After launching the world’s most cost-effective purifier (the Sqair), we started testing the HEPA lifespan. I’ve long been skeptical of the lifespan numbers purifier companies give because (1) they provide no empirical justification for the number and (2) they give the same numbers in Switzerland or Delhi. Yet clearly the HEPA would last less time in Delhi’s “impressive” pollution.
To correct this with the Sqair, we used two ways to estimate lifespan—one lab and one real world.
Method 1: The Lab Test to Measure HEPA Lifespan
Method one was to send the Sqair to a third-party lab to use the testing method from the Chinese government standard.
Here’s how the official test works:
- Technicians burn cigarettes in a closed 30m2 room and test how much particulate the purifier can remove in 30 minutes as a measure of clean air output (CADR).
- They put the purifier in a small 3m2 box and burn 20 cigarettes, letting the purifier suck up all those particles.
- Test the effectiveness again.
- Burn another 10 cigarettes for the purifier to suck up.
- Test it again.
- Repeat after a total of 40, 50, and 60 cigarettes.
- Once the effectiveness drops to 50% of the original effectiveness, the test is done.
- Because the lab knows how much particulate is in a cigarette, they can estimate how much particulate the HEPA filter held when it dropped to 50% effectiveness.
- Use estimates of how much particulate a purifier would face in average conditions in China and use that to estimate the lifespan.
Once we know how much particulate the HEPA can hold, there’s a formula to convert that into hours of use. Of course, there are a LOT of assumptions that go into that—how leaky homes are, how bad outdoor pollution is, and what an acceptable level of indoor pollution is. I think the most honest thing to do is to put the calculator online and let users put in their own numbers, which Smart Air will do soon.
But using the lab data and the national standards to fill in the calculations produces a lifespan number for the Sqair HEPA: 1,531 hours. That’s a little over 6 months if used 8 hours a day.
(For the nerds out there, the Sqair has a CCM of 4468mg)
What’s Wrong with a Lab Test?
Lab tests are great because they’re in a controlled environment and because they are easy to reproduce. But at Smart Air, we like to verify lab tests with real-world tests because lab findings for air pollution don’t always hold true in the real world.
For example, most purifier users aren’t going to be smoking around their purifiers. That’s a problem because the particulate that cigarettes put out are very likely NOT identical in composition and size distribution to outdoor air pollution.
Method 2: Real-World Pollution
Thus, we set out to check the lab numbers against a test using real-world air pollution in Beijing.
Here’s how we did it. First, we tested effectiveness (CADR) in our closed testing room. Like the official CADR tests, we burned cigarettes and used a separate fan to make sure the particles were distributed around the room. This part is similar to the official lab tests.
What differed was the next step. Rather than using cigarettes to fill up the HEPA, we used real Beijing air pollution. For get real Beijing air pollution, we ran the Sqair on the balcony of the Smart Air office, with the window open.
Every 3-4 days, we ran a CADR lab test on the Sqair and then put it back out on the balcony. All told, we let the Sqair munch on 1,400 hours of Beijing air.
Results: Sqair HEPA Lifespan
Just before it reached 1,400 hours of usage, the effectiveness had dropped by 50%.
At 1,400 hours, that would give a lifespan of 175 days at 8 hours a day—or 5.83 months to be exact! Pretty close to the lab results of 6 months. The fact that we left the window open and exposed the Sqair to constant outdoor air could account for the slightly shorter estimate.
Adjusting the HEPA Lifespan to Your Own Use
If you’re using the purifiers for a longer or shorter time per day, you can take the number of hours divided by your daily usage to get the number of days the HEPA will last. If you’re using the purifier in a smaller space than the rated 40m2 or at a lower speed, the HEPAs will last even longer!
Read More on Lifespan
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.