HEPA filters are great at filtering particles such as dust, PM2.5 and PM10. They also do an incredible job of capturing nanoparticles including viruses and bacteria.
HEPA Filters are Not Sieves
The truth is, HEPA filters don’t work like this. At least, not for the tiny small particles like viruses and bacteria. These particles are so small, chemical forces stick them to the HEPA filter. These forces are called van der Walls forces. 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, but where’s the data to prove it? Good news. Scientists in Japan have tested this. They shot plutonium particles that were 100-200nm in size at HEPA filters. For reference 100-200nm is roughly the same size as the coronavirus (60-140nm).
They then measured how many were re-released back into the air over a 20 day period. They did this while blowing air forwards through the filter, and in a reverse direction. The scientists even measured whether ‘jolting’ the filter would dislodge more 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 the 0.03% of particles that 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. In this case, more particles were dislodged.
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?
Data shows that some particles can become dislodged from HEPA filters during use. Despite this, the health risk may be minimal, since:
- The number of particles dislodged in the forward-flow direction is far less than what would get through the HEPA filter anyway.
- If we’re considering viruses, 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.
Paddy is the CEO of Smart Air, running operations from Beijing. He has a Masters in aeronautical engineering from Bristol University, UK having specialised in aerodynamics. An advocate for open data, free information and transparent business, he spends his spare time promoting honest business and social enterprise.