Air purifiers with HEPA filtration efficiently capture particles the size of (and far smaller than) the virus that causes COVID-19, and many readers have asked whether air purifiers can help protect against infection. The answer is yes, in theory, but as of July 2020, it remains unclear how much practical impact the machines could have. Dating back to April, researchers have expressed concern that the virus may be transmitting through the air, among other ways. In early July, a significant number of experts argued in a letter to the WHO that the virus is airborne; since then, the WHO has begun to acknowledge the possibility.
Back to purifiers: 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.