HEPA

HEPA is an efficiency standard of air filters. A HEPA filter is an air filter meeting such a standard.
A HEPA filter must satisfy certain levels of efficiency. Common standards require that a HEPA air filter must remove—from the air that passes through—at least 99.95% or 99.97% of particles whose diameter is equal to 0.3 μm, with the filtration efficiency increasing for particle diameters both less than and greater than 0.3 μm. HEPA filters capture pollen, dirt, dust, moisture, bacteria, viruses, and submicron liquid aerosol. Some microorganisms, for example, Aspergillus niger, Penicillium citrinum, Staphylococcus epidermidis, and Bacillus subtilis are captured by HEPA filters with photocatalytic oxidation. A HEPA filter is also able to capture some viruses and bacteria which are ≤0.3 μm. A HEPA filter is also able to capture floor dust which contains bacteroidia, clostridia, and bacilli.
HEPA was commercialized in the 1950s, and the original term became a registered trademark and later a generic trademark for highly efficient filters. HEPA filters are used in applications that require contamination control, such as the manufacturing of hard disk drives, medical devices, semiconductors, nuclear, food and pharmaceutical products, as well as in hospitals, homes, and vehicles.

Mechanism

HEPA filters are composed of a mat of randomly arranged fibers. The fibers are typically composed of polypropylene or fiberglass with diameters between 0.5 and 2.0 micrometers. Most of the time, these filters are composed of tangled bundles of fine fibers. These fibers create a narrow convoluted pathway through which air passes. When the largest particles are passing through this pathway, the bundles of fibers behave like a kitchen sieve which physically blocks the particles from passing through. However, when smaller particles pass with the air, as the air twists and turns, the smaller particles cannot keep up with the motion of the air and thus they collide with the fibers. The smallest particles have very little inertia and move randomly as a result of collisions with individual air molecules. Because of their movement, they end up crashing into the fibers. Key factors affecting its functions are fiber diameter, filter thickness, and face velocity, which is the measured air speed at an inlet or outlet of a heating ventilation and air conditioning system. Face velocity is measured in m/s and can be calculated as the volume flow rate divided by the face area. The air space between HEPA filter fibers is typically much greater than 0.3 μm. Unlike sieves or membrane filters, where particles smaller than openings or pores can pass through, HEPA filters are designed to target a range of particle sizes. These particles are trapped through a combination of the following three mechanisms:

Diffusion; particles below 0.3 μm are captured by diffusion in a HEPA filter. This mechanism is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 μm in diameter. The small particles are effectively blown or bounced around and collide with the filter media fibers. This behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either interception or impaction; this mechanism becomes dominant at lower airflow.
Interception; particles following a line of flow in the air stream come within one radius of a fiber and adhere to it. Mid size particles are being captured by this process.
Impaction; 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 effect increases with diminishing fiber separation and higher air flow velocity.

Diffusion predominates below the 0.1 μm diameter particle size, whilst impaction and interception predominate above 0.4 μm. In between, near the most penetrating particle size 0.21 μm, both diffusion and interception are comparatively inefficient. Because this is the weakest point in the filter's performance, the HEPA specifications use the retention of particles near this size to classify the filter. However it is possible for particles smaller than the MPPS to not have filtering efficiency greater than that of the MPPS. This is due to the fact that these particles can act as nucleation sites for mostly condensation and form particles near the MPPS.

Gas filtration

HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds, chemical vapors, or cigarette, pet or flatulence odors call for the use of an activated carbon or other type of filter instead of or in addition to a HEPA filter. Carbon cloth filters, claimed to be many times more efficient than the granular activated carbon form at adsorption of gaseous pollutants, are known as high efficiency gas adsorption filters and were originally developed by the British Armed Forces as a defense against chemical warfare.

Pre-filter and HEPA filter

A HEPA bag filter can be used in conjunction with a pre-filter to extend the usage life of the more expensive HEPA filter. In such setup, the first stage in the filtration process is made up of a pre-filter which removes most of the larger dust, hair, PM10 and pollen particles from the air. The second stage high-quality HEPA filter removes the finer particles that escape from the pre-filter. This is common in air handling units.

Specifications

HEPA filters, as defined by the United States Department of Energy standard adopted by most American industries, remove at least 99.97% of aerosols 0.3 micrometers in diameter. The filter's minimal resistance to airflow, or pressure drop, is usually specified around at its nominal volumetric flow rate.
The specification used in the European Union: European Standard EN 1822-1:2019, from which ISO 29463 is derived, defines several classes of filters by their retention at the given most penetrating particle size : Efficient Particulate Air filters, High Efficiency Particulate Air filters, and Ultra Low Particulate Air filters. The averaged efficiency of the filter is called "overall", and the efficiency at a specific point is called "local":

Efficiency	EN 1822	ISO 29463	Retention	Retention
EPA	E10	—	≥ 85%	—
EPA	E11	ISO 15 E ISO 20 E	≥ 95% ≥ 99%	—
EPA	E12	ISO 25 E ISO 30 E	≥ 99.5% ≥ 99.9%	—
HEPA	H13	ISO 35 H ISO 40 H	≥ 99.95% ≥ 99.99%	≥ 99.75% ≥ 99.95%
HEPA	H14	ISO 45 H ISO 50 U	≥ 99.995% ≥ 99.999%	≥ 99.975% ≥ 99.995%
ULPA	U15	ISO 55 U ISO 60 U	≥ 99.9995% ≥ 99.9999%	≥ 99.9975% ≥ 99.9995%
ULPA	U16	ISO 65 U ISO 70 U	≥ 99.99995% ≥ 99.99999%	≥ 99.99975% ≥ 99.9999%
ULPA	U17	ISO 75 U	≥ 99.999995%	≥ 99.9999%

See also the different classes for air filters for comparison.

Specifications for respirators

For respirators, MSHA and NIOSH define HEPA as filters blocking ≥ 99.97% of 0.3 micron DOP particles, under 30 CFR 11 and 42 CFR 84. Since the transition to 42 CFR 84 in 1995, use of the term HEPA has been deprecated except for powered air-purifying respirators. However, by definition, ANSI Z88.2-2015 considers N100, R100, P100, and HE as HEPA filters.

Marketing

Some companies use the marketing term "True HEPA" to give consumers assurance that their air filters meet the HEPA standard, although this term has no legal or scientific meaning. Products that are marketed to be "HEPA-type," "HEPA-like," "HEPA-style" or "99% HEPA" do not satisfy the HEPA standard and may not have been tested in independent laboratories. Although such filters may come reasonably close to HEPA standards, others fall significantly short.

Efficacy and safety

In general terms, HEPA filters experience the most difficulty in capturing particles in the size range of 0.15 to 0.2 μm. HEPA filtration works by mechanical means, unlike ionic and ozone treatment technologies, which use negative ions and ozone gas respectively. So, the likelihood of potential triggering of pulmonary side-effects such as asthma and allergies is much lower with HEPA purifiers.
To ensure that a HEPA filter is working efficiently, the filters should be inspected and changed at least every six months in commercial settings. In residential settings, and depending on the general ambient air quality, these filters can be changed every two to three years. Failing to change a HEPA filter in a timely fashion will result in it putting stress on the machine or system and not removing particles from the air properly. Additionally, depending on the gasketing materials chosen in the design of the system, a clogged HEPA filter can result in extensive bypassing of airflow around the filter.

Applications

Biomedical

Medical filtration systems use extreme ultraviolet light units that effectively kill bacteria, mold and viruses using panels with an anti-microbial coating to kill off the live bacteria and viruses trapped by the HEPA 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.

COVID-19

HEPA filters are capable of removing viruses including COVID-19 from the air harboring the live virus in the filter.
Extreme ultraviolet light must be incorporated into the air purifier in order to kill airborne pathogens such as viruses like COVID-19, mold & bacteria. As such, hospitals saw a surge in adoption during the pandemic in order to mitigate infection risks.
To combat supply chain and cost issues hindering adoption of HEPA filters during the COVID-19 pandemic, a professor at University of California, Davis, created a simple do-it-yourself air purifier design called crbox. It involves arranging 4 HEPA filters in a cubic shape, the bottom being made out of cardboard, sealing the filter sides with tape and adding a fan on top.
In addition, the COVID-19 Pandemic resulted in a surge of new air purifier products from new and established brands such as Dyson or Xiaomi hitting the markets.