Can HEPA filters capture nanoparticles?

HEPA Under 0.3 Microns

Many big purifier companies claim only their HEPA filters capture nanoparticles, like this Molekule ad.


Molekule air purifier review


Although this claim is false, even journalists repeat this myth. Why is this belief so pervasive?


HEPA Purifier Microns Molekule


One reasonable explanation is that the definition of “HEPA” leads people down that route. For example, this definition on Wikipedia says HEPA filters must remove: “99.97% of particles that have a size greater than or equal to 0.3 µm.”


HEPA filter particle capture size 0.3µm


It only mentions particles 0.3 microns and above. So what about nanoparticles? Nanoparticles range from 0.001 microns to 0.1 microns. 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 it.


Air Dog purifier microns review


Why Our Intuitions About Filters Are Wrong

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.


HEPA filters - do they capture particles 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.


HEPA filter straining capture method


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. (Scientists call this Brownian Motion.) That means they fly in random zigzag patterns.

Brownian motion capture method of particles in HEPA filter


Nanoparticles are so small that they can fit through the fibers in filters, but they get stuck anyway. Flying in zigzag patterns mean they end up hitting the fibers and getting stuck.


interception - diffusion - impaction capture method of particles on HEPA filter fiber


When a filter captures a particle in this way, scientists call it “diffusion.”


How Effective Is Diffusion at Capturing Nanoparticles?


OK, so diffusion captures nanoparticles. But how many nanoparticles? Maybe 50% of nanoparticles? It turns out that diffusion is far more effective than that. According to NASA, HEPA filters capture “virtually 100% of particulates.”


NASA nanoparticles HEPA capture


But where’s the data? Researchers at the University of Minnesota tested this question with weaker fiberglass furnace filters and higher-grade HEPA filters. In their test, they shot particles of silver from 3 to 20 nanometers at the filters.

The results showed that filters captured 99.99% of particles smaller than 5 nanometers.


HEPA Filters Capture Nanoparticles


Their results demonstrate that the miracle of Brownian motion isn’t just a HEPA thing. This principle works for any fiber filter, including furnace filters (also called “MERV filters”). In the graph above, the MERV filters captured about 99% of particles at 10 nanometers and 99.9% at 4 nanometers. That’s impressive for a cheap fiber filter.


MERV Filter Furnace


Brownian motion works for masks too! For example, scientists tested 3M masks and found they were 96% effective down to 0.007 microns (7 nanometers). Researchers at the University of Massachusetts found similar results in their mask tests.


Mask Nanoparticle Capture Effectiveness Efficiency


Whence The Focus on 0.3 Microns?


So why does Wikipedia mention the 0.3-micron particle size? Rather than being the lower limit, these larger 0.3-micron particles are right in the middle. It turns out, particles in this middle region are the hardest to capture.

That’s because 0.3 micron particles don’t fly in zigzag patterns much. They’re too big. But at the same time, they’re big enough that they don’t easily get caught in the fibers (fancy names when particles get caught in the fibers: “impaction” and “interception”).


HEPA Particle Capture 0.3 Microns


Why Should I Care?


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:

Molekule claim of HEPA filters only capturing 0.3µm


And here’s IQAir explaining why their HyperHEPA filter is better than an “ordinary filter.”


IQAir claim HyperHEPA capture nanoparticles



The National Head of Sharp claimed on Quora that ordinary HEPA filters can’t capture anything below 0.3 microns.


Sharp Quora claiming HEPA filter capture nanoparticles


I suspect that some people really do get so far into designing purifiers (and ads) without discovering this fact about filtration. Even a journalist at Wired repeated this myth. 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 virtually all sizes of solid particles.

Explore more: What types of filters do I need to protect myself from all types of pollutants?


The Sqair air purifier Kickstarter

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John M

I like the idea of a higher efficiency merv filter as Im on dollar power. What grade of merv would you recommend?


I was seriously considering buying one of the (much) more expensive air purifiers mentioned in the blog post! Thanks for writing!


I was just getting ready to buy an IQ Air or Airdoctor because they claim the .003 micron filtration! Now that I have read your article I am so confused. My main goal is purifying for mold exposure after remediation and everything I read talks about the mycotoxins being TINY and hard to capture so I have been trying to find the best filter for ultrafine particles like mycotoxins. Now what? IQ Air is expensive and from what this article says the filter they use isn’t any better than all the others. I need the best purifier for mold and… Read more »


I have two questions..

If HEPA can remove ultrafine particulates, why didn’t all the air purifier advertise this feature?

This is what I think a good way to prove…have a fan with HEPA filter pointing downward, put activated charcoal granules on the upside of HEPA, and turn on the fan. If there is no fine dust coming out, then HEPA can trap ultrafine particulates.

Hope it makes sense.


The reason of my suggestion is that I have tried the above experiment and failed, ie, there were fine dusts coming out. However, it is possible that my air purifier isn’t very tight and have a lot of gaps to let particulates pass through.

It will be interesting if somebody else with a better HEPA air purifier try it.

Dan Perry

Paddy – very interesting and counterintuitive. I’m glad you do so much research!