Thomas Talhelm

Thomas is an Associate 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 across the world breathe clean air without shelling out thousands of dollars for expensive purifiers.

Q&A: Do ACs Bring in More Dirty Air?

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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 …

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Are Particle Counters and Government Machines the Same?

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My particle counter is a beast–I’ve loved it. But I’ve always wondered how the counts of laser particle counters like mine:     …compare to the measurements of the huge stationary air quality monitoring stations that governments use, like this one in New Zealand:     If you look at …

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Smoking is Worse than Outdoor Air Pollution

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A while back, I posted data I collected from places around Beijing showing that indoor air is consistently cleaner than outdoor air. When I analyzed that data, I excluded places that allow smoking, but I’m posting the data here now: Even on days where outside air was bad (AQI ~ …

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Particle Counting in the Atlantic

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Particle Counting and the DIY workshop made it into the Atlantic. Hooray for getting the word out about how to protect your health in China without shelling out big money.     More workshops are in the planning stages. Stay tuned.

Does the Air Conditioner Bring in Dirty Air From Outside?

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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.

air conditioner

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.

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How Safe Is Indoor Air?

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I recently did some research on whether indoor air is as bad as outdoor air. Before that, I had a conversation with a friend in Beijing that went something like this:

Friend: I’m not sure if I can make badminton tomorrow. I have a basketball game in the day.

Me: Oh man, do you play outside?

Friend: Nah, it’s inside.

Me: Oh, phew. Good.

Friend: Wait, why do you say that?

Me: Oh, the air is way worse outside. I used to feel like I had asthma after playing basketball outside.

Friend: Really? No, they’re not that much different. I saw it’s just 20% different.

 

Seeing as how nerds cannot let matters of fact go, I started using my particle counter to take measurements of inside and outside air at different locations around Beijing. This answer is important: it tells you if it’s any safer to exercise indoors and how much damage you’re doing to your lungs by choosing that seat outdoors at your favorite cafe or restaurant.

 

The Test

So I took measurements in six locations around Beijing, in apartments, cafes, and my gym. I only chose bad days (pollution concentration above the WHO standard of 25), and I avoided days where it rained (because rain can cause quick changes in air quality). Here’s what I found:

1indoor

On average, indoor air had only 36% of the pollution outdoors.

 

Things were a little worse for the smaller .5 micron particles, but still much better than outside:

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On average, indoor air had only 51% of the .5 micron particulates of outside air. My guess is that the .5 micron data was worse than 2.5 micron data because it’s easier for smaller particles to get into your home and stay suspended in the air.

There is a lot of variation between places. For the 2.5 micron particles, the locations varied from 14% to 58%. Dr. Saint Cyr also found significant variation between two apartments he lived in, 50% to 70%.

 

Conclusion: Indoor air is relatively safer

In terms of particulate pollution, you’re safer snagging an indoor seat and working out indoors, particularly on bad days (I’ve seen some argue that we are particularly vulnerable when we work out because we breathe more deeply than normal).

But remember that doesn’t mean indoor air is safe, just better than outside. For example, if your air at home had 40% of Beijing’s concentration last night at 11pm (8/15), you would’ve had 64 g/m3 in your home, which is more than twice the WHO standard of 25.

 

As usual, I’m posting more on my methods and raw data below.

 

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Creative Workarounds DIY Purifiers

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Adam in Suzhou sent me a picture of a creative solution to a filter problem. The fan he had at his house had an opening that was slightly larger than the HEPA. So Adam cut out a piece of cardboard to close up the edges. Great idea!     Adam …

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Q&A: How Long Can HEPA Filters Last?

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passport4adragon 提问: Hi, I’m in Shanghai and recently put together your DIY purifier and bought the same fan and filter you suggested – I think it’s working! A couple of questions – I bought the exact same filter you did on Taobao – how often should it be replaced if …

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Smart Air low cost purifiers

Smart Air is a social enterprise and certified B-Corp that offers simple, no-nonsense air purifiers and provides free education to protect people from the harms of air pollution. We are proud to be the only certified B-Corp dedicated to fighting air pollution.

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