When I was coughing through Beijing’s airpocalypse and thinking about buying an air purifier, I wanted to know something first–do air purifiers really get the small particles of PM2.5?
I wondered because I saw lots of air purifier tests on pollen, but pollen is generally 10 microns or larger, whereas I was really concerned about the stuff below 2.5 microns.
So I went out and bought a laser particle counter. This one tests down to particles 0.5 microns and above.
Then I set up a simple DIY purifier in my bedroom in Beijing like this:
And here’s what happened to the 0.5 micron particles in my room after I turned it on:
I’ve found similar results with three other major purifiers I’ve tested. On average,.
What about even smaller particles?
For the longest time, I thought the effect stopped there. When I read about HEPA filters (the filters in these machines), they always said they capture particles 0.3 microns and above. So I figured they just don’t get smaller particles.
Well it turns out I was flat wrong. It’s actually easier for HEPA filters to capture particles smaller than 0.3 microns (thanks to ). As just one example, (like me) who has a particle counter that measures down to 0.01 microns. He tested seven major purifiers for 10 minutes.
The different purifiers he tested removed about as much of these ultra-fine particles (light blue) as larger particles (dark blue).
Bottom line: HEPA purifiers remove PM2.5—even the smallest particles of PM2.5.
So breathe easy!
P.S. Big purifier companies often use this misconception about HEPA filters to sell you own their own “proprietary” filters, like in the.
Now that you’ve seen the data, you won’t fall for this trick!
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.