How HEPA Filters Work

Rightfully regarded as the most important component in a purifier, the HEPA (or HEPA-like) filter is your main weapon in the fight for clean air, and the one you’re most likely to have heard of.

There are two ways to understand the term “HEPA”:

1) The slightly confusing, technical way

HEPA refers not to a particular technology or type of filter, but to a standard of air filtration. It stands for “High Efficiency Particulate Air…”

…filter.

“HEPA filter.” You need to add the “filter” part yourself.

So what’s the standard for a HEPA filter? Well, it depends who you ask.

The American EPA says that to qualify as a HEPA filter, it must be able to catch at least 99.97% of particles with a diameter of 0.3µm that pass through it.

The European standard is different, and a bit more nuanced:

This is shamelessly pulled from Wikipedia. Great site, highly recommended.

See, they have a slew of categories ranging from G1 (the G stands for “Guess,” as in “I guess it’s doing something?”) to U17, which refers to ULPA, which in turn stands for “Ultra Low Penetration Air.” ULPA filters are what go into the cleanest cleanrooms. Before 2008, there was no E class—A HEPA filter was considered anything from 85% to 99.995% efficient. That’s a big difference!

HEPA is one category below ULPA, but that doesn’t necessarily make it less effective. In fact, even a HEPA filter might be overkill for your apartment. If you’re still not convinced, just look to the International Space Station: HEPA is good enough for them.

2) The “everybody calls it that” way

The other way that the term “HEPA” is used is to describe any air filter that traps particulate matter. Just like anything that you put over your cuts is a Band-Aid, and anything you blow your nose into is a Kleenex. It’s incorrect usage, of course, because those are brand names, not product names, but it’s common enough that you should know that Band-Aid often refers to any brand of adhesive bandage. Similarly, HEPA sometimes refers to PM air filters in general.

FUN FACT: A brand name that is used to describe a general product is called a “proprietary eponym.” Other examples include Aspirin, Cheerios, AstroTurf, Hula-Hoop, Zipper, Jacuzzi, Super Glue, Popsicle, and Jello (Jell-O).

To avoid getting into trouble, some companies use terms like “HEPA-type” or “HEPA-like.” That might seem kind of deceptive, but think about how hard it would be to explain a wireless internet device without using the trademark “Wi-Fi®.” Rather than explain the concept of WLAN from scratch, it would be much easier to connect to customers just by saying “It’s like Wi-Fi®,” because that’s what people have heard of.

Because “HEPA” is so strongly associated with air filters in general, many companies—Good ones and bad ones—Use the term to describe their product, irrespective of the extent to which they adhere to the true(…?) HEPA standards referred to above. In fact, according to the US Patent and Trademarks Office, there are 104 registered trademarks that use the term “HEPA:”

HEPA-STEALTH is what the Navy Seals use to purify the air deep behind enemy lines.

Some of these products are excellent. Some are basically as effective as a balled-up t-shirt. The key takeaway is that people are going to throw the term “HEPA” around, and it’s up to you to look into what that means in each case. Don’t get too caught up on labels.

And don’t be that guy that says “Well I don’t have a Kleenex to lend you, but I do have a store brand facial tissue.” Ugh.

Okay, now do they actually work?

If you’ve ever seen heavy winds launch hundreds of ping pong balls into a rosebush, where most of them become embedded (who hasn’t?), you’ve essentially seen how a HEPA-flavored filter works. Just shrink the ping pong balls down into solid particulates, and shrink the rosebush branches down into microglass or synthetic fibers.

Straight from Wikipedia again. This one is part of the very cool Philip Greenspun illustration project.

Layers and layers of matted fibers are woven tightly into a fabric, which is arranged into a pleated shape and put inside of a frame. The fibers create this tangled forest of hooks and snags for particles to become trapped in as air passes through. There are three mechanisms by which this trapping works:

Impact: The particle is unable to maneuver through the maze and becomes wedged between two filter fibers. For that reason, it’s normally only large particles that are trapped by impaction. This is like kicking a soccer ball into a tree, and it doesn’t come back down.

Interception: As air passes through the filter, medium-sized particles bounce off of the fibers of the filter medium. Every time the particle changes direction, it loses some energy, and it eventually just settles by clinging to a filter fiber.

Diffusion: The smallest particulates avoid being trapped altogether. The way HEPA filters stop them is by slowing them down enough that they become “lost” in the mess of tangled fibers. They become held inside the filter—but not settled—through the phenomenon of Brownian Motion. The particles just kind of float in there.

HEPA filters are not extremely complex (although a good one can be very hard to make). Any filter that can effectively slow down particles as small as 0.3µm, randomize their direction and still maintain high airflow will be suitable for cleaning your air. Filters come in a wide variety of quality ranges, so be sure to do your own homework when it comes to things like airflow-to-efficiency ratio, and whether the material is microglass or synthentic. Knowing this key information is the best way to make sure that the one you end up using does what it claims it can do.