| HEPA | |
|---|---|
 HEPA filter corrugated internal structure and aluminium support along with the description of its functioning principle (interception, impact and diffusion of dust particles through a dense non-woven fiber material) | |
| Overview | |
| Other names | High-Efficiency Particulate Air, High-Efficiency Particulate Arresting filter, HEPA |
| Standards | DOE NE F 3-45, EN 1822, ISO 29463 |
HEPA (/ˈhɛpə/,high efficiency particulate air[1], orhigh efficiency particulate arresting[2]) is anefficiency standard ofair filters.[3] AHEPA 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% (ISO, European Standard)[4][5] or 99.97% (ASME,U.S. DOE)[6][7] ofparticles 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.[8] HEPA filters capturepollen,dirt,dust,moisture,bacteria (0.2–2.0 μm),viruses (0.02–0.3 μm), and submicron liquidaerosol (0.02–0.5 μm).[9][10][11] Somemicroorganisms, for example,Aspergillus niger,Penicillium citrinum,Staphylococcus epidermidis, andBacillus subtilis are captured by HEPA filters withphotocatalytic oxidation (PCO). A HEPA filter is also able to capture some viruses and bacteria which are ≤0.3 μm.[12] A HEPA filter is also able to capture floor dust which containsbacteroidia,clostridia, andbacilli.[13]
HEPA was commercialized in the 1950s, and the original term became a registeredtrademark and later ageneric trademark for highly efficient filters.[14] HEPA filters are used in applications that requirecontamination control, such as the manufacturing of hard disk drives, medical devices, semiconductors, nuclear, food and pharmaceutical products, as well as in hospitals,[15] homes, and vehicles.
HEPA filters are composed of amat of randomly arranged fibers.[16] The fibers are typically composed ofpolypropylene orfiberglass with diameters between 0.5 and 2.0 micrometers. Most of the time, these filters are composed of tangled bundles of finefibers. 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 (Brownian motion). Because of their movement, they end up crashing into the fibers.[17] 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 (HVAC) system. Face velocity is measured in m/s and can be calculated as the volume flow rate (m3/s) divided by the face area (m2). The air space between HEPA filter fibers is typically much greater than 0.3 μm. Unlikesieves ormembrane 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 (they stick to a fiber) through a combination of the following three mechanisms:
Diffusion predominates below the 0.1 μm diameter particle size, whilst impaction and interception predominate above 0.4 μm.[18] In between, near the most penetrating particle size (MPPS) 0.21 μm, both diffusion and interception are comparatively inefficient.[19] Because this is the weakest point in the filter's performance, the HEPA specifications use the retention of particles near this size (0.3 μm) to classify the filter.[18] 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 asnucleation sites for mostlycondensation and form particles near the MPPS.[19]
HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses andodor molecules. Circumstances requiring filtration ofvolatile organic compounds, chemical vapors, orcigarette, pet orflatulence odors call for the use of anactivated carbon (charcoal) or other type of filter instead of or in addition to a HEPA filter.[20] Carbon cloth filters, claimed to be many times more efficient than the granular activated carbon form atadsorption of gaseouspollutants, are known as high efficiency gas adsorption filters (HEGA) and were originally developed by the British Armed Forces as a defense againstchemical warfare.[21][22]
A HEPA bag filter can be used in conjunction with a pre-filter (usually carbon-activated) to extend the usage life of the more expensive HEPA filter.[23] In such setup, the first stage in thefiltration 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 inair handling units.[citation needed]
HEPA filters, as defined by the United States Department of Energy (DOE) standard adopted by most American industries, remove at least 99.97% of aerosols 0.3 micrometers (μm) in diameter.[24] The filter's minimal resistance to airflow, orpressure drop, is usually specified around 300 pascals (0.044 psi) at its nominalvolumetric flow rate.[7][25]
The specification used in theEuropean Union: European Standard EN 1822-1:2019, from which ISO 29463 is derived,[4] defines several classes of filters by their retention at the given most penetrating particle size (MPPS):Efficient Particulate Air filters (EPA),High Efficiency Particulate Air filters (HEPA), andUltra Low Particulate Air filters (ULPA). The averaged efficiency of the filter is called "overall", and the efficiency at a specific point is called "local":[4]
| Efficiency | EN 1822 | ISO 29463 | Retention (averaged) | Retention (spot) |
|---|---|---|---|---|
| EPA | E10 | — | ≥ 85% | — |
| E11 | ISO 15 E ISO 20 E | ≥ 95% ≥ 99% | — | |
| 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% |
| 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% |
| U16 | ISO 65 U ISO 70 U | ≥ 99.99995% ≥ 99.99999% | ≥ 99.99975% ≥ 99.9999% | |
| U17 | ISO 75 U | ≥ 99.999995% | ≥ 99.9999% |
See also the different classes forair filters for comparison.
| HEPA | |
|---|---|
| Other name(s) | High-Efficiency Particulate Air, High-Efficiency Particulate Arresting filter, HEPA |
| Regulated by | United States Department of Energy,Mine Safety and Health Administration,National Institute for Occupational Safety and Health |
| Regulation | 30 CFR 11,42 CFR 84,DOE NE F 3-45 |
| NIOSH schedule | TC-21C |
Forrespirators,MSHA andNIOSH define HEPA as filters blocking ≥ 99.97% of 0.3 micronDOP 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 forpowered air-purifying respirators.[26] However, by definition, ANSI Z88.2-2015 considers N100, R100, P100, and HE as HEPA filters.[27]
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.[28] 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.[29]
In general terms (and allowing for some variation depending on factors such as the air-flow rate, the physical properties of the particles being filtered, as well as engineering details of the entire filtration-system design and not just the filter-media properties), HEPA filters experience the most difficulty in capturing particles in the size range of 0.15 to 0.2 μm.[30] HEPA filtration works by mechanical means, unlike ionic andozone treatment technologies, which use negativeions and ozone gas respectively. So, the likelihood of potential triggering of pulmonary side-effects such asasthma[31] andallergies is much lower with HEPA purifiers.[32]
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.[33]
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Medical filtration systems use extremeultraviolet 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.[citation needed]
Some of the best-rated HEPA units have an efficiency rating of 99.995%, which assures a very high level of protection againstairborne disease transmission.[citation needed]
HEPA filters are capable of removing viruses including COVID-19 (particles of 60-140 nanometer diameter) 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 atUniversity of California, Davis, created a simpledo-it-yourselfair purifier design called crbox(Corsi–Rosenthal Box). 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 asDyson orXiaomi hitting the markets.
Manyvacuum cleaners 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 andhouse dust mitefeces) which trigger allergy and asthma symptoms. For a HEPA filter in a vacuum cleaner to be effective, the vacuum cleaner must be designed so thatall 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" may 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 asimilar construction to HEPA, but without the filtering efficiency. Because of the extra density of a true HEPA filter, HEPA vacuum cleaners require more powerful motors to provide adequate cleaning power.[citation needed]
Washable filters or reusable filters have proven to be very effective in trapping and keeping Airborne contaminants for millennia. Many washable reusable filters today are cleverly designed & engineered a substrate that electronically draw material to the substrate & attaching to that substrate, lock into place until washed away. Washable reusable filters meet air quality standards, are cost effective, environmentally friendly, no replacement required & little maintenance VS. true HEPA filters are expensive, with regular replacement damaging to the environment and can Harbor contagions which can breed and multiply within the filter substrate.[citation needed]
A high-quality HEPA filter can trap 99.97% of dust particles that are 0.3 microns in diameter. For comparison, a human hair is about 50 to 150 microns in diameter. So, a true HEPA filter is effectively trapping particles several hundred times smaller than the width of a human hair.[34] Some manufacturers claim filter standards such as "HEPA 4," without explaining the meaning behind them.[citation needed] This refers to theirMinimum Efficiency Reporting Value (MERV) rating.[citation needed] 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 16, and measures a filter's ability to remove particles from 10 to 0.3 micrometer in size. Filters with higher ratings not only remove more particles from the air, but they also remove smaller particles.
Heating, ventilation, and air conditioning (HVAC)[35] is technology that uses air filters, such as HEPA filters, to remove pollutants from the air either indoors or in vehicles. Pollutants include smoke, viruses,powders, etc., and can originate either outside or inside. HVAC is used to provide environmental comfort and inpolluted cities to maintain health.[citation needed]
Modern airliners use HEPA filters to reduce the spread of airbornepathogens in recirculated air. Critics have expressed concern about the effectiveness and state of repair of air filtering systems, since they think that much of the air in an airplane cabin is recirculated. Almost all of the air in apressurized aircraft is, in fact, brought in from the outside, circulated through the cabin and then exhausted through outflow valves in the rear of the aircraft.[36] About 40 percent of the cabin's air goes through a HEPA filter and the other 60 percent comes from outside the plane. Certified air filters block and capture 99.97 percent of airborne particles.[37]
In 2016, it was announced that theTesla Model X would have the world's first HEPA-grade filter in a Tesla car.[38] Following the release of the Model X, Tesla has updated theModel S to also have an optional HEPA air filter.[39]
The idea behind the development of the HEPA filter was born fromgas masks worn by soldiers fighting in World War II. A piece of paper found inserted into a German gas mask had a remarkably high capture efficiency for chemical smoke. The British ArmyChemical Corps duplicated this and began to manufacture it in large quantities for their own service gas masks. They needed another solution for operational headquarters, where individual gas masks were impractical. The Army Chemical Corps developed a combination mechanical blower and air purifier unit, which incorporated cellulose-asbestos paper in a deeply-pleated form with spacers between the pleats. It was referred to as an "absolute" air filter and laid the groundwork for further research to come in developing the HEPA filter.[40]
The next phase of the HEPA filter was designed in the 1940s and was used in theManhattan Project to prevent the spread of airborneradioactive contaminants.[41] The US Army Chemical Corps and National Defense Research Committee needed to develop a filter suitable for removing radioactive materials from the air. The Army Chemical Corps asked Nobel LaureateIrving Langmuir to recommend filter test methods and other general recommendations for creating the material to filter out these radioactive particles. He identified 0.3 micron size particles to be the "most penetrating size"—the most difficult and concerning.[42]
It was commercialized in the 1950s, and the original term became a registered trademark and later a generic trademark for highly efficient filters.[14]
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 industry, hospitals, health care, nuclear fuels, nuclear power, andintegrated circuit fabrication.[citation needed]
