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Craving a breath of fresh air...even while inside? There's one important feature you should look for if you or someone in your household or office is considering investing in an air purifier, report two new Mayo Clinic studies.

Whether due to the airborne nature of COVID-19 and people aiming to reduce potential transmission risk or the fact that we're now spending about 90%(!) of our time indoors, the air purifier market is booming. Air purifier sales have risen over the past year and are expected to jump by more than $8 billion over the next three years.

While there are specific certification programs that shine a spotlight on air cleaners that are proven to remove pollution from the air, there's one very specific feature you should look for in the best air purifiers, these Mayo Clinic studies say: A high-efficiency particulate air (HEPA) filter.

Since dust, mold, pollen and aerosol particles (including those possibly contaminated with coronavirus or other viruses or bacteria) can vary in size, they can be tough to catch, trap and remove from the air. But these filters can remove 99.97% or more of all of the above, according to the U.S. Environmental Protection Agency (EPA)—and confirmed by the Mayo Clinic researchers.

The medical pros conduct exercise stress tests regularly for heart patients at their facilities. For this new Chest Journal study, they discovered that even with the increased exertion and aerosols floating around the room after the heavy breathing during said stress tests, a HEPA filter was able to capture all the aerosols and reduce the time needed to clear the air between patients.

"Our work was conducted with the support of Mayo Cardiovascular Medicine leadership who recognized right at the start of the pandemic that special measures would be required to protect patients and staff from COVID-19 while continuing to provide quality cardiovascular care to all who needed it," says Thomas Allison, Ph.D., director of Cardiopulmonary Exercise Testing at Mayo Clinic in Rochester in a report published on the Mayo Clinic News Network. "Since there was no reliable guidance on how to do this, we put a research team together to find answers through scientific testing and data."

To examine this, Allison and team added a plastic tent to their lab to control and monitor the air. Laser particle counters detected aerosols circulating around the eight stress test participants as each individual rode on an exercise bike. The volunteers wore gear that measured heart rate, oxygen consumption and ventilation. As expected, aerosol concentrations increased exponentially as people pedaled and breathed more rapidly and harder, and exercise at or above 50% of the volunteer's typical resting heart rate showed substantial increases in aerosol concentration.

"I think we have proven dramatically what many suspected—that is why gyms were shut down...Exercise generates millions of respiratory aerosols during a test, many of a size reported to have virus-carrying potential. The higher the exercise intensity, the more aerosols are produced," Allison says.

Now here comes an important addition: As part of a follow-up study in Chest Journal, Allison and team tried to determine the best and quickest way to remove these particles from the air. Six new volunteers completed the same 20-minute exercise bike test as in the previous study, first without, then with a portable HEPA filter running.

"We translated CDC (Centers for Disease Control and Prevention) guidelines for aerosol mitigation with enhanced airflow through HEPA filters and showed that it worked amazingly well for exercise testing. We found that 96% plus or minus 2% of aerosols of all sizes generated during heavy exercise were removed from the air by the HEPA filter. As a result, we have been able to return to our practice of performing up to 100 stress tests per day without any recorded transmission of COVID in our exercise testing laboratories," Allison says.

Even if you're not buying an air purifier for your home gym and just use it for regular bedroom or office use, it still holds true that HEPA filters are helpful to filter out contaminants in the air. Looking to invest wisely after learning about this news? This Germ Guardian True HEPA Filter Air Purifier (buy it: $93.99, Bed Bath & Beyond) and this Honeywell HEPA Extra-Large Room Air Purifier (buy it: $209, Amazon) each have five-star reviews.