A fox running into a hedge has a better chance of getting through than a virus particle through a HEPA filter. What may look and feel like paper is composed of hundreds of layers of tangled fibers dense enough to trap small particles yet porous enough for air to pass through.
HEPA stands for high-efficiency particulate air, and a HEPA designation is the gold standard of air filtration used in hospitals, clean rooms and in the homes of allergy sufferers and the smoke sensitive.
Air filter efficiency (μe) can be expressed as:
μe = nt / nu = (nu - nd) / nu
where:
nt = particles trapped
nu = particles upstream
nd = particles downstream
A HEPA filter’s remarkable effectivity in filtering out small particles the size of viruses has aroused public interest in the time of COVID-19 but how the filter actually works is usually missing or incorrectly reported by popular media. Therefore, most think of filters as sieves impeding small particles by collision or mechanical entrapment. But HEPA filters work because of the physics of diffusion where weak attractive forces of the thin fibers are just strong enough to pull in small particles. The microscopically thin fibers are large relative to small particles the size of viruses. The fibers pull the particles from their path and they get stuck to the surface of the fibers.
A Little History
HEPA filters predate the pandemic. During World War II, the Allies found that the carboard made of cellulose and asbestos in captured German masks did a terrific job of removing chemicals and smoke while still being porous enough to let soldiers breathe. The British copied the formulation for their own masks. The U.S. military made room air filters out of the gas mask filter medium, calling it “collective protection.” The Manhattan Project, which led to atomic and nuclear bombs, required a need to filter out radioactive particles and may have been the first to achieve 99.97 percent efficiency. The HEPA designation appears as a trademark in the 1950s but became a generic term.
Fast forward to the present and the COVID-19 pandemic has made HEPA the darling of filters. It is able to block particles as small as the SARS-CoV-2 virus, the cause of disease.
Homes, Hospitals and Clean Rooms
A HEPA media filter cabinet as would be used in a hospital or clean room. (Image courtesy of IAQSource.com.)
The HEPA designation corresponds to a minimum efficiency reporting value (MERV) rating of 16. By contrast, the ordinary dust filter found in homes has a MERV rating of 1 to 4. It is more effective in preventing damage to your heater or air conditioner than your lungs.
Hospitals use HEPA filtration systems but not everywhere. The CDC sort of recommends HEPA filters, saying HEPA filtration may be indicated for special-care areas of hospitals.
HEPA filter is a system. At its heart is the filter medium, most commonly a white and paper-like material that on closer inspection reveals a tangle of glass or plastic microfibers. A common material is borosilicate, which has fibers with diameters ranging from 2 to 500 nm. The total thickness of the filter medium is under 0.508 mm (500 million nm) thick1. The fibers are produced by a rapidly moving nozzle from which the molten medium quickly dries in the air. The filter medium is folded like an accordion and put in a rigid frame. The frame must be tightly secured in the filter system. A fan pulls air through the filter medium.
The construction technique, material, length and diameter of the fibers are left to individual manufacturers. Only the efficiency is regulated. A filter medium can be made of other materials. NASA uses dense beds of charcoal particles in space vehicles to get HEPA-level efficiency. In order to get a HEPA designation in the U.S., a HEPA filter must be able to remove at least 99.97 percent of the particles in the air, whether they be dust, smoke, bacteria or viruses of 0.3 µm or 300 nm mass median diameter (MMD). The European Committee for Standardization is much more finicky. It has five grades of HEPA, H10 through U17, with H12 being equivalent to the American HEPA designation and the most effective, U17, achieving 99.999995 percent efficiency2.
A 2016 NASA study, which compared HEPA filters with fibers to packed granular material (charcoal) filters, found HEPA filters to be effective in removing particles the size of the SARS-CoV-2 virus with better than 99.97 percent efficiency. The NASA study did not address viruses specifically.
The SARS-CoV-2 virus, the cause of COVID-19, has a mean body diameter between 65 nm to 12 nm with growths that extend 9 to 12 nm from the surface of the main body.
HEPA filters may not be suitable for all HVAC systems, warns ASHRAE, as not all fans may be powerful enough to handle the pressure drop across the more tightly packed filters and the special mounting required to prevent leaks.
HEPA in Consumer Products
Some vacuum cleaners use HEPA filters, such as Dyson and certain Phillip’s models.
Americans spent $350 million on air purifier units last year, according to the Wall Street Journal. Sales have tripled in the last 20 years. Now, 14 percent of all homes have at least one air purifier. The pandemic has resulted in a rush to buy HEPA. However, the effectiveness of HEPA filters has come under scrutiny. A Johns Hopkins study found that they did not reduce users’ symptoms. Also, Alpine Industries, which sells its Living Air air cleaner door to door for $600, was challenged in court by the FTC. Consumer Reports found a hang-around-your neck air cleaner to barely reduce the particles in the air.
HEPA filters are also used in high-end vacuum cleaners. Some sell for more than $800, such as models from Miele, Dyson and Shark, although one may find a Kenmore for around $300.
Will HEPA Filters Guard Against COVID-19
A homemade solution? Not necessarily. A rush to protect against COVID-19 has led to many DIY approaches. But leakage around the filter could let too much nonfiltered air through. Also, a normal box fan may overheat and burn out trying suck air through HEPA’s densely packed fibers. (Image courtesy of Pinterest.com, not the author’s home)
The SARS-CoV-2 is not a hardy virus. Outside the host’s body, it cannot survive for long in a dry environment because of an external lipid layer. It’s spread primarily through the air expelled (exhaled, coughed or sneezed) from the breathing passage of an infected host by viruses in droplets. Most of the big droplets soon fall to the ground within the nominal safe distance (6 ft or 2 m) but the smallest can stay airborne longer—up to 3 hours according to the New England Journal of Medicine)—which is enough time to go greater distances and be caught by air filtration systems. As the smallest droplets are two orders of magnitude bigger than the virus itself, a lesser filter (lower MERV rating) may be deemed sufficient except that droplets get smaller fast. A 20 nm droplet will shrink to half of its size in less than half a second at room temperature3. The smallest droplets exhaled, on the order of 50nm, should leave the SARS-CoV-2 naked and exposed but with enough time to get into a host unless intercepted by a HEPA filter and removed from the air.
HEPA filters, with their nano-porosity, at best can be considered an ultra-safe option for the SARS-CoV-2 virus, the cause of COVID-19. Although, it is conceivable that enough virus particles could be floating around in the air either in very small droplets or by themselves around a source of infection, we found no studies conclusively found them to stop the spread of COVID-19. Therefore, a recommendation to use HEPA filters is made from an overabundance of caution.
But, it can’t hurt, right?
Studies show dilute amounts of the virus, such as a thousand floating around a room, could be considered a small enough “viral load” that results in no infection or an infection so mild the victim has no symptoms. So, in the end, although HEPA may not be have proven themselves significantly slowing or stopping the spread if COVID-19, their use certainly cannot hurt.
References
1Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials, J.L. Perry, Marshall Space Flight Center, Huntsville, Alabama; J.H. Agui, Glenn Research Center, Cleveland, Ohio; R. Vijayakumar, Aerfil LLC, Liverpool, New York
Can HEPA Air Purifiers Capture the Coronavirus?, Tim Heffernan, New York Times, July 9, 2020
2EU EN standard 1822 vs US HEPA filters, Stack Exchange
3Dynamics of Airborne Influenza A Viruses Indoors and Dependence on Humidity, Wan Yang, Linsey C. Marr, PLOS ONE, June 24, 2011