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.
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.
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.
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.
Filtration Rate of MPPS
ISO / European (ISO29463 / EN 1822)
US Standard (MIL-STD-282)
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'.
EPA (efficient particulate air filter)
85% – 99.95%
HEPA (high efficiency particulate air filter)
99.95% – 99.999%
ULPA (ultra low penetration air filter)
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.
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 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.
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.'
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.
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.
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