the air stream come within one radius of a fibre and adhere to it.
  2. Impaction, where larger particles are unable to avoid fibres by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fibre separation and higher air flow velocity.
  3. Diffusion, an enhancing mechanism that is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter; this behaviour is similar to Brownian motion
    Brownian motion
    Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...

     and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities.

Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the Most Penetrating Particle Size (MPPS) 0.3 μm, both diffusion and interception are comparatively inefficient. Therefore, the HEPA specifications use the retention of these particles to define the filter.


HEPA filters, as defined by the DOE
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 standard adopted by most American industries, remove at least 99.97% of airborne particles
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

 0.3 micrometer
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

s (µm) in diameter. The filter's minimimal resistance to airflow, or pressure drop
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

, is usually specified around 300 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 at its nominal flow rate.

The specification usually used in the European Union is the European Norm EN 1822:2009. It defines several classes of HEPA filters by their retention at MPPS:
HEPA class retention (total) retention (local)
E10 > 85 % ---
E11 > 95 % ---
E12 > 99.5 % ---
H13 > 99.95 % > 99.75 %
H14 > 99.995 % > 99.975 %
U15 > 99.9995 % > 99.9975 %
U16 > 99.99995 % > 99.99975 %
U17 > 99.999995 % > 99.9999 %

The original HEPA filter was designed in the 1940s and was used in the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...

 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
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...

, pharmaceutical processing, hospitals, health care, nuclear fuels, nuclear power, and electronic microcircuitry (computer chips).

Today, a HEPA filter rating is applicable to any highly efficient air filter that can attain the same filter efficiency performance standards as a minimum and is equivalent to the more recent NIOSH N100 rating for respirator filters. The United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 (DOE) has specific requirements for HEPA filters in DOE regulated applications. Products that claim to be "HEPA-type", "HEPA-like", or "99% HEPA" do not satisfy these requirements and may not have been tested in independent laboratories.

Biomedical applications

HEPA filters are critical in the prevention of the spread of airborne bacterial and viral organisms and, therefore, infection. Typically, medical-use HEPA filtration systems also incorporate high-energy ultra-violet light units to kill off the live bacteria and viruses trapped by the filter media. Some of the best-rated HEPA units have an efficiency rating of 99.995%, which assures a very high level of protection against airborne disease transmission.

Vacuum cleaners

Many vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...

s also use HEPA filters as part of their filtration systems. This is beneficial for asthma and allergy sufferers, because the HEPA filter traps the fine particles (such as pollen and dust mite feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

) which trigger allergy and asthma symptoms. For a HEPA filter in a vacuum cleaner to be effective, the vacuum cleaner must be designed so that all the air drawn into the machine is expelled through the filter, with none of the air leaking past it. This is often referred to as "Sealed HEPA" or sometimes the more vague "True HEPA." Vacuum cleaners simply labeled HEPA have a HEPA filter, but not all air necessarily passes through it. Finally, vacuum cleaner filters marketed as "HEPA-like" will typically use a filter of a similar construction to HEPA, but without the filtering efficiency. Because of the extra density of a HEPA filter, HEPA vacuum cleaners require more powerful motors to provide adequate cleaning power.

Newer models claim to be better than the first models because of being "washable". Generally washable filters are expensive. Some manufacturers claim filter standards such as "HEPA 4", without explaining the meaning behind them. It refers to their Minimum Efficiency Reporting Value
Minimum Efficiency Reporting Value
Minimum efficiency reporting value, commonly known as MERV rating is a measurement scale designed in 1987 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers to rate the effectiveness of air filters...

 (MERV) rating. These ratings are used to rate the ability of an air cleaner filter to remove dust from the air as it passes through the filter. MERV is a standard used to measure the overall efficiency of a filter. The MERV scale ranges from 1 to 20, and measures a filter's ability to remove particles from 10 to 0.3 micrometre in size. Filters with higher ratings not only remove more particles from the air, they also remove smaller particles.


Modern airliners use HEPA filters to reduce the spread of airborne pathogens in recirculated air. Test results from a Department of Transportation
Department of Transportation
The Department of Transportation is the most common name for a government agency in North America devoted to transportation. The largest is the United States Department of Transportation, which oversees interstate travel. All U.S. states, Canadian provinces, and many local agencies also have...

 (DOT) study conducted on 92 randomly selected flights showed that bacteria and fungi levels measured in the airplane cabin are similar to or lower than those found in the common home. These very low microbial contaminant levels are due to the complete exchange of inside cabin air 10 to 15 times per hour and the high filtration capability of the recirculation system. The filters used on current airliners are similar to HEPA filters used in industrial “clean rooms” and in critical wards of hospitals, such as organ transplant and burn units. By comparison, the filtration systems in typical buildings are not capable of removing microbial contaminants including bacteria and viruses.

See also

  • Air purifier
    Air purifier
    An air purifier is a device which removes contaminants from the air. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke...

  • Electrostatic precipitator
    Electrostatic precipitator
    An electrostatic precipitator , or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas using the force of an induced electrostatic charge...

     - trap particles with high voltage
  • HEGA
    HEGA is an acronym for "High Efficiency Gas Absorption". Originally developed by the British military as a defense against chemical warfare, HEGA is a carbon cloth filter. This carbon cloth has been claimed to be many times more efficient than the granular activated carbon form at adsorption of...

     - carbon cloth filter developed by the British military against chemical warfare
  • Hypoallergenic vacuum cleaner
    Hypoallergenic vacuum cleaner
    An hypoallergenic vacuum cleaner removes particles from flooring, carpet, and furniture, as well as the air surrounding the cleaner by suction, creating a non-allergic state.- Existing technology :...

     - vacuum cleaner with high efficiency air filter
  • Respirator
    A respirator is a device designed to protect the wearer from inhaling harmful dusts, fumes, vapors, or gases. Respirators come in a wide range of types and sizes used by the military, private industry, and the public...

  • ULPA
    ULPA is an acronym for "ultra-low penetration air". An ULPA filter can remove from the air at least 99.999% of dust, pollen, mold, bacteria and any airborne particles with a size of 120 nanometres or larger...

     - Removes 99.999% of dust, pollen, mold, bacteria and particles larger than 120 nm

General references

External links

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