Jump to content

HEPA

From Wikipedia, the free encyclopedia
For other uses, see HEPA (disambiguation).

HEPA
Schematic of HEPA filter
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 namesHigh-Efficiency Particulate Air, High-Efficiency Particulate Arresting filter, HEPA
StandardsDOE NE F 3-45, EN 1822, ISO 29463

HEPA (/ˈhɛpə/, high efficiency particulate air) filter,[1] also known as a high efficiency particulate arresting filter,[2] is an efficiency standard of air filters.[3]

Filters meeting the HEPA standard 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] 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.[8] HEPA filters capture pollen, dirt, dust, moisture, bacteria (0.2–2.0 μm), viruses (0.02–0.3 μm), and submicron liquid aerosol (0.02–0.5 μm).[9][10][11] Some microorganisms, for example, Aspergillus niger, Penicillium citrinum, Staphylococcus epidermidis, and Bacillus subtilis are captured by HEPA filters with photocatalytic 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 contains bacteroidia, clostridia, and bacilli.[13] HEPA was commercialized in the 1950s, and the original term became a registered trademark and later a generic trademark for highly efficient filters.[14] 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,[15] homes, and vehicles.

Mechanism

[edit]
Four diagram each showing the path of small particle as it approaches a large fiber according to each of the four mechanisms
The four primary filter collection mechanisms: diffusion, interception, inertial impaction, and electrostatic attraction (air flow direction from left to right)
Classic Collection Efficiency Curve with Filter Collection Mechanisms

HEPA filters are composed of a mat of randomly arranged fibers.[16] 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 (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. HEPA filters in very high level for smallest particulate matter. 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 (they stick to a fiber) through a combination of the following three mechanisms:

  1. 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.
  2. 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.
  3. 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.[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 as nucleation sites for mostly condensation and form particles near the MPPS.[19]

Gas filtration

[edit]

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 (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 at adsorption of gaseous pollutants, are known as high efficiency gas adsorption filters (HEGA) and were originally developed by the British Armed Forces as a defense against chemical warfare.[21][22]

Pre-filter and HEPA filter

[edit]

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 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.[citation needed]

Specifications

[edit]
A portable HEPA filtration unit used to clean air after a fire, or during manufacturing processes

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, or pressure drop, is usually specified around 300 pascals (0.044 psi) at its nominal volumetric flow rate.[7]

The specification used in the European 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), and Ultra 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 for air filters for comparison.

Specifications for respirators

[edit]
HEPA
Other name(s)High-Efficiency Particulate Air, High-Efficiency Particulate Arresting filter, HEPA
Regulated byUnited States Department of Energy, Mine Safety and Health Administration, National Institute for Occupational Safety and Health
Regulation30 CFR 11, 42 CFR 84, DOE NE F 3-45
NIOSH scheduleTC-21C
Main article: NIOSH air filtration rating

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.[25] However, by definition, ANSI Z88.2-2015 considers N100, R100, P100, and HE as HEPA filters.[26]

Marketing

[edit]

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.[27] 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.[28]

Efficacy and safety

[edit]

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.[29] 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[30] and allergies is much lower with HEPA purifiers.[31]

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.[32]

Applications

[edit]
Hospital staff modelling a powered, air-purifying respirator (PAPR) fitted with a HEPA filter, used to protect from airborne or aerosolised pathogens such as tuberculosis

Biomedical

[edit]

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 ultraviolet light units or panels with anti-microbial coating to kill off the live bacteria and viruses trapped by the 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 against airborne disease transmission.[citation needed]

COVID-19

[edit]

Vacuum cleaners

[edit]
HEPA original filter for Philips FC87xx-series vacuum cleaners

Many vacuum 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 and house dust mite feces) 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" 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 a similar 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]

Some newer models claim to be better than the earlier ones with the inclusion of "washable" filters. Generally, washable true HEPA filters are expensive. 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.[33] Some manufacturers claim filter standards such as "HEPA 4," without explaining the meaning behind them.[citation needed] This refers to their Minimum 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

[edit]
HEPA filter effect inside home HVAC system: without (OUTdoor) and with filter (INdoor)

Heating, ventilation, and air conditioning (HVAC)[34] 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 in polluted cities to maintain health.[citation needed]

Vehicles

[edit]

Airlines

[edit]

Modern airliners use HEPA filters to reduce the spread of airborne pathogens 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 a pressurized 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.[35] 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.[36]

Motor vehicles

[edit]

In 2016, it was announced that the Tesla Model X would have the world's first HEPA-grade filter in a Tesla car.[37] Following the release of the Model X, Tesla has updated the Model S to also have an optional HEPA air filter.[38]

History

[edit]

The idea behind the development of the HEPA filter was born from gas 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 Army Chemical 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.[39]

The next phase of the HEPA filter was designed in the 1940s and was used in the Manhattan Project to prevent the spread of airborne radioactive contaminants.[40] 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 Laureate Irving 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.[41]

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, and integrated circuit fabrication.[citation needed]

See also

[edit]

References

[edit]
  1. ^ "GLOSSARY". HEPA Corporation. Archived from the original on 2020-04-20. Retrieved 2021-05-14.
  2. ^ "HEPA". The Free Dictionary. Archived from the original on 2020-04-20. Retrieved 2021-05-14.
  3. ^ "Efficiency of the HEPA air filter: HEPA filter quality and bypassing". Air-Purifier-Power. Archived from the original on 2020-04-20. Retrieved 2021-05-14.
  4. ^ a b c "INTERNATIONAL ISO STANDARD 29463-1—High-efficiency filters and filter media for removing particles in air". International Organization for Standardization. October 15, 2011. Archived from the original on March 8, 2021. Retrieved May 16, 2021.
  5. ^ European Standard EN 1822-1:2009, "High efficiency air filters (EPA, HEPA and ULPA)", 2009
  6. ^ American Society of Mechanical Engineers, ASME AG-1a–2004, "Addenda to ASME AG-1–2003 Code on Nuclear Air and Gas Treatment", 2004
  7. ^ a b Barnette, Sonya. "Specification for HEPA Filters Used by DOE Contractors — DOE Technical Standards Program". www.standards.doe.gov. Archived from the original on 2020-04-20. Retrieved 2019-06-05.
  8. ^ Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks (PDF). Cincinnati, OH: National Institute for Occupational Safety and Health. April 2003. pp. 8–12. doi:10.26616/NIOSHPUB2003136. Archived (PDF) from the original on February 10, 2020. Retrieved 2020-02-09.
  9. ^ Godoy, Charlotte; Thomas, Dominique (2020-07-02). "Influence of relative humidity on HEPA filters during and after loading with soot particles". Aerosol Science and Technology. 54 (7): 790–801. Bibcode:2020AerST..54..790G. doi:10.1080/02786826.2020.1726278. ISSN 0278-6826. S2CID 214275203.
  10. ^ Payet, S.; Boulaud, D.; Madelaine, G.; Renoux, A. (1992-10-01). "Penetration and pressure drop of a HEPA filter during loading with submicron liquid particles". Journal of Aerosol Science. 23 (7): 723–735. Bibcode:1992JAerS..23..723P. doi:10.1016/0021-8502(92)90039-X. ISSN 0021-8502. Archived from the original on 2021-05-16. Retrieved 2021-03-05.
  11. ^ Schentag, Jerome J.; Akers, Charles; Campagna, Pamela; Chirayath, Paul (2004). SARS: CLEARING THE AIR. National Academies Press (US). Archived from the original on 2021-01-05. Retrieved 2021-03-04.
  12. ^ Chuaybamroong, P.; Chotigawin, R.; Supothina, S.; Sribenjalux, P.; Larpkiattaworn, S.; Wu, C.-Y. (2010). "Efficacy of photocatalytic HEPA filter on microorganism removal". Indoor Air. 20 (3): 246–254. Bibcode:2010InAir..20..246C. doi:10.1111/j.1600-0668.2010.00651.x. ISSN 1600-0668. PMID 20573124.
  13. ^ Guo, Jianguo; Xiong, Yi; Kang, Taisheng; Xiang, Zhiguang; Qin, Chuan (2020-04-14). "Bacterial community analysis of floor dust and HEPA filters in air purifiers used in office rooms in ILAS, Beijing". Scientific Reports. 10 (1): 6417. Bibcode:2020NatSR..10.6417G. doi:10.1038/s41598-020-63543-1. ISSN 2045-2322. PMC 7156680. PMID 32286482.
  14. ^ a b Gantz, Carroll (2012-09-21). The Vacuum Cleaner: A History. McFarland. p. 128. ISBN 9780786493210. Archived from the original on 2021-05-16. Retrieved 2020-11-11.
  15. ^ "About HEPA". hepa.com. Archived from the original on 2020-04-20. Retrieved 2019-06-05.
  16. ^ Gupta, Shakti Kumar; Kant, Sunil (2007-12-01). Modern Trends in Planning and Designing of Hospitals: Principles and Practice. Jaypee Brothers. p. 199. ISBN 978-8180619120. OCLC 1027907136.
  17. ^ Christopherson, David A.; Yao, William C.; Lu, Mingming; Vijayakumar, R.; Sedaghat, Ahmad R. (July 14, 2020). "High-Efficiency Particulate Air Filters in the Era of COVID-19: Function and Efficacy". Otolaryngology–Head and Neck Surgery. 163 (6): 1153–1155. doi:10.1177/0194599820941838. PMID 32662746. S2CID 220518635.
  18. ^ a b Woodford, Chris (May 21, 2008). "How do HEPA air filters work?". Explain That Stuff. Archived from the original on 2020-04-20. Retrieved May 15, 2021.
  19. ^ a b da Roza, R. A. (December 1, 1982). "Particle size for greatest penetration of HEPA filters—and their true efficiency". U.S. Department of Energy Office of Scientific and Technical Information. doi:10.2172/6241348. OSTI 6241348. Archived from the original on May 16, 2021. Retrieved May 15, 2021.
  20. ^ Khan, Faisal I; Ghoshal, Aloke Kr. (November 2000). "Removal of Volatile Organic Compounds from polluted air" (PDF). Journal of Loss Prevention in the Process Industries. 13 (6). Elsevier: 527–545. Bibcode:2000JLPPI..13..527K. doi:10.1016/S0950-4230(00)00007-3. ISSN 0950-4230. Archived (PDF) from the original on February 15, 2017. Retrieved May 15, 2021.
  21. ^ Glover, Norman J. (May 2002). "Countering chemical and biological terrorism". Civil Engineering. 72 (5). New York City: American Society of Civil Engineers: 62–67. ISSN 0885-7024. OCLC 926218714. ProQuest 228557557.
  22. ^ Jonathan (August 19, 2016). "Air Purifier Acronyms - Stripping Out The Tech Jargon". Air Enhancing. Archived from the original on 2020-04-20. Retrieved May 16, 2021.
  23. ^ "Air Purifier Pre-Filter Replacement: The Prefilter Experiments". Air-Purifier-Power. Archived from the original on 2020-04-20. Retrieved May 16, 2021.
  24. ^ Perryman, Oliver (December 3, 2020). "Do HEPA filters or air purifiers remove carbon monoxide?". Dehumidifier Critic. Archived from the original on May 16, 2021. Retrieved May 16, 2021.
  25. ^ Spelce, David; Rehak, Timothy R; Meltzer, Richard W; Johnson, James S (2019). "History of U.S. Respirator Approval (Continued) Particulate Respirators". J Int Soc Respir Prot. 36 (2): 37–55. PMC 7307331. PMID 32572305.
  26. ^ ANSI/ASSE Z88.2 - 2015 American National Standard Practices for Respiratory Protection (PDF), April 2015
  27. ^ Bretag, Scott (March 18, 2020). "Air Conditioners, HEPA Filters, And Airborne Allergens". Pulse Electrical. Archived from the original on March 10, 2021. Retrieved May 16, 2021.
  28. ^ "HEPA-Type Filter: The Great Pretender". Air-Purifier-Power. Archived from the original on February 25, 2021. Retrieved May 16, 2021.
  29. ^ Christopherson, David A.; Yao, William C.; Lu, Mingming; Vijayakumar, R.; Sedaghat, Ahmad R. (2020-07-14). "High-Efficiency Particulate Air Filters in the Era of COVID-19: Function and Efficacy". Otolaryngology–Head and Neck Surgery. 163 (6): 1153–1155. doi:10.1177/0194599820941838. ISSN 0194-5998. PMID 32662746. S2CID 220518635.
  30. ^ Dunkin, Mary Anne (2010-04-30). "HEPA Filter Benefits for Allergy Relief". WebMD. Reviewed by Nayana Ambardekar. Archived from the original on 2021-03-29. Retrieved 2021-05-16.
  31. ^ "How do HEPA Filters Help in Cleaning Indoor Air - Complete Guide". Pure Air Hu. Archived from the original on 2019-10-20. Retrieved 2021-05-16.
  32. ^ Kelly, Tammy (March 14, 2018). "How Often Should A HEPA Filter Be Changed". Janitized. Archived from the original on March 8, 2021. Retrieved May 16, 2021.
  33. ^ Anand, Mohit (November 23, 2020). "Understanding the Home Air Purifier Technology in Use Today". Honeywell Connection. Archived from the original on May 16, 2021. Retrieved May 16, 2021.
  34. ^ Hvac. Merriam-Webster. Archived from the original on January 15, 2021. Retrieved May 16, 2021.
  35. ^ Smith, Patrick (July 22, 2012). "The truth about cabin air". AskThePilot.com. Archived from the original on May 6, 2021. Retrieved May 16, 2021.
  36. ^ Read, Johanna (August 28, 2020). "How clean is the air on planes?". National Geographic. National Geographic Partners. Archived from the original on May 6, 2021. Retrieved May 16, 2021.
  37. ^ "Putting the Tesla HEPA Filter and Bioweapon Defense Mode to the Test". Tesla, Inc. 2016-05-02. Archived from the original on 2021-04-27. Retrieved 2021-05-16.
  38. ^ Völcker, John (2016-04-12). "2016 Tesla Model S gets styling update, 48-amp charger, new interior options, $1,500 price increase (updated)". Green Car Reports. Archived from the original on 2021-02-24. Retrieved 2021-05-16.
  39. ^ First, Melvin W. (March 1, 1998). "HEPA Filters". Journal of the American Biological Safety Association. 3 (1). American Biological Safety Association: 33–42. doi:10.1177/109135059800300111. ISSN 1091-3505. S2CID 207941359. Archived from the original on May 16, 2021. Retrieved May 16, 2021.
  40. ^ Ogunseitan, Oladele; Robbins, Paul, eds. (2011). Green Health: An A-to-Z Guide. Los Angeles: SAGE Publishing. p. 13. ISBN 9781412996884. OCLC 793012578. Archived from the original on 2021-05-17. Retrieved 2016-12-18.
  41. ^ "The History of HEPA Filters". APC Filters. November 21, 2019. Archived from the original on March 26, 2021. Retrieved May 16, 2021.

Further reading

[edit]

9382659989

[edit]
Wikimedia Commons has media related to High-Efficiency Particulate Arresting.
Retrieved from "https://en.wikipedia.org/w/index.php?title=HEPA&oldid=1265978716"