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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:

  1. 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.
  2. 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.

Does Air Conditioning Bring in Dirty Outside Air?

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A question that I get asked often (and that I have always wondered about) is whether my wall-mounted air conditioner is bringing in dirty air from outside. If so, it’d be safer not to use it, especially on really bad days.

My short answer is no. To explain why, I’ve got three points of evidence:

  1. How air conditioners work. Regular wall-mounted air conditioners in China do have a unit outside connected with tubes to the inside, but that tube is not bringing in outside air. It’s passing coolant, and letting heat escape outside.

So where does the air it’s blowing come from? If you look around your air conditioner, you’ll probably discover that it works like mine: it brings air from the top, runs it over the cooling coils, and blows it out the front. It’s recycling indoor air, not bringing in outdoor air.

  1. Tests of the air coming out of the air conditioner. (See a live test here.) I’ve held my particle counter up into the air coming out of my AC unit, and it’s no different from the ambient room air. I’ve also compared that air to outside air on very dirty days, and the air coming out of the AC is nowhere near as dirty as outside air.

(I did this test when I had just turned on my AC. If the AC were bringing in dirty air and I were to test the exhaust after I had been running the AC for a long time, then my whole room would be dirty, not just the exhaust.)

  1. Tests of the ambient room air before and after turning the AC on. Results? AC makes basically no difference.

Here’s what happened in one test after turning the AC on:


And here is the average effect over 7 different tests in my bedroom.


In each test, I ran my particle counter for 30 minutes to get a baseline. Then I turned on the AC for 30 minutes. Here I’m comparing the numbers just before I turned the AC on and 30 minutes later. As you can see, there’s basically no effect. If anything, PM 2.5 goes down slightly. This could be because of the coarse filter in the AC unit. Or it could be random variation.

Conclusion: If it’s hot outside, don’t sweat it. Use your AC.

Central AC: I should note that these tests are of wall-mounted AC units in China. Central air conditioning may work differently.

For those interested, I’m pasting the detailed data below.