We’ve all heard it before. Surely you can’t stop a virus with a filter because the gaps in the filter are too big and something as small as a virus would just pass through?
Well hold on!
It’s not that simple!
EPA (Efficient Particle Air Filter) and HEPA (Highly Efficient Particle Air Filter) really do capture tiny particles as small as viruses with incredible efficiency. Even though the gaps in them are much bigger than they are.
In fact – (fun fact) – the smaller the particle is, the better these filters are at capturing it.
Yes…. Really. You read that correctly. EPA and HEPA filters capture tiny nano particles more efficiently than, say, mere microscopic particles even though these are much, much smaller than the gaps in the filter itself.
It sounds counterintuitive, but these filters make use of a couple of bits of funky physics to achieve this. Here’s an explanation with, of course, an angle related to our unwelcome guest the Coronavirus…..
A Coronavirus particle is approximately 0.12 microns in diameter, and that is absolutely tiny. In fact, it’s been calculated that all the Coronavirus in the world wouldn’t fill a coke can.
It’s small. Really, really small, but….
Viruses and Germs don’t travel alone. They travel in aerosols which all of us breathe out. The WHO defines an aerosol as anything which can remain suspended in the air including dust or fungal spores. While there’s some debate about the physical definition of an aerosol a particle of 5 microns or smaller has usually been used as the benchmark. That’s bigger than a virus, but still minute and much smaller than the gaps in a filter, so what else does a filter have up its sleeve?
Firstly, filters are pleated to ensure that their available surface area for filtration is enormous in relation to their size. We don’t necessarily advise you to do this (in fact we’ve done it so you don’t have to), but if you were to unpleat a standard 12 x 20 HEPA filter you would be left with a strip of material that was approximately 9m or 30ft long. (Don’t do it. Just don’t.…)
This is a principle with analogies in nature. Your lungs and your intestines are far more effective at absorbing tiny oxygen and nutrient particles than a simple measurement of their size suggests they should be. This is because their surfaces are packed with “pleats” called alveoli (in your lungs) and villi (in your intestines) which massively increase their surface area.
The filter’s enormous surface area means a tiny particle has to pass through a lot more filter material than it would if the filter were unpleated, drastically increasing the chances that it will be trapped.
Secondly, unlike normal fabric, the fibres which make up a filter are packed together irregularly. This means that there is no neat, straight gap in a filter that a particle can travel through. If you want to picture it, think of a particle navigating an obstacle course, blocked by fibres at every turn.
Moreover, the outsized (very large) surface area surface area of the filter means a particle has to effectively pass through several obstacle courses without being trapped before it can find freedom on the other side. Hopefully you’re already forming a picture of why this is becoming unlikely!
This is the final trick, and also explains why filters become more efficient at trapping particles the smaller they are.
When air is passed through a filter, the air molecules knock the very tiny particles in it around randomly. This phenomenon is known as Brownian Diffusion, and means that small particles pinball around as they move. Even if you had a straight gap through a filter, small particles would be more likely to hit the sides of it than to pass straight through it.
This is where the final trick comes in. The fibres which make up the filter are made “sticky,” with a long lasting electrostatic charge that holds particles.
Have you ever rubbed a regular party balloon on a carpet and then watched in awe as your hair stands up when you pass it over your head? If not, try it.
Rubbing the balloon on the carpet has given it an electrostatic charge, and this attracts your hair.
Electrostatically charged fibres do the exact same thing to tiny particles. Random, Brownian movement makes tiny particles more likely to hit a fibre. As soon as they do they are captured and held fast by its electrostatic charge.
It’s game over.
The huge surface area of the filter, its random weave of electrostatically charged fibres and the random way small particles move make it almost impenetrable. Even to tiny viruses and the aerosols which carry them.
Almost impenetrable…. Of course some get through. That’s how randomness works, but here are some figures which show you just how good this is.
An E11 EPA Filter captures 95% of particles measuring 0.3 microns in diameter in air which passes through it, while an H13 HEPA Filter captures 99.97% of particles of the same size. A Study by NASA showed they’re even more efficient at capturing smaller particles (remember, Brownian Diffusion).
So all in all an Air Purifier with an EPA or HEPA filter is going to remove practically all the tiny particles, including virus and germ containing aerosols that pass through it. Even though the gaps in the filter are much bigger than the particles.