In some countries, wearing a mask is common practice. However, in others, it is a relatively new concept which has taken some getting used to. Naturally, there are plenty of sceptics, and ‘armchair epidemiologists’ are always quick to weigh in on the mask debate.

It is a lot to ask billions of people around the world to suddenly become mask experts. So, it is hardly surprising that there are lots of questions (and misinformation!) out there.

We recently looked at the science behind ‘medical silicone’ and why this material is so ideal for face masks. But what about filters? How do they protect us from coronavirus? To answer that, we first need to look at the size of coronavirus particles…

Under the microscope

Using electron microscopes, scientists have measured coronavirus particles (also known as ‘virions’) and found that the smallest particles are 0.06 microns — 1/1000th of a millimetre — and the largest are 0.14 microns.

However, FFP2 filters are rated to stop 94% of particles at 0.3 microns (the most-penetrating particle size and, therefore, the most difficult size to capture). As a result, many have claimed that these filters cannot possibly block virus particles because the virus is too small.

Clearly, the maths does not quite add up here, so how can these filters protect you from the virus?

As with most things in the world of science, there is much more to it than meets the eye. And these claims fail to grasp the fundamental principles of how virus particles behave and how face masks work. Mask filtering is not comparable to water flowing through a net or a sieve!

Firstly, virus particles do not exist alone — they are always bonded to something larger. When we breathe, talk, sneeze or cough, we generate water droplets or aerosols, which are all larger than 1 micron. The virus attaches to these particles and can then be collected by FFP2 (or equivalent N95) filters with very high efficiency.

Closer inspection

There is also a physical phenomenon known as ‘Brownian motion’, in which particles smaller than 0.3 microns move in an erratic, zig-zagging motion. This motion dramatically increases the chances of the particles being snared by the filter fibres.

Rather than merely passing through, particles are also drawn to the fibre in the FFP2 filters and trapped through something called ‘electrostatic absorption’. The material then loses its electrostatic properties as the charges are dissipated during usage or storage, which is why you are advised to change your mask or filters frequently.

So, although the pores in FFP2 filters are physically around 0.3 microns in size, that does not mean they can only stop particles equal to or larger than that. They are 94% efficient at stopping particles in their least efficient particle size range (0.3 microns), but they are actually even more effective at capturing particles either larger or smaller than that threshold.