If you or someone you know has asthma, allergies, or a weakened immune system, you’ve probably heard or read about HEPA filters. You might have gotten the idea that it was a good thing in some way, but most likely nobody stopped to explain exactly what a HEPA is, what it does, and what benefits it has compared to other kinds of air filter.

The shortest, simplest explanation is that HEPA stands for High Efficiency Particle Air. In other words, it’s just a kind of air filter that happens to work exceptionally well.

At a deeper level, though, the difference between HEPA and other kinds of filters becomes qualitative instead of quantitative. If you were a surgeon, for instance, you would feel very different about “fewer” bacteria in the operating room if “none” was also an option.

Although it’s rarely that simple in practice, HEPA filters make this possible.

The Technical HEPA Numbers

To be called HEPA, a filter must be capable of removing 99.97% of all airborne particles 300 nanometers (0.000012″) or larger in diameter. To put this into perspective, this is still larger than the largest viruses and some bacteria, but this doesn’t mean that HEPA filters are not effective at stopping these pathogens.

In the first place, when airborne the particles are typically attached to a (relatively) large water droplet or dust mote, which will be caught. Also, because of the way fluid dynamics and electrostatic forces work at that tiny level, 300 nanometers is kind of a magic number for HEPA filters – their least efficient particle size. Their capture rate looks something like this:

Note: the bottom of the graph actually represents an efficiency of 99.94%, not zero. A few internet-based sources claim that HEPA filters cannot remove particles smaller than 300nm from the air, but this is simply not true. If it were, the air filters installed on airliners to help prevent epidemics from spreading globally would be no more than a scam (several tests do prove that viruses can get through HEPA filters, but their levels are significantly reduced). Some specialized filters can actually catch as much as 99.9999% of anything that’s either larger or smaller than their least efficient size.

What Is a HEPA Filter’s Special Secret?

Although the HEPA standard doesn’t specifically prescribe how a filter should be made, almost all of them are constructed from glass fibers a few hundred microns across.

Instead of being woven in some way, these are pressed into sheets with each fiber facing in whatever direction it wants. These sheets are then arranged in a kind of sandwich pattern as follows:

In other words, air flows across rather than through these membranes.

The way it works is slightly counter-intuitive, but this actually functions better than a flat filter. For one thing, the large effective surface area of this filter means that it takes very long to clog, so that it can be used safely for longer. For another, this actually purifies air far more thoroughly.

The random arrangement of the fibers creates a great deal of turbulence in the air flow, without significantly harming flow rates. As it turns out, the combination of this turbulence and the tendency of airborne particles to stick to the fibers is far more efficient at trapping particles than forcing them through a microscopic net.

How Does a HEPA Filter Help Those with Allergies and Asthma?

The first thing to understand is that filters marketed as “true HEPA” or “HEPA certified” can be very different from those labeled “HEPA style” or anything similar. In the former case, the products have been tested and verified in an independent laboratory, while in the latter the manufacturers presumably just tried their best.

However, buying even one of the cheaper products out there – either a stand-alone air purifier, a whole-home filtration system, or even an allergy-friendly vacuum cleaner – can have a major effect on the levels of airborne allergen levels inside your home. Getting one of these is certainly recommended.

Various studies attest to the effectiveness of HEPA filters, with positive results for anyone suffering from hay fever, asthma, or other respiratory conditions. It is not a substitute for normal cleaning, and it’s important to match its flow rate to the size of your home, but it is almost certain to help.

Comparing HEPA, Electrostatic, and Activated Carbon Filters

In many air purification systems, more than one type of filter is combined in a way that makes the best use of their complementary abilities. In one of the more extreme examples, gas masks usually contain a HEPA filter to remove nuclear fallout, bacteria, and viruses, as well as a carbon filter that eliminates chemical toxins.

You may not need that much protection, but if you’re shopping for an air purifier, it’s good to know how each filter type performs:

Each manufacturer and model is different, making it essential to read specifications and customer reviews before spending money on a unit for your home (which may cost anything between $500 and $5,000). Understanding a little about the technical side makes it much easier to interpret these, so it makes sense to compare the most commonly used kinds of filters side by side. Or, simply go to our comparison of today’s top rated air purifiers.