What is a HEPA filter?Updated a year ago

What does HEPA mean and where did it come from?
HEPA, which stands for High Efficiency Particulate Air, is a designation used to describe filters that are able to trap 99.97 percent of particles that are 0.3 microns. Though the HEPA standard and certification process wasn’t established until 1983, development of HEPA filters dates back to World War II, when American scientists with the Manhattan Project created the first HEPA filter to capture radioactive particles released during the creation of the atomic bomb.

Why 0.3 microns?
That micron size (0.3) is referred to by scientists as the MPPS, or the most penetrating particle size. Scientists have found that particles of that size evade air filters more than larger or smaller particles. We’ll get into why in a little bit.

What are HEPA filters made from and how do they work?
Most modern HEPA filters consist of interlaced glass fibers that are twisted and turned in myriad directions to create a fibrous maze. As particles traverse this web, they’re taken out of circulation in the following ways:

 Direct Impaction: Large contaminants, such as certain types of dust, mold, and pollen, travel in a straight path, collide with a fiber, and stick to it.

Sieving: The air stream carries a particle between two fibers, but the particle is larger than the gap, so it becomes ensnared.
Interception: Airflow is nimble enough to reroute around fibers, but, thanks to inertia, particles continue on their path and stick to the sides of fibers.
Diffusion: Small, ultrafine particles move more erratically than larger ones, so they’re more likely to hit and stick to fibers.

What about when particles penetrate a HEPA filter? Are they going straight to my lungs?
Don’t worry. There are other technologies that work in concert with HEPA filters to ward off many super-small contaminants, such as smoke, fumes, and other chemicals. Activated carbon filters, like those in Airmega’s Max2 filters for example, use small pores to capture some chemicals, odors, and smoke that a HEPA filter might not catch.

I keep hearing the term 'True HEPA.' Are there different forms of HEPA?
There are quite a few terms marketers use to describe air filters. While True HEPA is technically a marketing term, it’s used by many to differentiate between American and European HEPA standards. In Europe, a filter only needs to capture 85 percent of particles sized at 0.3 microns—compared to the 99.97-percent U.S. standard—to be HEPA-certified. The American standard is, thus, often referred to as “True HEPA.” “HEPA-type,” “Ultra HEPA,” and other HEPA variations aren’t recognized by the accrediting bodies in the U.S. or Europe.

What if a filter advertises itself as capable of capturing 99.9 percent of small or large particles—that’s good, right?
Not necessarily. A filter that says it can suck up 99.9 percent of large particles may be poor at capturing ultra-fine ones. Likewise, a filter that touts its ability to trap the smallest of particles isn’t telling you how well it captures particles that are 0.3 microns, which we know are the most troublesome.

Where are HEPA filters used?
Like in the Manhattan Project, HEPA filters were originally intended to be used in lab and factory settings. Today, they’ve also made their way into consumer products, including cars, vacuum cleaners, and—you guessed it—air purifiers.

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