tmthyliu 提问: Just built a DIY air filter, pretty excited to have clean air in the house! I was wondering though, how does this work when the air conditioner is on? Does it make a difference at all? I don’t know how ACs work but it seems they pump more air into my room (where does the air come from, outside??). Would it make sense to strap a filter on the AC unit? When you ran your tests did you have the AC on in the same room? Thanks for the amazing work!
Excellent question! I long wondered whether ACs bring in dirty outside air. I tested that question with my particle counter and the answer is no:
And a video test here:
To the two other questions:
- Does the fact that the AC is moving air influence purifier effectiveness? I suspect if you were blowing air around AND running your purifier, you might actually get slightly better performance because it’s taking that clean air and spreading it around the room. But that’s just a hunch.
- Would it make sense to strap a filter onto an AC? Yes! I wouldn’t strap a HEPA there because the AC isn’t strong enough for it, and you’d really weaken the cooling effect of the AC. But a weaker filter could help.
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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.