Since the pandemic, HEPA filters have been all over the news.  As per the CDC’s recommendation, hospitals, malls, and schools around the country are using HEPA filters to lower transmission rates in indoor places.  But what is it about a HEPA filter that makes them so special?

Why were HEPA filters created?

During the 1940’s, a classified government project called the Manhattan Project was formed to produce the first nuclear weapons. As part of the project, scientists needed to create a filter that was able to remove very small radioactive particles that had been emitted during the manufacturing of atomic bombs.

Scientists used gas masks worn by US soldiers as the model for how to create the filter.  The masks contained a special type of complex paper made from asbestos and cellulose fibers that filtered out airborne toxins in order to keep the soldiers safe during gas attacks.

The Army Chemical Corps designed an air purifier using that same filtration paper to be used in air purifiers at army headquarters along the front line. This early filtration system was the precursor to modern-day HEPA filters.

Today’s HEPA filters are made of either plastic or fiberglass and can be found in HVAC systems, vacuums, and air purifiers.

What does HEPA stand for?

“HEPA” stands for “high-efficiency particulate air” (filter). To be labeled a True HEPA filter, it must be able to trap 99.97 percent of particles that are 0.3 microns.

Why 0.3 microns?

That micron size (0.3) is referred to by scientists as the MPPS, or the most penetrating particle size. Particles of that size are particularly difficult for air purifiers to filter out.

What about particles that are bigger or smaller than .3 microns?

Although the standard for HEPA filters is that they need to be able to filter out 99.97% or more of all particles which are 0.3 microns in diameter, they are actually capable of filtering out particles of almost any size. A True HEPA filter can trap dust, mold, smoke, pet allergens, PM2.5 (dangerous particles that are 2.5 microns or smaller), and even bacteria and viruses… which is exactly why the CDC has recommended them for help in slowing the spread of COVID-19.

How are HEPA filters able to handle so many different sizes of particles?

HEPA filters trap particles in different ways depending on their size.

Particles larger than 1 microns: As air flows through the filter, the larger particles are heavy enough that the airflow from the filter propels them forward into the fibers of the filter where they get stuck.

Particles that are .3  to 1 micron: Particles this size can fit between the gaps in the filter. But, they are too heavy and slow to follow the air flow around the HEPA filter and end up getting stuck in the fibers.

Particles smaller than .3 microns: While HEPA filters only need to filter particles of .3 microns, the truth is that they are also very effective at filtering out smaller ones. Because of a phenomena known as Brownian Motion, tiny particles bounce wildly off of other larger particles in the air in random patterns that send them careening off into different directions. It’s these zigzag patterns that cause them to hit the fibers of the HEPA filter and get stuck.

What does this mean for keeping indoor air clean?

Here is a list of common indoor pollutants and their size:

  • Dust Mite Debris: 0.5 to 50 microns
  • Household Dust: .05 to 100 microns
  • Human Hair: 70 to 100 microns
  • Bacteria: .35 to 10 microns
  • Spores from plants: 6 to 100 microns
  • Mold: 20 to 200 microns
  • Smoke: .01 to 1 microns
  • COVID: .06 to 1.4 microns
  • Influenza: .08 to .12 microns

As you can see, these are all in the range of what HEPA filters can trap.

Are all HEPA filters the same?

Because of the demand for high quality air purifiers, many companies are using the term “HEPA” to describe their filters.  However, a True HEPA filter must meet the United States Department of Energy’s standard of removing 99.97% or more of all particles which are 0.3 microns in diameter.

Air purifiers that use terms such as “HEPA like” or other such phrases have not passed the HEPA standard.. These terms are essentially meaningless and are used in order to confuse consumers into purchasing lower quality filters and air purifiers. Usually, these types of filters are less dense and unable to capture the smallest and most dangerous particles.

Whether you are purchasing an air purifier to remove COVID or other pollutants, look for the HEPA seal to ensure that you are getting the cleanest possible air.