How long do Smart Air HEPAs last? My quest to give a solid empirical answer has now completed 130 days! Gus is still dutifully tracking the effectiveness of the HEPA he’s been using in his bedroom since January (with a trip to the US in the middle), calculating the percent reduction in particulates every night.
I reported a few weeks ago that the HEPA was just as effective at Day 1 and Day 90. Now after 130 days and 951 hours of usage, here’s what the HEPA looks like:
Results: At 100 days, the overall effectiveness declined by about 4%. Even so, the HEPA is still removing 80% of .5 micron particles from the overall room air:
- On the Original DIY, the Smart Air HEPA lasted about 100 days without losing any effectiveness. That was for a total of 729 hours of usage or an average of about 8 hours a day (7.29 to be precise).
- Between days 100-130, the effectiveness dropped by about 4%. I think it’s up to you to decide whether a 4% drop is enough to make you change the HEPA.
- You can adjust these numbers to the number of hours you use the machine. For example, if you run the DIY 24 hours a day, you can get about 30 days with full effectiveness and at least 40 days with a roughly 4% reduction.
- As I’ve argued before, I prefer this type of test over the context-less numbers that some big purifier companies give because (1) it is based on air in China and (2) it gives the concrete number of hours you can use it rather than just the number of months. Of course, there will be some variation across different seasons, apartments, and cities in China.Limitations:
- This number doesn’t necessarily apply to the Cannon. On the one hand, the Cannon filter might have a shorter life because it pushes more air and therefore should collect particles faster. However, our Cannon HEPAs have a pre-filter, which extends the life of the HEPA. The next step is to do longevity tests with the Cannon!
- Gus changed apartments on Day 102, which is not ideal. However, the average effectiveness didn’t seem to be affected by his move (see the points at Day 100 and Day 110). We’ll repeat the tests in the future, and that will give us more reliable estimates.
For now, the test continues! Stay tuned for the next report.
I’m posting the raw data and detailed methods below for fellow nerds.
Basics: Gus did the tests overnight while he slept. The room door was closed during the tests. I calculated effectiveness by averaging the number of .5 micron particles over the last three hours before Gus woke up. Then I divided that number by the baseline measurement before Gus turned on the purifier.
Effect of Apartment-mate’s Purifier: As I wrote in my last update, we discovered on Day 57 that Gus’s roommate was running a purifier 24 hours a day in his room. This may artificially boost our estimate of the lifespan. It also artificially lowered the baseline particle counts and thereby lowered the estimates of effectiveness for the first 57 days.
Therefore, the raw data points for days 0-60 would give people a false sense that the HEPA increases in effectiveness after 60 days. To correct for that, I averaged difference between Days 0-57 and Days 58-90. The original graph is in my earlier post, and all of the raw data is available here.
Outdoor Fluctuation: On Day 94 and Day 113, the outdoor PM 2.5 concentration increased by almost 100 micrograms during the test, so I removed these days from the calculation. Keeping those datapoints lowers those two 10-day estimates by about 3%. This data is retained in the raw data file.
Room Change: For days 1-101, Gus was in his old apartment, which had a 12.3 m2 bedroom. For days 102-130, he was in his new apartment. Obviously that’s not ideal, but his landlord was taking back his old apartment! We will repeat the tests in his new apartment from scratch after this test ends.
Calculating room size for Gus’s new bedroom is a little tricky because the ceiling is slanted. The floor area is 13.7 m2–slightly larger than his old room. But the slope of his ceiling gives it a volume of 26.3 m3 compared to 33.5 m3 if it were perfectly rectangular and 30.1 m3 in his old room.
Thus, his new room may be easier to purify, although I suspect the roof is not very well sealed, so it may leak more than his old room.
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.