(They all aced the high-speed test, as did many models.) The best air purifiers in our comprehensive ratings earn an Excellent or Very Good rating for particle reduction at a low speed while keeping noise to a minimum. And because these run day and night, we measure noise levels, in decibels, at every speed, and calculate annual operating costs for filter replacements and energy use to run the machine 24 hours a day. (Human hair has a diameter of 100 microns.) We use a particle counter to measure the change in particle concentration as the air purifier runs for 15 minutes at the highest speed, and then at a lower speed.

To test air purifiers, we inject smoke and dust particles into a sealed chamber and measure how well each model removes particles between 0.1 and 1 micron. On the flip side, Kollontai says that the worst air purifiers in our ratings struggle to clean the air at both low and high speeds, adding, “Oftentimes, poor performance in removing particles like smoke and dust is accompanied by a lot of noise, which can be bothersome in a living area.”

“They are able to do so at a relatively low noise level, which is great since they are operating around the clock.” “The best air purifiers we’ve tested are able to remove small particles from your air whether at high or low speed,” says Misha Kollontai, who oversees the testing of air purifiers at Consumer Reports. And that could mean a higher concentration of dust and particulate matter floating around. With the temperatures soaring this summer and the AC cranking, chances are that fresh air hasn’t been circulating much in your home.

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