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 clean the ambient room air? For example, if you have a really tiny filter and a huge room, the filter could work properly but still not be strong enough to make a difference.
Answering the second question is more difficult because you need a controlled environment. You can’t open and close windows during the test, and you need to test the air for a longer period of time. Fortunately for you, I’m a nerd, and I’ve been doing these tests for fun for the past few weeks.
Testing with my Particle Counter
According to my particle counter, here’s what the filter did in an hour:
I’ve also tested the effect by running tests with the particle counter on hourly mode. These tests show that the downtrend continues over several hours:
(Note that the hourly test and minute test are on different days.)
The particle counter also gives data on 0.5 microns – even smaller particles. Here’s what that looks like over eight hours:
From my perspective, air purifier companies like IQ Air are taking advantage of how little we know about air pollution. We also need expert instruments to tell whether the filter is working or not. When consumers don’t know how to assess the products we buy, we often use price to tell us whether the product is good. That happens with expensive wines all the time. I’m convinced you can breathe safe air in China for far less than filter companies want you to believe.
Finally, for data nerds like me, I’m including more details on the tests here:
The test above was done starting at 11:30 pm (I’m a night owl) on 6/16/2013. The outside AQI in Beijing was 230 according to the US embassy’s AQI Twitter feed. (The outside air improved the next day, but results were similar on a later test where AQI went up slightly from 195 to 202 during the test. Details to follow here.)
By comparisons of my particle counter’s tests of outside air to US embassy AQIs, an AQI of 230 would convert to about 2,650 on the 2.5 micron count on my reader. Remember, the particle counter gives the raw number of particles 2.5 micrometers and above per .01 cubic feet. The US embassy takes mg/m3 and converts that to an AQI. Therefore, the numbers are different, although they correlate.
I did the test in my bedroom with the doors and windows closed. The room is 13.5 meters squared, with two windows.
Astute readers have asked whether I let the particle counter run a bit to get a stable reading before turning on the filter. The particle counter tends to take a 5-10 minutes to get stable readings. To be conservative, I gave it about an hour:
(Update: This question is answered with much more data in my new tests where I left the particle counter on for 6 days, and I turned the DIY Cannon on a timer to turn on every day for 2 hours.)
Explaining the results
The spike at the top was when I entered the room to turn the air purifier on and reset the machine, so it may just be noise or it may be the dust I kicked up by walking around. A more stable reading for that time would probably be about 230.
The uptrend prior to turning the filter on may have been because I was running my dehumidifier prior to the tests, and I have some small filters in that. (Yes, for some reason my house gets very humid–at times over 80%–despite the fact that Beijing is a semi-desert. I think it’s a problem with the plumbing system.)
Regardless, this data suggests that the effect of the filter was NOT a confound of calibration.
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.