# Does AC Bring in PM2.5?

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It’s that time of year when Beijing is starting to sizzle, if this person roasting fish on her car is any indication.

But air pollution isn’t taking a summer break here.

It’s not airpocalypse levels, but the buildings nearby are starting to look fuzzy.

That made me wonder. If I turn on the air conditioner, is that going to bring in dirty outdoor air?

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 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.)

### #2 Tests of the ambient room air before and after turning the AC on

In each test, I ran my particle counter for 30 minutes to get a baseline. Then I turned on the AC for 30 minutes.

Results? AC makes basically no difference.

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

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.

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

### #3 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 circulating coolant.

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.

### What if I have a central air system?

If you’re in a room with a central air system, things get trickier. You’ll have to do some research to find out what air is coming out of those vents.

Some systems bring in a lot of outdoor air. Some bring in none. Some systems filter that air. Some don’t.

Here’s how you can get to the bottom of it:

1. Check out the system (if you can). Does it have filters? Does it have an outdoor air intake?

2. Ask the management (if they even know!).

3. Test the air coming out. Here’s how I’d recommend doing it.

Smart Air provides home tests for particulate and gas pollutants.

Open Data

I made the original test data available for fellow nerds here.

Caveat about me: I’m not an expert in air pollution. I’m a social psychologist.

But living through Beijing’s pollution got my nerd spirit running about air quality, and I soon got into making low-cost DIY purifiers and started a social enterprise (Smart Air) to help get low-cost purifiers to more air-breathers. With the help of dedicated nerds and volunteers, Smart Air ships simple, no-bull purifiers to air-breathers in IndiaChinaMongoliaNepal, and the Philippines.

Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.