Demystifying HEPA Filters

Updated: Jun 16, 2020


Lately, you may have heard the acronym HEPA in the news and many social media posts on COVID-19 prevention. You may be wondering what it means, and how it affects you.


What is a HEPA Filter?

HEPA stands for High-Efficiency Particulate Air, and it's a very specific rating for air filters.

HEPA filters are rated to filter out 99.97% of particles 0.3µm (microns) in diameter.


How Do Filters Work?

Air filters remove particles through 4 different methods: inertial impaction, interception, diffusion, and electrostatic attraction.

  1. Inertial impaction: a particle's inertia causes it to detour from the air stream and collides into a filter fiber.

  2. Interception: a particle collides with a filter fiber as it is carried by the air stream.

  3. Diffusion: random motion of a particle causes it to collide with a filter fiber.

  4. Electrostatic attraction: electrostatically charged filter fibers attract and capture small particles.

Figure 1. Filtration mechanisms.

From Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks (p. 10), by Earnest, G. S., Gressel, M. G., Mickelsen, R. L., Moyer, E. S., Reed, L. D., Karwacki, C. J., ... & Persily, A. K., 2003, Cincinnati, Ohio: National Institute for Occupational Safety and Health (NIOSH).


Particles larger than 0.2 microns are filtered via inertial impaction and interception. Particles smaller than 0.2 microns are filtered diffusion.


What about particles smaller than 0.3 microns?

This is a question that is often asked after reading what the HEPA filter rating is. 0.3 microns is the particle diameter size chosen for filter ratings because filters have the most difficulty removing particles of this size. The 99.97% filter efficiency of a HEPA filter for particles 0.3 microns in diameter is the HEPA filter's lowest filtration efficiency for any particle diameter. In other words, filtration efficiencies for particles greater than or lower than 0.3 microns are higher than 99.97%, as shown in the graph below.


Figure 2. Filtration efficiencies of air filters.

From Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks (p. 21), by Earnest, G. S., Gressel, M. G., Mickelsen, R. L., Moyer, E. S., Reed, L. D., Karwacki, C. J., ... & Persily, A. K., 2003, Cincinnati, Ohio: National Instit