One of the biggest questions when I started doing the DIY was how long the HEPAs last. Rather than just give people a nice-sounding answer, I wanted to see what the data says, so Gus has been dutifully turning on his Original DIY every day in his 12.3m2 Beijing bedroom and tracking how effective it is each day in real Beijing air
Around Day 100, effectiveness dropped 4%. Around another Day 170, effectiveness dropped another 5-10%. But for the benefit of science, Gus has risked his own lungs by continuing the test for another 60 days.
Gus turns on the Original DIY while he sleeps each night and tracks effectiveness using a Dylos particle counter. I calculate effectiveness as the percentage decrease in the number of particles .5 microns (and above) over the course of the night.
Here’s what a sample day looks like:
To smooth out variability over time, I averaged the effectiveness over every 10 days. For the full details on the methods, check out my earlier post.
The death crawl continues. At Day 200, effectiveness dipped to just below 50%.
At the risk of Gus’s health (especially given the coming onslaught of winter air), I voted for Gus to stop the test. So the test is over!
In real Beijing air, the Smart Air HEPA lasted at 100 days at about 8 hours a day at full effectiveness (729 hours to be exact). It lasted through 140 days (1,028 hours) with a slight 4% drop, which is when I would change the filter.
Recommendation: I recommend changing the HEPA every 140 days at about 8 hours a day or every 1,000 hours. You can adjust the number based on how many hours you use it per day.
Limitations: We did the test in Beijing, so HEPAs probably last a few more weeks in places with cleaner air (but still not “clean air”) like Guangzhou and Hong Kong.
The results may also be different for the Cannon because it pushes more air. The next longevity test will be with a Cannon!
As always, I’m publishing the raw data–all 200 days!
Thomas is a new Assistant 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.