What Are HEPA Filters and How Do They Work?

What is a HEPA filter and how does a HEPA filter work?
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The term “HEPA filter” can be confusing. What does HEPA stand for? What is a HEPA filter made from, and what actually makes it a HEPA filter?

What Exactly Is a HEPA Filter?

“HEPA” stands for “high-efficiency particulate air” (filter). It sounds complicated, but HEPA filters are nothing fancy. They were invented back in the 1940s when scientists were developing the atomic bomb, and they’re just a mat of randomly aligned fibers, made from either glass or synthetic materials. The synthetic material is similar to what’s used in quick-dry T-shirts.

Polyester material HEPA filter t-shirt

On the other hand, fiberglass filters are made from glass – that means things like silica, alumina, calcium oxide, boron oxide, magnesium oxide, sodium oxide.

What’s important about HEPA filters is they’re incredibly effective at capturing almost every size of particle. They can capture viruses, bacteria, pollen, PM2.5, allergens, and more.

To see what the randomly mat of fibers look like, here’s a closeup of the HEPA filter Smart Air makes for The Sqair air purifier, and then alongside it a scanning electron microscope photo for a view at the nano-level.

Closeup HEPA filter material
electron microscope HEPA filter

What Makes a Filter a HEPA Filter?

OK, so if HEPA filters were invented in the 1940s, and they’re nothing fancy, then what makes any old filter a HEPA filter? Turns out using the term “HEPA” has a strict set of requirements. In Europe, HEPA filters must remove 99.95% (ISO) of particles. In the US, they must remove 99.97% of particles.

Standard Filtration Rate of MPPS
ISO / European (ISO29463 / EN 1822) ≥ 99.95%
US Standard (MIL-STD-282) ≥ 99.97%


When following the ISO or European rules, filters that capture ≥85% of particles but less than 99.95% are called ‘EPA filters’ or ‘efficient particulate air filter’. Filters that capture more than 99.999% of particles are called ‘ULPA filters’ or ‘ultra low penetration air filter’.


Designation Filtration Rate
EPA (efficient particulate air filter) 85% – 99.95%
HEPA (high efficiency particulate air filter) 99.95% – 99.999%
ULPA (ultra low penetration air filter) ≥99.999%

OK, but what does “particles” mean? All particles? A particular size of particles? Turns out HEPAs are tested against the “most penetrating particle size” (MPPS)–more on that in a bit.

How Do HEPA Filters Work? 

The answer to this question is the most fascinating thing about HEPA filters. Most of us might start out thinking HEPA filters work like a net, like this.

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!

Direct Impact & Straining

That intuition is true for big particles. By “big,” we’re talking typically larger than 1 micron. For comparison, a human hair is about 50 microns wide. So 1 micron is actually quite small.

But these “large” particles fly into a HEPA filter, they’re too big to get through, so they get stuck. Scientists have a name for that. When particles get stuck between two fibers, they call it “straining.”

HEPA filter straining capture method


What happens for particles smaller than 1 micron? Let’s look at the next size range down: 0.3 – 1 microns. We’re talking about the size of a bacteria.

Size of coronavirus particle pm2.5 and bacteria


Particles this size can fit between the gaps in the filter. But they have a problem. They’ll try to follow the air around a HEPA filter fiber, but they are a bit heavy. So some of them don’t move fast enough and thus end up getting stuck. Scientists call this “interception.”

Interception capture HEPA particles


OK, so do the particles below that size get through? For the really small particles (less than 0.3 microns), the science gets weirder. These particles that small have so little mass that they actually get bounced around like a pinball when they hit gas molecules (that’s called Brownian Motion). So they move in random zigzag patterns.

Brownian motion capture method of particles

These particles are so small they could easily fit through HEPA filters. But sadly (for their freedom) and happily (for our lungs), they don’t fly in straight lines. Because they fly in zigzag patterns, they end up hitting the fibers and getting stuck. Scientists call that diffusion.

Here’s how all three capture mechanics work:

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

And here’s how the three different mechanics work for different particles sizes. Straining and impact capture large particles; interception captures medium particles; and diffusion captures the smallest particles.

HEPA filter capture mechanics diffusion impact intersection

What’s The Most Penetrating Particle Size?

Did you notice the dip in the last graph? That happens because diffusion works really well under about 0.3 microns. Straining and other mechanics that are more intuitive work really well above 0.3 microns.

Where those two mechanics intersect is the hardest particle size to capture. That’s because it’s not quite so small for diffusion to work its full strength, and it’s not quite large enough for straining to work its full strength. Call it the “weak spot” of HEPA filters. This dip in the graph is called the most penetrating particle size.

Most penetrating particle size HEPA filter

The dip in the graph most commonly occurs around the 0.3 microns mark, which is why people mention 0.3 microns all the time.


HEPA Filters Can Capture Nanoparticles


See how the line goes up to the left of the dip in the graph.

HEPA filter capture mechanics diffusion effective capture nanoparticles

That’s Brownian motion and diffusion in action. Diffusion is incredibly effective at capturing nanoparticles. That means HEPA filters are also incredible effective at capturing nanoparticles too.

Read more: can HEPA filters capture nanoparticles? »

Can HEPA filters also capture gases and VOCs?

Since HEPA filters are so great at capturing nanoparticles, it’s reasonable to think they might capture VOCs and gases too. Unfortunately this is not the case. Since gases are not particles, they move and behave differently in the air. To capture VOCs and gases you’ll need a carbon filter for that.

Read more: what combination of air filters do I need to protect myself from all pollutants? »

Can I clean a HEPA filter?

Cleaning HEPA filters with water, or vacuuming them or bashing them isn’t recommended, as it may damage the HEPA filters fibers. Smart Air tested washing HEPA filters in the Smart Air Lab. Learn more about the harms of cleaning your HEPA filter here.


Bottom line on HEPA filters

HEPA filters are high efficiency filters that typically capture over 99.5% of all particulate pollution. They’re made from either plastic (PP+PET) or fiberglass, and can capture things like pollen, viruses, bacteria, mold and PM2.5. Diffusion also means they are highly effective at capturing nanoparticles too.

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