When the “airpocalypse” struck Beijing, I got out my credit card to give $1,000 to IQ Air. But the price just didn’t seem right. I soon learned that HEPA filters are what capture particulates (even in the expensive machines), and they were invented way back in the 1940s. So I made my own and started doing tests.
Could a fan and a HEPA remove as much particulate as an IQ Air? The IQ Air is one of the most expensive purifiers on the market, and it advertises “the cleanest air guaranteed.” On top of that, it uses more than twice as much energy as the DIY Cannon, so surely it can capture more particles?
Thankfully, a kind soul donated his IQ Air Health Pro (13,900 RMB), and I ran 11 room tests on the highest setting. Then I compared it to a Blue Air 203/270E (3,600 RMB), Philips AC4072 (3,700 RMB), Original DIY (200 RMB), and Cannon (450 RMB).
I tested all of the purifiers in the same 15m2 bedroom in Beijing, with the same particle counter, for the same amount of time (overnight). To calculate effectiveness, I compared the number of .5 micron and 2.5 micron particles before turning on the purifier (baseline) to the average of the last four hours before I woke up. (More methodological details here.)
Over 11 tests, the IQ Air removed 91% of the .5 micron particles and 95% of the 2.5 micron particles. That’s good, but not any better than the Cannon:
I was particularly surprised because the IQ Air was noticeably louder than the Cannon:
With this new data, we can use my earlier tests in the same room to compare the DIY to the three major brands:
All of the filters significantly reduced particulates, but the 470 RMB Cannon removed as many particles as the highest-performing big brand. Even the 200 RMB Original DIY was 6% behind the Blue Air on the .5 micron particles and 4% behind on the 2.5 micron particles.
More money = more effectiveness?
Among the different brands, there seems to be no relationship between price and particulate removal. The cheapest name-brand purifier was the Philips, and it removed the most particulates:
According to his data, 6 RMB 3M masks like the kind I use now outperform the 349 RMB Respro masks I used to use. With the 3M 9501, I can buy 58 masks for the price of one Respro AND block a higher percentage of particles:
Bottom line: clean air doesn’t have to cost an arm and a leg, whether it’s in your home or strapped to your face.
As always, I’m posting the raw data and more details on the testing methods below for fellow nerds. And in a follow-up post, I’ll talk about the other important ways to compare purifiers: HEPA replacement costs, gas removal, and extra features.
Open Data and Methods
I turned the purifier on the highest setting before I went to bed at night and turned it off after I woke up. To measure particulates, I used a Dylos particle counter, which measures particles .5 microns and above and 2.5 microns and above per .01 cubic foot. The particle counter took one measurement each hour.
I calculated effectiveness as the percentage of particles the purifier removed from the room air. The baseline was the particle count before turning on the purifier. The final count was the average number of particles over the last four hours before I woke up. I prefer this over comparisons to outside air because:
- Indoor air is cleaner than outdoor air, even without a purifier (although that doesn’t mean it’s safe).
- The baseline room number takes into account how dirty the outdoor air is because indoor particulates go up and down with outside air.
However, the drawback is that outdoor particulates sometimes go up after the test starts (lowering estimates of effectiveness) and sometimes go down (raising estimates). However, these average out over multiple tests, and the results are similar if you look only at days where outdoor particulates were relatively stable.
The bedroom is 15 m2, located in Chaoyangmen, Beijing. The doors and windows were closed while I slept, but I opened the door at various times in the very beginning of the test before I went to sleep. Although the doors and windows were closed, the apartment isn’t new, and the seal isn’t great.
For more details on the methods, see the end of my earlier post. All methods were identical to my earlier tests.
After all the tests were done, the IQ Air said the pre-filter had 1,931 hours of use left, the carbon filter had 3,077 hours, and the HEPA had 1,910 hours. Thus, the filter was in its prime.
Of the 11 tests, one day was a strong outlier. On May 22nd, the IQ Air got only 68% of the .5 micron particles. Normally when I see poor results, it’s because the outdoor air got a lot dirtier during the night. But on May 22nd, the outdoor concentration fluctuated between 74 and 110 micrograms, which isn’t out of the ordinary.
Because I couldn’t see any reason the results were poor that night, I left the data in. If I redo the analysis with that day included, the results are only slightly better: 93.0% of .5 micron particles and 96.5% of the 2.5 micron particles. That would put the IQ Air about equal with the Philips and still 4% below the Cannon.
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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.