HEPA filters are a nearly perfect solution for air pollution. They are backed by tons of scientific tests; they’re pretty cheap; they use about as much energy as a lightbulb; they even have documented health benefits.
Just about the only remaining problem is that you have to throw out the HEPA filters every few months. Can you wash HEPA filters? If so, that’d be great because we could re-use them.
Some Experts Say: Wash HEPA Filters
On the question-answer site Quora, I’ve seen industry insiders say you can clean HEPA filters by banging the HEPA to get the dust out:
…or you can clean HEPA filters by vacuuming them
…or even washing them with water. Yet other experts say you can NEVER wash HEPA filters.
But one thing I haven’t seen is anyone actually test the question. Well now I’ve got data. That’s because Smart Air engineer CK tested the best way to clean a HEPA filter. Here’s CK’s story.
Vacuuming HEPA Filters
First I tried vacuuming a HEPA. I took a dirty HEPA filter, ran tests on its performance, vacuumed it, and then ran more performance tests. In all, I tested 8 dirty HEPAs given to us by avid Smart Air users! They all used in China for between 2-4 months.
Particle Capture: I measured the percent of 0.3 micron particles captured at the outlet of the HEPA before and after washing. 0.3 micron particles are a great benchmark because they are the hardest particle size for HEPAs to capture.
Air Flow: I measured air speed coming out of the HEPA before and after washing.
CADR: Then I used particle capture and air flow to estimate CADR.
On average, vacuuming increased CADR by about 10%. But there was also a lot of variance, ranging from a 14% increase to a 3% decrease. Based on that data, I’d say cleaning a HEPA by vacuuming might help a little, but there’s also a decent chance it’ll do nothing at all.
What’s The Vacuum Doing?
HEPAs are made of thin fibers. The vacuum is probably pulling out some of the larger dust particles from the HEPA filter, but at the same time some of these fibers as well. You can see what looks like fibers that have been pulled loose in this picture.
Vacuuming bottom line: There may be a slight benefit of vacuuming HEPAs, but the benefit is small, and it can also damage the HEPA. Therefore, we do not recommend vacuuming.
Test: Trying to Wash HEPA Filters
So, what if we wash them instead? I used the same 3 filters that were vacuumed before and washed them under a shower head. Then I gave them a day to dry.
Washing HEPA Results
I stopped testing after 3 trials because the results were so conclusive: washing HEPA filters with water decreased effectiveness by an average of 32%.
Why Is It Bad to Wash HEPA Filters?
To get to the bottom of why washing was so bad for the HEPAs, I looked at wind speed and particle capture before and after washing.
First, washing actually increased the windspeed.
Great, right!? But then particle capture actually went down a lot after washing.
Washing the HEPA filters probably broke some of the fibers or pushed the gaps between the fibers larger. That would explain why more air passed through after washing, but also more particles!
Washing bottom line: Washing HEPA filters with water significantly decreases performance so is not recommended.
Bottom Line: Can You Wash HEPA Filters?
Vacuuming HEPAs may improve performance, but the effect is small. Washing HEPAs strongly decreases performance. We do not recommend washing HEPAs.
Based on this data, it seems it really is not possible to clean your HEPA filter – either by washing your HEPA filter or vacuuming your HEPA filter. We still recommend replacing your HEPA when its effectiveness drops. When is this? We tested this with the Smart Air DIY air purifier found replacing after 140 days is best. For the larger HEPAs, like those in the Blast and Blast Mini purifiers the huge surface area means they can last over 1 year.
Own another purifier? Learn why the standard numbers might not always be accurate.
You can learn more about the testing methods and view the original data in the supplemental data article.
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Choon Khin is a Smart Air engineer from Singapore, studying chemical engineering at the National University of Singapore