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 remove 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.

The meaning of PM2.5 in Air Purifier

PM2.5 refers to fine inhalable particles with dimensions of 2.5 micrometers or smaller. PM_2.5 is more dangerous because it can get into the deep parts of your lungs — or even into your blood.

Do Air Purifiers Remove PM2.5?

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, they removed over 80–90% of 0.5-micron particles from my room air in overnight tests.

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 Brownian Motion). As just one example, here’s a test from a dedicated nerd (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).

So breathe easy!

P.S. Big purifier companies often use this misconception about HEPA filters to sell you their own “proprietary” filters, like in the popular Molekule purifier.

Now that you’ve seen the data, you won’t fall for this trick!