HEPA filters are great at filtering particles such as dust, PM2.5 and PM10. They even do an incredible job of capturing more than 99.9% nanoparticles like viruses and bacteria.
What HEPA filters don’t do, however, is destroy the particles. This has lead to a lot of marketing hype claiming HEPA filters cannot destroy particles, but new fancy technologies that can.
The logic seems reasonable. If a HEPA filter is like a sieve, then when you turn it upside down the particles can fall out. Here’s the Smart Air Lab’s super simple rendition of how we think air filters might work, using a sieve and tea leaves.
HEPA Filters are Not Sieves
The truth is, HEPA filters don’t work like this, at least – not for the really small particles like viruses and bacteria. These particles are so small chemical forces called van der Walls force stick them to the HEPA filter. It’s the same chemical magic that makes geckos stick to surfaces.
What The Data Says on HEPA Filters Releasing Dirty Particles Back into the Air
Okay, so that’s the theory behind HEPA filters and how they work, but where’s the data to prove it? Fortunately, scientists in Japan have tested this. They shot plutonium particles at HEPA filters, and then measured how many of these particles were re-released back into the air over a 20 day period.
The plutonium particles tested were approximately 100 – 200nm in size. For reference, that’s roughly the same size as the coronavirus which measures 60-140nm in size.
After firing plutonium particles at the filter, they measured how many of these particles were released when blowing air in both directions through the filter, and at different speeds. The scientists even went as far as to ‘jolt’ the filter to see if that would lead to any further dislodging of particles.
They found that when blowing air forwards through the filter, there was a very minimal re-release of particles back into the air. Much lower than what would normally get through a 99.97% filter anyway.
Things got interesting when they blew air through a really full (well used) filter in a reverse direction. A larger number of particles were dislodged in this case.
This makes sense, clogged/full HEPAs are more likely to release particles back into the air. But only when air is blown through the filter in a reverse direction. This is an extreme case, unlikely to occur unless a used filter is re-inserted into an air purifier in the wrong direction. However, the scientists concluded that filters should be handled with care when removing and replacing them.
Are particles dislodged from HEPA filters really a health risk?
The data above shows that some particles can become dislodged from HEPA filters during use, however they shouldn’t be considered a health risk:
- The number of particles dislodged in the forward-flow direction is far less than what would get through the HEPA filter anyway
- For viruses, data shows they typically die within 3-24hrs on dry surfaces. That means that any viruses that do escape the HEPA filter will most likely be dead by the time they do.