Does the Air Conditioner Bring in Dirty Air From Outside?

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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Comparing Air Purifiers

Dr. Saint Cyr recently pointed me to great tests of air purifiers from the Shanghai Consumer Protection Bureau: The reason this type of research is so badly needed is that Western research (like this report from Consumer Reports) focuses on allergens, not general industrial air pollution. Allergens are probably more … Read more

DIY Filter Compared to Expensive Filters

UPDATE: I now have directly comparable data with the DIY Filter, Blue Air, and Philips: Direct comparison tests.

In earlier posts (1, 2), I showed that you can make effective an DIY filter to clean air pollution out of your home. But just because it is effective doesn’t mean it’s as effective as the Ferrari filters. How effective is the DIY filter compared to the expensive filters?

I’ll attempt to answer this question from several angles in different posts.


Comparison with Dr. Saint Cyr’s Test

Here’s one method: comparisons with the published data from the blog of the Beijing doctor Dr. Saint Cyr (which was an original inspiration for me that air purifiers could actually reduce air pollution). He ran tests of an 11,000 RMB IQ Air and a 6,000 RMB Blue Air. I used the same calculation of effectiveness and compared his results for the pricey machines with tests of my DIY filter. Here are the results:

DIY Filter comparison

To make the comparisons more precise, I compared my filter results only to the tests he reports from the IQ Air and Blue Air on max power in his smaller room.

Dr. Saint Cyr calculated reductions based on outside air quality, which I do not think is the best method. I think it makes more sense to calculate reductions based on the room air before and after using the filter because my room air is significantly cleaner than outside air, even before using the filter. But to make the data comparable, I calculated effectiveness using Dr. Saint Cyr’s method.

Now, the comparisons aren’t perfect. Dr. Saint Cyr’s smaller room is still 6.5 meters larger than mine. He also doesn’t say how long he ran his filters and whether the doors were closed. My tests were with the doors closed and overnight, so the filter had several hours to run.

(Update: via email Dr. Saint Cyr said his tests were with doors closed and testing times of 1 hour+.)

But even if the difference in room size bumps mine down 5% and the others up 5%, that would mean 11,000 RMB and 166 RMB get you similar results. To illustrate that, I calculated a graph of how much you’re paying for each percentage reduction in air pollution:

DIY filter cost per reduction

Of course, the ideal test would use all three filters in the same room with the same particle counter. To that end, I will hopefully get the chance to borrow a friend’s Panasonic filter and test it in my home. I’ll post those results when I get them.


Conclusion: A DIY Filter can be as effective

Although the comparisons are imperfect, they show that a DIY filter is at least roughly comparable to expensive filters at a far lower cost. This illustrates my larger point: all you really need to combat particulate air pollution is a HEPA filter. In fact, Dr. Saint Cyr found the same thing in his tests of a much simpler Hunter brand HEPA filter. In a room with the doors closed, his HEPA got 91% of the pollution–similar to what I found.

For data nerds like me, I’m posting more details on my data and methods here:

Test Details:

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DIY Purifier Tests

How do I know if a DIY purifier works? I think you can break this down into two questions: Is the air coming out of the DIY purifier clean? The test results are as clear as can be. The DIY purifier shoots out very clean air. But is that enough to actually … Read more

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