Multiple companies claim only their HEPA filters capture nanoparticles. But this is simply not true. Read on to find out why.
Wikipedia’s article on HEPA filters states:
“Standards require that an air filter must remove […] 99.97% of particles that have a size greater than or equal to 0.3 µm.”
You’ll see it only mentions particles of a size greater than or equal to 0.3µm.
So what about nanoparticles? Nanoparticles range from 0.001 microns to 0.1 microns in size. Can HEPA filters capture these?
This exact question has led to years of confusion and false advertising claims about HEPA filters, so let’s get to the bottom of this.
How do HEPA filters work?
The answer is surprising. It’s surprising because our intuition about filters is wrong. Like most people, I had the intuition that HEPA filters work like a net. If a particle is smaller than the holes in the net, it gets through. Makes sense!
That intuition is true for big particles. When large particles fly into a HEPA filter, they’re too big to get through, so they get stuck.
But if we zoom down to the really small particles—like nanoparticles—things start getting weird. Nanoparticles are so small that they bounce around like a pinball when they hit gas molecules (this phenomenon is called Brownian Motion). That means they fly in random zigzag patterns.
Nanoparticles are so small that they can fit through the fibers in filters, but they get stuck anyway because they fly in zigzag patterns. Those zigzag patterns mean they end up hitting the fibers and getting stuck.
When a filter captures a particle in this way, they call it “diffusion.”
How effective is diffusion at capturing nanoparticles in HEPA filters?
Is diffusion something that works some of the time? Maybe with 50% of nanoparticles? It turns out that diffusion is far more effective than that. Scientists from NASA have tested HEPA filters and found that they capture well over 90% of particles even down to 5 nanometers .
The University of Minnesota conducted filtration tests for a fiberglass filter medium which “approaches the HEPA regime for small particle size”, meaning this filter performs similarly to HEPA. In the experiment, silver particles of size 3 to 20 nanometers were shot at the filter medium.
The results showed that even for particles smaller than 5 nanometers, 99.99% were captured.
So why does Wikipedia mention the 0.3-micron particle size, and why is it so important? It’s actually the larger 0.3-micron particles are actually the hardest particles to remove. They’re much more difficult to remove than those nanoscale particles! This is because at 0.3 microns, diffusion is not that prevalent, and the particles are small enough that not all are captured from direct impact and interception.
The miracle of Brownian motion isn’t just a HEPA thing. This principle works for any fiber filter, including furnace filters (MERV filters) and even masks! For example, scientists tested 3M masks and found them to be more than 95% effective down to 0.007 microns, or 7 nanometers.
Why is this important?
Purifier companies can prey on our faulty intuition and use it to overcharge us. For example, the Molekule purifier makes claims like this on their website:
And here’s IQAir explaining why their HyperHEPA filter is better than an ‘ordinary filter’:
Representatives from Sharp have also taken to Quora to postulate that ordinary HEPA filters can’t capture anything below 0.3 microns.
I suspect that some people really do get so far into designing purifiers (and ads) without discovering this fact about filtration. So some of these tricks may be ignorance rather than deception. But now you know the science!
Bottom line – can HEPA filters capture nanoparticles?
A simple HEPA filter will capture nanoparticles, and essentially any other particles as well.
Paddy graduated in aeronautical engineering from Bristol University, and now runs Smart Air’s operations from Beijing. He’s an advocate for open data, free information and transparent business.