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.
2. Tests of the air coming out of the air conditioner.
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 (See a live test here). 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.)
3. 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, the larger 2.5 micron particles go down slightly. My guess is this is because the coarse plastic filter in the AC unit captures up some large particles.
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.
Here’s an Excel chart with my original data, including test times, outside AQI as measured by the US embassy’s Twitter feed, and (at times) outside measurements with my particle counter. Remember that the scales between these two types of counters are completely different.
If the AC were bringing in outside air, I would expect the largest effects on really bad days. But if you look at the tests on the very bad days (8-15 was the worst), the numbers are no different.
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.