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. So wouldn’t it be great if we could clean them and re-use them?
People have lots of opinions! On the question-answer site Quora, I’ve seen people say you can bang the HEPA to get the dust out:
…or vacuum it
…or even wash it. Then I’ve seen other people say you can NEVER wash a HEPA.
But one thing I haven’t seen is anyone actually test the question. Well now I’ve got data. That’s because CK from Smart Air decided to test it out. Here’s CK’s story.
Vacuuming the HEPA
First I tried vacuuming the HEPA. I went to our storeroom, took a dirty HEPA filter, tested it, vacuumed it, and tested it again. We have loads of dirty HEPA, thanks to several kind volunteers! The HEPAs were used in China for 2-4 months. In all, I tested 8 dirty HEPAs.
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: I used particle capture and air flow to estimate CADR.
On average, vacuuming increased relative 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 it 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 and some of these fibers. You can see what looks like fibers that have been pulled loose in this picture.
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.
Washing the HEPA
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.
I stopped testing after 3 trials because the results were so conclusive: washing the HEPA decreased effectiveness by an average of 32%.
Why Was Washing So Bad for the HEPAs?
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 the HEPA.
First, washing actually increased the windspeed.
Great, right!? But then particle capture actually went down a lot after washing.
Washing the HEPAs 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!
Bottom Line: 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, we still recommend replacing the DIY HEPA after 140 days.
Particle Capture Tests
For the particle capture tests, CK used a Met One GT-531 particle counter. This particle counter is best for HEPA outlet tests because it measures down to 0.3 micron particles—the hardest particles to capture. CK noted 0.3 micron and 2.5 micron particle counts with and without the HEPA.
Importantly, this particle counter has a pump to regulate air intake. That’s important because increasing the air flow in a particle counter will mean the particle counter counts more particles, simply because more air has passed through it!
There was decent variation in the 0.3 micron efficiency between the 8 HEPAs. We randomly inspect several HEPAs from each batch we receive from the manufacturer to make sure they capture 99.9% of particles 0.3 microns and above. Yet some of the readings of these old HEPAs were below 99.9%.
I can think of two reasons for that variation. First, there are slight variations in the test—particularly in how close the air inlet of the particle counter gets to the HEPA. That affects whether any air from near the HEPA (not coming through the HEPA) gets in. That doesn’t let many particles in, but when you’re trying to tell the difference between 99.9% and 99.0%, a few particles make a huge difference.
Second, these HEPAs were used, strapped in, then unstrapped, shipped to Beijing by Smart Air volunteer DIYers, and strapped on to another fan. All that movement and handling could make some of the gaps between HEPA fibers larger.
The Met One tests for 60 seconds and reports output as the cumulative number of particles counted (rather than the average).
Air Speed Tests
CK used a Pyle anemometer to take wind speed measurements at four points on each HEPA—north, south, east, and west. Wind speed varies a fair amount between the different HEPAs. That can be due to the dynamics of the air coming out of the fan (it’s surprisingly not uniform!), damage from being shipped and handled, and how much particulate is stuck in the filter.
CADR (in meters cubed per hour) was estimated using the following formula:
AVERAGE(wind speed in m/s)*AVERAGE(percent particle capture of 0.3 micron particle capture)*area of the outlet*3600/100
For reference, the smoke CADR of a Blue Air 203 is 155 (according to AHAM).
The original data is available as an Excel file and as an image below:
P.S. What about using compressed air to clean the filter? Check out the end of Thomas’s Quora post for the answer.