What Are HEPA Filters and How Do They Work?

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 is a HEPA Filter?

“HEPA” stands for “high-efficiency particulate air” (filter). It sounds complicated, but HEPA air 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 used in the air filter is similar to what’s used in quick-dry T-shirts.

Polyester material HEPA filter t-shirt

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

fiberglass air filters

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

To see what the randomly aligned 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 view at the nano-level with the help of a microscope.

Closeup HEPA filter material
Closeup of Smart Air HEPA filter
Closeup HEPA filter material
Microscopic view of 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.

StandardFiltration 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’. These air filters do not meet the filtration requirement to be considered a ‘HEPA air filter’.

On the other hand, filters that exceed the requirements of a ‘HEPA air filter’ and capture more than 99.999% of particles are called ‘ULPA filters’ or ‘ultra low penetration air filter’.

DesignationFiltration 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!

HEPA Filter Method 1: Large Particles

That intuition is true for big particles. By “big,” we’re talking typically larger than 10 micron. For comparison, a human hair is about 50 microns wide. So 10 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

HEPA Filter Method 2: Smaller Particles

What happens for particles smaller than 10 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

HEPA Filter Method 3: Really Small 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 particle 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 For a HEPA Air Filter?

So the larger the particle, the better a HEPA filter filters out the particle? Not so fast! Did you notice the dip in the last graph? That happens because diffusion works really well under about 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 the Coronavirus?

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.

HEPA as compared to alternatives

HEPA is the international gold standard for filtering particles. Studies have shown the dangers of ionizers as it creates air pollution. Because (1) most ionizers are too weak to have an effect, (2) they produce harmful ozone, PM2.5, Harmful VOC Gases and Formaldehyde (3) they shoot out negative ions which cause particles to stick to surfaces in your home, rather than actually removing them.

Most of the major air purifier companies use ionizers in their air purifiers, including Xiaomi, Blueair, Levoit, and many more. The reason is: It is a cheap way to get a boost in CADR (metric measuring the effectiveness of air purifier) as a cheap ionizer can slightly improve the efficiency of the HEPA filter. Unfortunately, much of the time the user is unaware an ionizer is being used and unaware of the possible harmful effects.

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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14 thoughts on “What Are HEPA Filters and How Do They Work?”

  1. Can HEPA improve breathing due to sinus and blocked nose? Are there any better filters better than HEPA?

    • Hi there! Song here from Smart Air. HEPA filters can improve to some extent if the sinus and blocked nose is induced by small particles or typical allergens. What’s important about HEPA air filters is they’re incredibly effective at capturing almost every size of particle. We would insist that HEPA filters are most suitable for air purification based on the above description.

    • Hi Julian! Song here from Smart Air.
      Good question! HEPA filters have to meet strict standards which differ from region to region. It’s necessary for users to check the certifications before purchase.

  2. I have read that the fibres from the filter can be dangerous if they are breathed in. Is this correct?

  3. Hepa filters are great and reliable and I use a HEPA filter during the year to reduce my allergies, I suffer a lot during the spring and summer months from really aggressive allergies that knock me out all day and since getting a good HEPA filter I’ve had no issues with my allergies.

    I recommend buying a hepa system with a carbon filter as well, I have another unit in my dark room with a carbon filter to trap any formaldehyde and other chemical gasses from film development.

  4. If HEPA filters can effectively filter almost everything below 0.3 microns, can it also filter VOC gas particles which are much smaller than 0.3 microns?

      • Do you carry a filter capable of capture fumes from air? … specially speaking of fumes from fermentation . I need to modify A/C units from a comercial facility where Tea is put on fermentation process for days  this A/C units have copper coils that are bursting due to the fermantation in the air being heated.  I want to install an air purifier unit to capture the fumes of  fermentation  in the inlet side going to the A/C units. 


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