Can HEPA Filters Capture Nanoparticles?

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 small 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?

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