When I did my first experiments, several people told me not to publish the data. “Don’t give it away for free,” they told me. “Use it to make money!” I decided then that my main goal wasn’t to make money. I almost got tricked into paying $1,000 for clean air, and I wanted to help people avoid getting tricked too, so I published the data anyway.
Of course, publishing the instructions online has made it easy for people to copy the idea. 还我蓝天 (Huanwo Lantian) was one of the first to follow in our footsteps, selling a DIY filter a few months back. They even use a screen capture of Gus’s appearance on Chinese TV on their shop:
I was curious to see how their filter works, so I decided to order one off of Taobao and put it to the test.
Now I’m in an awkward position because I found that their HEPA was not working nearly as well as my Smart Air HEPAs. It’s awkward because, if I publish the data, will people think I’m just trying to attack a competitor?
In the end, I think it’s better to publish the results and be honest about my conflict of interest. At the very least, I think people have an interest in knowing how well other DIYs work–especially when some of those websites use graphs that are lifted from my site, which can mislead people into thinking the test results are from their machines.
And as always, I’m publishing my raw data and testing methods at the end of this post, so fellow nerds can replicate my studies.
Method: My collaborator Anna used the same methods as our earlier tests in her 15m2 room. Anna did five overnight tests with the same Dylos Pro particle counter, and I calculated effectiveness as the percent reduction of particles in the room air, averaging the last three hours (more info). Then I compared the results to my earlier tests in the same room.
Here’s what I found:
Results: The 还我蓝天 DIY removed 21% fewer particles 0.5 microns and above and 11% less 2.5 micron particles than the Original.
Is it the fan? The 还我蓝天 fan is slightly smaller than the Smart Air Original, so one explanation could be that the 还我蓝天 fan is just moving less air. Anna tested that by strapping the 还我蓝天 HEPA onto our Original fan.
Here’s what I found:
Result: There wasn’t much difference. With the new fan, it was getting 4% more 0.5 micron particles and 2% less 2.5 micron particles. Thus, the fan doesn’t seem to be the reason.
Is it the HEPA? The second possibility is that the 还我蓝天 HEPA isn’t as good. Anna tested the HEPA by doing air outlet tests with a Met One GT-521, which measures down to 0.3 microns. Anna tested the air coming out of the HEPA for 10 seconds, and I averaged the results from three tests. (More details at the end of the post.)
Here’s what I found:
Results: The 还我蓝天 HEPA performed worse, about 7% lower than the HEPA standard. The major source of the 还我蓝天’s poor performance seems to be the quality of the filter.
Conclusion: In room tests, the 还我蓝天 DIY removes about 21% fewer particles than the Original DIY, and the data suggests that the reason is that 还我蓝天 HEPA is lower quality.
Now, don’t get me wrong. The 还我蓝天 DIY is making the room air cleaner. I’d rather have a 还我蓝天 than nothing. But the results show that this DIY copycat is cutting corners by using cheap HEPAs.
As always, I’m posting the raw data and more detail on the methods for fellow nerds.
For the methodology of the overnight room tests, see my earlier post. Anna ran five tests of the 还我蓝天 DIY and six tests of the 还我蓝天 HEPA + Original fan. The raw data is below.
- The Met One gives counts for particles 0.3 microns and above, which is important for HEPA tests because 0.3 microns is the particle size that is hardest for HEPAs to capture.
- The Met One uses a pump, which is better for regulating air flow than the fan in the Dylos. I think the fan is adequate for most tests, but pumps help keep a steady air flow when the particle counter is in a stream of quickly moving air (which is what happens during HEPA tests).
Cumulative numbers: All of the particle counters I’ve used display the current number of particles, but the GT-521 displays the cumulative number of particles, so the numbers keep increasing, even if the room air is getting cleaner.
To deal with that, I divided the cumulative number of particles by the number of seconds in the test.
Baseline: To get a baseline number, Anna held the particle counter about a foot away from the purifier while the purifier was off. Anna ran the particle counter for 10 seconds and divided the cumulative number of particles by 10.
Timing: The particle counter starts immediately after hitting the start button, but starting the test right away is a bad idea because we want to avoid moving during the test. Therefore, Anna hit start, moved the counter so that the inlet was about half an inch from the center of the HEPA, and then waited until the particle counter collected a fresh 10 seconds of data.
Anna set the particle counter to collect data for 10 seconds, pause for 10 seconds, and then collect data for another 10 seconds. Anna used the latter 10 seconds so that we could avoid the effect of moving the particle counter into place.
Calculation: Anna repeated all of these steps three times for each HEPA, and I averaged the results of the three tests.
Room Test Raw Data
Here is the raw data for the 还我蓝天 plus Original fan:
Fluctuations in Outdoor Air Pollution
The outdoor air seemed to fluctuate more widely in these tests than during most of my earlier tests. So to test for the effect of outdoor air fluctuations, I analyzed days where the outdoor air was above the WHO limit (25 micrograms) and didn’t fluctuate more than 15 micrograms. Data from 3/16 met those criteria, starting at 149 micrograms and fluctuating a maximum of 6 micrograms.
On that day, the 还我蓝天 removed 66% on 0.5 microns and 77% of 2.5 microns. That’s within 2% of the overall average of 67% and 79%.
I’ve had similar conclusions when I did this type of analysis in my previous tests: changes in outdoor air pollution can strongly affect data on any single day, but averaging the data over multiple days leads to surprisingly robust estimates.
HEPA Test Raw Data
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.