Some purifier companies recommend running your purifier all day. But is that really necessary? If so, isn’t that a big waste of your filters and energy?
To get to the bottom of it, I set a Smart Air Cannon on a timer to turn on for two hours everyday in a 13.5m2 Beijing bedroom:
I put a particle counter in the room to take measurements every minute. I did the test while I was on vacation, so there was no influence of me opening and closing doors.
After six days, I came back and saw how long it took to the Cannon to clean the air each time it came on. Here’s what six days of data looked like for the small .5 micron particles:
Over the six test days, the air in Beijing became progressively worse. But on each day, it was clear when the Cannon turned on and off. The dropoffs were sharp, showing the Cannon was working quickly.
I averaged over the six test days to find out how long it took the Cannon to clean the air on average.
On average, the Cannon cut 0.5 micron particulate in half in 10 minutes. By 20 minutes, it removed 80%.
Bottom line: Powerful purifiers like the Cannon clean the air very quickly, so I see no need to run the purifier while I’m not at home.
Can I turn it off while I sleep?
The data can also answer another question a few people have asked me: “I don’t want to hear the fan while I sleep, so can I run it for an hour and then turn it off while I sleep?
In the tests, the air got dirty very quickly after the Cannon turned off (even though the windows and doors were closed). Dirty air is entering our homes constantly, even though we can’t see it.
Bottom line: I do NOT recommend turning the purifier off while you sleep.
As always, I’m publishing the raw data and more details on the methods below. I’ll also be publishing data from similar tests in a much larger room (30.5m2) and for tests when I’m at home and moving in the room.
Methods: I used a regular timer to have the Cannon turn on between 3pm and 5pm every day. I didn’t choose those two hours for any particular reason.
Room: The room is a 13.5 m2 bedroom in the Chaoyangmen neighborhood of Beijing. The doors and windows were closed at all times.
I did the tests while I was on vacation, so it is a very controlled test, although reductions may take long if people are opening doors and coming in and out of the room. For that reason, I’ve repeated the tests while I was at home, and I’ll be publishing that data soon.
Data: Because the raw data file is very big, I’m making it available as a download here.
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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.