HEPA Filter: What do you need to know?

The below article talks about everything you need to know about HEPA Filters, along with some frequently asked questions about air quality and how HEPA filters play an important role in improving indoor air quality. 

What is a HEPA Filter?

A HEPA filter, a.k.a. “high-efficiency particulate air” filter is a mechanical air filter with enhanced capabilities to remove airborne contaminants such as toxic chemical particulates, viruses, dust, pollen, bacteria, etc. HEPA filters can filter out approximately 99.97% of airborne contaminants with a minimum diameter size of 0.01 microns (Heffernan T., 2020) (EPA, n.d.).

History of the HEPA Filter

HEPA filter, a.k.a. High-efficiency particulate air filter is a mechanical pleated air filter that was designed in the 1940s by Arthur D. Little as part of a classified government project under the Manhattan Project that was elemental to creating the first atomic bomb developed during World War 2. The invention of the HEPA filter was seen as a large step into innovative technology which would be used to separate out air particles that had been contaminated by radioactive materials. 

The findings by researchers back then pointed to the condensed nuclei of radioactive iodine to be the most toxic, therefore, designers and researchers focused on being able to create filters that absorbed solid particles formed through the condensation of liquid or gaseous matter. It was then found out that there is a higher incidence of particles with diameter sizes above 0.3 microns, due to which it was decided that the HEPA filter was consistent and efficient in removing particles that are minimum of 0.3 microns. This also lead to the conclusion that the particulate matter with a diameter of 0.3 microns was the most penetrating particle size (MPPS) The high rates of filtration, ~99% from the HEPA filter, caused it to be named as an absolute filter and was commercialized into the economy. 

Over the next few decades, the HEPA filter would be extensively studied across varying fields which contribute to its usage in various sectors from manufacturing plants, aerospace needs, pharmaceutical needs, etc. Differences and improvements in designs and materials used for the filter improved its effectivity in removing smaller particles than were previously recognized and led to the recent pleasant discovery by NASA that it could remove up to 100% of particles of sizes 0.01 microns. The HEPA filter has contributed to great successes such as the lunar landing, the introduction of silicone chips, etc. 

Filtration technology has come a long way following the discovery of HEPA filters, and these have been crucial to understanding the workings of different forms of matter, as well as played important roles in significant milestones for human civilization over the years. 

(C-Vac, n.d.) (Heffernan t., 2020)

HEPA Efficiency 

HEPA filters are tested meticulously to ensure effective filtration and ventilation, especially since it would be used in scenarios where toxic airborne contaminants maybe on the rise. Therefore, the standards that HEPA filters are tested against ensures that any filter employed is able to filter out ~99.97% of toxic airborne particles. Some tests that are taken up include Mil-Std-282 or the “hot” or “thermal” DOP test, IEST-RP-CC-007, etc. HEPA filters stand out from other mechanical filtration systems through different elements such as:

  • HEPA filters have a unique fiber glass filter media that is capable of trapping particles through different methods such as interception, impaction, or diffusion. 
  • The finely woven nature of HEPA filters provide it with a higher efficiency rate in comparison to other filters; and is capable of removing a wide variety of pollutants such as pet dander, bacteria, pollen, dust, etc.
  • The efficiency of HEPA filters allows for it to be used in areas where clean air is critical such as clean labs, hospitals, etc. 

It must be noted that with the advancement of technology at the rates we observe, various companies have tried to manufacture HEPA filters under various guises of it being HEPA-like, HEPA-type, hospital grade HEPA, etc. However, these filters are not held to the same standards as the traditional HEPA filters, and could just provide with filtration from 90 to 99% of airborne contaminants of larger sizes (C-Vac, n.d.). 

Maintaining a HEPA Filter 

HEPA filters vary in their design depending on manufacturers, however, in mist cases it is a rigid frame into which a closely pleated filter medium is inserted and sealed. Traditionally, HEPA filters use all-glass paper composed of extremely large number of randomly oriented micro-fibers to achieve its effectiveness of adsorption of sub-micron particles. The folded structure of the medium used is aimed towards increasing the surface area of the filter, forcing for the particles to keep colliding with the filter, undergoing repeated filtration, giving out cleaner air. 

HEPA filters are ridiculously efficient in removing airborne pollutants, however, once they reach saturation, these pollutants will start dispersing from the filters, undoing all the work done to improve the air quality. The efficiency of the filter only remains as long as the filter is able to effectively absorb particles through the weaves of the filter medium, and once it is clogged by previously filtered debris, the filter ecomes less efficient in maintaining adequate air quality levels. It is for this reason that manufacturers and other experts recommend changing filters periodically, so as to ensure optimal efficiency. 

While replacing HEPA filters may seem like a arduous and time-consuming process, and even seen as unsustainable in the long-run; it is the healthier option as washable HEPA filters may not have the same efficiency as the replaceable ones. HEPA filters must not be washed unless it is specifically tagged as being reusable or washable, normal HEPA filters can be lightly dusted, broomed, or vacuumed to remove any excess or large volumes of dust that maybe hindering its productivity, but it usually must not be washed, unless it has been specifically stated by the manufacturer as being safe. 

It must be noted that EPA states that HEPA filters in recent times have been made by more sustainable materials such as coarse glass fibers, coated animal hair, vegetable fibers, along with other synthetic fibers (such as polyester or nylon), synthetic foams, metallic wools, or expanded metals and foils. These materials are often compressed to form the mediums that is used for the HEPA filters. Therefore, there is hope in the future as to the sustainability of the filters that is used, and there are researches around the world that study the efficacy of HEPA filters against varying kinds of pollutants, and how it can be used to create a healthier and safer environment for all. 

Minimum Efficiency Reporting Values (MERVs)

The MERV rating was developed as a test method by the American Society of Heating, Refigerating, and Air Conditioning Engineers (ASHRAE). A MERV rating shows the capability of different filters to remove airborne pollutants, and denotes the efficiency of the filter to remove particles of varying sizes. The MERV rating is an industrial standard used to identify the standing of a filter in terms of comparison of efficiencies, filter trapping capabilities, etc. The higher is the rating, the higher is the efficiency of the filter to trap toxic contaminants. All HEPA filters have a rating of 17 or higher on the MERV scale (Iso-Aire, n.d.). 

Source: EPA, n.d. 

MERV RatingAverage particle size (in microns)Efficiency
1 – 43 – 10< 20%
63 – 1049.9%
83 – 1084.9%
101 – 350% – 64.9%
3 – 10≥ 85%
121 – 3 80% – 89.9% 
3 – 10≥ 90%
140.3 – 175% – 84%
1 – 3≥ 90%
160.3 – 1≥ 75%

Some common standards that are used for HEPA filters

As previously mentioned, HEPA filters have varying standards to adhere to, depending on the manufacturers codes, as well as to industrial codes. In general, below is a list of specifications or standards that are most commonly used to assess the efficacy of a HEPA filter. 

Source: C-Vac, n.d.

ANSI Z9.2 Fundamentals Governing the Design and Operation of Local Exhaust SystemsThis code is often applied to vacuum cleaners as the exhaust system would not usually specify any HEPA filtration efficiency, construction or test requirements. 
ASME N509Nuclear Power Plant Air-Cleaning Units and Components
ASME N510Testing of Nuclear Air Treatment Systems
ASME NQA-1Quality Assurance Requirements for Nuclear Facility Applications
ASME AG-1Code on Nuclear Air and Gas TreatmentThe standards were initially written for nuclear power plants and nulear fuel cycle facilities, and includes standards for design, fabrication, inspection, and testing of air cleaning equipment and components. Although special filters can be manufactured to this code, standard HEPA filters, such as those used in most vacuum cleaners do not meet this code. 
Mil-F-51068Filters, Particulate (High-Efficiency Fire Resistant) This military specification covers eight sizes and types of HEPA filters which must be qualified to be furnished under this specification. 
Mil-F-51079 Filter, Medium, Fire-Resistant, High-EfficiencyA military specification standard covering one grade of high-efficiency, fire-resistant, filter medium normally used for compliance with Mil-F-51068
UL-586 Standard for High-Efficiency, Particulate Air Filter Units It is a standard for HEPA filters including details and requirements on construction and environmental test requirements where samples are tested for minimum performace characteristics. 
UL-900Test Performance of Air Filter Units This standard particularly investigates the combustibility and smoke generation of a filter, not the ability of a filter to remove airborne particles.

Frequently Asked Questions (FAQs): What is a HEPA filter?

Can HEPA filters remove disease-causing viruses such as the COVID-19 virus? 

Various studies conducted under different settings such as hospitals with patients or controlled circumstances show that HEPA filters, HVAC filters, etc. are capable of removing airborne disease-causing viruses such as the coronavirus. A study conducted found that the coronavirus is sized at 0.125 microns in diameter (Heffernan T., 2020), which falls squarely into the filtering capacity of HEPA filters, as proven by NASA through their recent studies that showed that HEPA filters are capable of removing approximately 100% of particulates of 0.01 microns, and this proves effectively that it has startling efficiency to remove airborne viruses that can cause COVID-19 as well as absorb any aerosolized droplets from the carriers of the virus. 

Should HEPA filters be cleaned or replaced; and which option is more effective in improving indoor air quality? 

In general, it is recommended by most companies to replace your HEPA filter than to clean them, in order to ensure the efficiency of the product. Generally, HEPA filters are not washable, unless they are specifically labeled as being washable. A HEPA filter can be dusted, brushed, or lightly vacuumed to remove the excess dust, and usually, it wouldn’t tamper with the product’s efficiency. 

However, washing these filters could cause a stretch in the weave of the material, allowing for airborne pollutants to pass through the filter, thus decreasing indoor air quality. While washable filters may not perform immediately poorly, it is to be noted that there is no industry standard for washable HEPA filters, due to which it is recommended to replace your HEPA filters for maximum efficiency (Grabianowski E., 2018). 

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C-Vac. (n.d.). The Basics of HEPA filtration. HEPA Construction. Viewed on 01-13-2022. http://www.c-vac.com/construction.html  

C-Vac. (n.d.). The Basics of HEPA filtration. HEPA Efficiency. Viewed on 01-13-2022. http://www.c-vac.com/efficiency.html 

C-Vac. (n.d.). The Basics of HEPA filtration. History of the HEPA Filter. Viewed on 01-13-2022. http://www.c-vac.com/history.html 

Environmental Protection Agency (EPA). (n.d.). What is a HEPA Filter? Indoor Air Quality (IAQ). Viewed on 01-13-2022. https://www.epa.gov/indoor-air-quality-iaq/what-hepa-filter-1 

Grabianowski E. (2018, September 26). How to clean a HEPA Filter and what you should know beforehand. Molekule Blog. Viewed on 01-13-2022. https://molekule.science/how-to-clean-a-hepa-filter-and-what-you-should-know-beforehand/ 

Heffernan T. (2020, November 18). Can HEPA Air Purifiers capture Coronavirus? Wirecutter. NY Times. Viewed on 01-13-2022. https://www.nytimes.com/wirecutter/blog/can-hepa-air-purifiers-capture-coronavirus/ 

Iso-Aire. (n.d.). What are the differences between a MERV 13 and a HEPA Filter? Viewed on 01-13-2022. https://www.iso-aire.com/blog/what-are-the-differences-between-a-merv-13-and-a-hepa-filter 

Sobieski. (n.d.). HEPA Filteration: Upper echelon of indoor air quality options. Viewed on 01-13-2022. https://www.sobieskiinc.com/blog/hepa-filtration-upper-echelon-indoor-air-quality-options/  

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