A case for “HEPA” and Air Filtration

By Dr. Stefan Wagener SM (NRCM), CBSP, RBP (16)

November 24, 2022

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One of the many lessons learned during the last two years of Covid was that one of the most significant ways the virus was transmitted was through the air. Not just the air carrying big droplets that don’t go far, but the air with tiny droplets of virus suspended and moved throughout the room or the facility. That realization highlighted the need for clean air in our work, schools, and home environments. In addition, it brought out many new or semi-new methods, tools and approaches to get the air cleaned. We have UV lights, ozone generators sold as air cleaners (1), air treatment with chemicals, etc. As the EPA says:..” The most effective ways to improve your indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air…”(2). One of the other most efficient and straightforward non-chemical or radiation-based air treatments is air filtration. Since viruses and bacteria, in effect, are particles, we can use HEPA filters!

Let me, however, make one point very clear. The misinformation about what HEPA filters do and do not is unbelievable. Most of it is centred around misbelief, lack of knowledge, or even intentional misinformation that a HEPA filter is not working below a specific size of particles. What do I mean by that? We hear and read the following statement: “A HEPA filter has a filtration efficiency of 99.97% at a particle size of 0.3mm (micrometre)”.

That’s good, right? Yes, it tells us that when 10.000 particles, each having a size of 0.3mm, travel through a HEPA, only 3(!) come out at the other end. That is very efficient! Now, we still need to know what 0.3mm is in comparison, and it also does NOT tell us what happens with smaller or larger particles. We only hear that it’s 99.97% at 0.3mm. To give you a few examples, human hair is in the 100mm range, pollen in the 30mm range, bacteria are around 3mm, viruses like SARS-CoV2 (Covid) have a size of 0.1mm, and RSV is around 0.2mm.

Clearing up misinformation

Unfortunately, many folks and companies alike take this 99.97% at 0.3mm specification and tell it in another (often wrong) way. They say that a HEPA filter only goes down to 0.3mm and does not work below that. They even publish comparison charts and promote their products by publishing this misinformation. Let’s clear this up!

First, a little history:

HEPA is an acronym standing for “High-Efficiency Particulate Air.” It describes a specific type of filter developed in the early 40s to protect against chemical, biological, and radiological warfare agents (3). That filter is a pleated mechanical filter meaning it uses a folding pattern in its design to filter the air. It is usually made from special fibres designed for filtration. In other words, the HEPA filter relies purely on physically removing particles in the air. No chemicals, radiation, or other potentially harmful ways, just plain physical filtration. The HEPA filter has a very long and successful history, and it is and will be the gold standard for a long time in air treatment for Level 4 laboratories worldwide. These work with Ebola and other highly infectious and deadly viruses. Think about this for a minute. It better be good if it is the gold standard for high security and safety laboratories.

How does the HEPA work:

Since a HEPA filter works by physically removing particles, we need to discuss what that “physics” is. If we have a large particle, say in the millimetre range, it will be trapped by direct “impact” on the fibres (also called “inertial impact”). It is like trying to drive your car directly through a densely wooded area where the trees and the branches are the fibres. You do not get very far! Once particles get smaller, they are not stopped by impact anymore but by “interception.” It’s like you are not driving the car this time, you are shooting an arrow into the woods, and yes, it will be just a matter of time before the arrow gets intercepted by a branch or tree. If particles get even smaller, they attach to the fibres by diffusion. No car, no arrow, this time, it’s the butterfly (figuratively speaking) going up and down and left and right through the woods before it sits on a flower or leaf. If the particle gets even smaller, it is all about electrostatic attraction. Opposite charges attract each other. We know about that one J

When scientists investigated the performance of HEPA filters, they found something very interesting. Diffusion and electrostatic attraction worked very well with particles that were in the 0.01 to 0.1 mm (~virus size) range, while interception and impact worked very well above 0.4 mm. They found a particle size range between 0.1 and 0.4 mm, where the HEPA filters were not the best, and the lowest efficiency was around 0.3mm. However, for a HEPA filter, that still means it is 99.97% efficient! Now you understand why a HEPA filter is measured at 0.3mm. Any certified HEPA filter will be at least 99.97% efficient at 0.3mm, and what’s even more critical, its efficiency goes up towards 100% when particles are smaller or larger.

Any certified HEPA filter will be 99.97% efficient at 0.3mm, and what’s even more critical, its efficiency goes up to 100% when particles are smaller or larger

The HEPA gets tested at its “worst” filtration efficiency, which is still a fantastic 99.97%. To make a long story short, the HEPA will filter out viruses in the air, bacteria, dust, pollen, and other particulate contamination.

What it does not do, is filter out any chemical smells or odours. To help with that, many HEPA filters are also attached to a carbon filter that can take care of that. However, a HEPA filter is an excellent way of keeping your air clean, and it can be placed inside your HVAC system or on your desk to control particles right at their source. Is there a downside? Yes! Because the HEPA filter is so efficient, it can load up quickly with dirt and dust in the air. In HVAC systems, HEPA filters typically have pre-filters that take care of the big stuff and can be quickly and less expensively changed.

Interestingly, the efficiency of the HEPA gets better when it gets loaded (dirty). The problem is that the air cannot move anymore, and your fan will not be able to pull enough air through it. So, we often replace a perfectly working filter because of the lack of air movement.

I hope this short dive into some physics and filtration has helped clarify the use and function of an oldie but goodie, the HEPA filter. The next time you see a product that is compared to a HEPA filter, take a close look at the claims and their accuracy.

Let’s put science back into cleaning, disinfection, and infectious disease prevention!