What is a HEPA Filter? | How Do They Work?

What is a HEPA filter?

While it’s vital to have a properly working HVAC system, you need the right filters to ensure superior air quality. However, knowing the differences between the various filters can be confusing. To choose the perfect option for your home or business, it’s important to understand the wide variety of filters on the market.

Today, a HEPA filter is among the most popular options. Keep reading to learn more about these filters, including how they work and where they're utilized.

What does “HEPA filter” mean?

What are they made from? How do they function?

Interception

As particles travel around fibers, they can eventually get caught up and stick to the sides of them. This is referred to as interception.

Sieving

This refers to the action of particles getting trapped on fibers when there isn’t any room to pass through. While the airflow may bring particles through several fibers, sieving occurs when particles are too large to make it through a fibrous gap.

Diffusion

While there are many different sizes of particles, the smaller ones tend to move around at random. These small particles are more likely to stick to the filter's fibers. This is called diffusion.

Direct impaction

This is the more straightforward method, where large contaminants move in a straight path before hitting and sticking to a fiber. The common particles that follow this path include pollen, mold, and certain types of dust.

Can particles smaller than 0.3 microns pass through? Are they dangerous?

A HEPA filter traps particles at 0.3 microns because scientists have determined this is the most penetrating particle size, or MPPS. Particles both smaller and larger than 0.3 microns tend to get trapped more easily.

While you may be concerned about some particles making it through a HEPA filter, there’s no need to worry—several different filters have additional technologies to ward off the peskiest of particles. For example, some filters are made with activated carbon, which has small pores that trap smoke, odors, and certain chemicals. This acts as a second line of defense to a HEPA filter.

What are “True HEPA” filters? Do they work differently?

As many different providers have made their own version of a HEPA filter, there are various marketing terms at play. One of which is “True HEPA."



This term is used to describe American HEPA filters, which work a little differently than some alternatives in other parts of the world. For example, in European countries, their HEPA standard involve filters that only trap 85 percent of particles at 0.3 microns. This compares to the 99.97-percent standard here in the United States. That said, the American standard for these filters is referred to as “True HEPA.”