High-efficiency particulate air, also known as high-efficiency particulate absorbing and high-efficiency particulate arrestance, is an efficiency standard of air filter. 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% (European Standard) or 99.97% (ASME, U.S. DOE 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. See the Mechanism and Specifications sections for more information.
HEPA was commercialized in the 1950s, and the original term became a registered trademark and later a generic term for highly efficient filters. HEPA filters are used in applications that require contamination control, such as the manufacturing of disk drives, medical devices, semiconductors, nuclear, food and pharmaceutical products, as well as in hospitals, homes and vehicles.

Can HEPA Air Purifiers Capture the Coronavirus?

The virus that causes COVID-19 is approximately 0.125 micron (125 nanometers) in diameter. It falls squarely within the particle-size range that HEPA filters capture with extraordinary efficiency: 0.01 micron (10 nanometers) and above. Many media outlets have incorrectly stated that HEPA filters don’t filter below 0.3 micron and therefore could not capture airborne coronaviruses. That’s wrong. This NASA study of HEPA filtration is quite technical, but the graph on page 7 and the preceding paragraph do a good job of explaining why HEPA filters are actually most efficient—almost 100% at 0.01 micron—at capturing ultrafine particles below the 0.3-micron HEPA test standard.

But that doesn’t necessarily mean an air purifier will protect you. As of June 16, 2020, the position of the CDC is that the coronavirus is primarily transmitted by person-to-person contact and by contact with virus-laden droplets expelled through coughing and sneezing. This is where the definition of “airborne” gets tricky. As we report on this topic in real time, we can only go with the most recent evidence, and it seems clear that community spread can occur even when people are taking a reasonable level of precaution—whether in churches or among Major League Baseball teams.

That’s a key reason HEPA purifiers must not be considered a first line of defense against the COVID-19 virus. “The big thing with trying to say that a HEPA filter would do any good is whether you’re getting anything to the filter or not,” said Kathleen Owen, a consulting engineer with nearly 40 years of experience in air filtration. “If it turns out—and this is the big if; I’m not sure you should even mention it—but if there’s stuff that’s getting into the air, HEPA would catch it.”

What’s less clear is whether a HEPA purifier could catch the virus prior to the point of infection, or for that matter, what level of exposure to the virus causes an infection to begin with. It’s possible HEPA purifiers will prove only marginally useful in the fight against coronavirus. We are following this topic closely, and we appreciate our readers’ ongoing support in helping us maintain accurate, current information.

For the most recent guidance, we recommend that you continue to follow the CDC’s advice, including social distancing, wearing a face covering outdoors, washing your hands frequently, and treating frequently touched surfaces with disinfectants.

Tim Heffernan

Effective for Global Trends

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