Understanding HEPA Air Filters and Covid-19

by Jim Rosenthal

The biggest problem that HEPA filters have in capturing COVID-19 particles is not their size. It is the fact that they do not make it to the filter.

How HEPA Filters Can Reduce the Probability of Covid-19 Spread

“HEPA filters remove particles down to 0.3 microns. COVID-19 viruses are 0.125 microns so a HEPA filter cannot remove them from the air.”

The above statement is often found in news articles about COVID-19. It is WRONG! HEPA filters remove over 99% of 0.125 micron sized particles!

Even in well researched pieces people keep making the same mistake. It is clear that there is a widespread misunderstanding of how air filters work. The purpose of this article is to give a concise explanation of how air filters remove particles and to clear up this misunderstanding.

Air filters capture particles by five mechanisms:

  1. Straining
  2. Impaction (Impingement)
  3. Interception
  4. Diffusion
  5. Electret charge

Most people think that all filters work on the basis of the first mechanism – straining. This is where the particle is too big to fit between two fibers and is pulled out of the airstream much like a colander catches noodles. This misconception is the basis for most of the problem in understanding air filters.

With a basic understanding of how filters work, let’s look again at the oft repeated mistake on HEPA filters. (HEPA stands for High Efficiency Particulate Arresting filter.) The definition of a HEPA filter is one that “removes 99.97% of the particles at 0.3 microns.” The reason 0.3 microns is used is that this is the most difficult size of particle to remove. It is called the Most Penetrating Particle Size (MPPS).

For particles less than 1 micron the two filtration mechanisms that come into play are interception and diffusion. As the size of the particles go down, the effectiveness of interception decreases. On the other hand, in the smallest sizes diffusion is very effective but decreases as the size of the particles goes up. The result is an efficiency curve for a HEPA filter that looks like this:

If we assume the particle size of COVID-19 is 0.12um, the efficiency of a HEPA filter is 99.97%. It is not zero as many report in the media.

Let’s take a look at the efficiency curve for a HEPA filter as described in news articles:

Once one understands how filters work, it seems a bit ridiculous. The chart shows efficiency going from 99.97% at 0.3 microns to 0% at 0.25 microns. A HEPA filter will clearly capture an extremely high percentage of Covid-19 even assuming a 0.12 target particle size. And this is a big assumption. In all probability the target size for the virus is significantly higher. Here’s why:

Viruses do not “live” on their own. They invade organisms by taking over the RNA of their host. They are a parasite. An analogy that aptly describes their behavior is to think of viruses as “software.” The hosts they invade are the “hardware.” The current thinking is that the combination of viable viruses and the host is greater than one micron. Particles greater than one micron in size will still stay in the air for an extended period of time (several hours), but they are much easier for filters to capture than sub-micron size particles.

The biggest problem that HEPA filters have in capturing COVID-19 particles is not their size. It is the fact that they do not make it to the filter. A very good HVAC system might provide 6 Air Changes per Hour (ACH). This means that all of the air in the space will go through the system every ten minutes. A “super spreader” in a room is continuously giving off particles. So even in the best circumstances some people in a room could still be exposed. However, the probability of exposure is dramatically reduced. And at the end of the day that is what it is all about. In today’s pandemic world there is always the possibility of exposure. It is important to make decisions that reduce the probability of that exposure. HEPA filters are certainly a good option to do so.

Actual HEPA Efficiency

(Click to enlarge.)


(Click to enlarge.)

Jim Rosenthal

Jim Rosenthal

Jim Rosenthal is the Chairman and Chief Executive Officer of Air Relief Technologies, Inc. He has over 20 years of experience in the air filtration and environmental control industries and has been active in the National Air Filtration Association (NAFA) – serving as its President in 2009-2010. He is a Certified Air Filter Specialist (CAFS) by NAFA. He is also involved in air filter test standards and is currently a voting member of the ASHRAE 52.2 Committee. Jim has given over 50 presentations and training seminars on air filtration and indoor air quality.

Other Articles in this Series

This article appeared as part of the feature story in the August / September 2020 print issue of Southern PHC. Featured alongside were COVID-19 & The HVAC Industry and Case Study: Covid-19 Associated with Air Conditioning in Restaurant.