The bar graph compares examples of the three basic types of mask: a cotton mask (as in the fabric ones you can wash), a surgical mask (these are typically blue and are disposable), and a mask that meets the USA standard for personal protective equipment (PPE) – this is called the N95 standard. The height of the bar quantifies the amount of protection offered, for example, a ten on this scale means a reduction in dose of particles by a factor of ten. In other words, 90% of the particles are filtered out. Masks are just air filters we wear on our faces. Note that the y axis is a log scale – there are large differences in the amount of protection! The data is from Duncan, Bodurtha and Naqvi.
The blue bars measure how well the material of which the mask is made of, filters out particles. This is for the particles that the material is worst at filtering, i.e., a worst-case situation. How good a material is at filtering particles, depends on the size of particles. So the blue bars are worst case measures of how good the material is at filtering virus. Cotton is not a good filtering material – perhaps unsurprising as it was developed to make jeans and t-shirts not filters. Surgical masks are better, because they are made of a non-woven material which is designed to filter. So the blue bar for surgical masks is a lot higher than for cotton.
However it is lot lower than for the material N95 masks are made of. The material this surgical mask is made from is nowhere near a good as filter as the material the N95 mask is made from. I think the problem is that the standard for surgical masks is rubbish. The only filtration requirement for a “Type II”* surgical mask, is to filter out 95% of an aerosolised bacterium. As COVID-19 is caused by a virus this is not very useful – especially as the virus SARS-CoV-2 is much smaller than bacteria. The N95, and the European equivalent, the FFP2 standard, are both much more rigorous.
It gets worse. Not only does the Type II mask standard require the material of which it is made to just filter most aerosolised bacteria, it says, as far as I know, nothing about how well the mask should fit. But a badly fitting mask is a bad mask. Any air going through gaps at the side of your cheeks or either side of your nose, and so not through the material of the mask, is not filtered.
The performance of the mask – not just the material but including fit to the face – is measured by the orange bars**. For the surgical mask, the orange bar measuring mask performance is lower than the blue bar measuring material performance. Surgical masks typically fit poorly, compromising their performance. Cotton masks by contrast may fit better, note here mask performance is a bit higher than material.
By contrast, the N95 (and FFP2) standards are not just standards for the material of the masks but how well they fit. So the orange bar measuring mask performance is very high. An N95 or FFP2 mask must be designed to fit your face snugly with few gaps because the standard requires this, a surgical mask does not. A cotton mask can be designed to fit your face well, but it is still made of cotton, which is still rather poor at filtering.
So, the N95 mask reduces the dose by a factor of over a hundred. Now, it will be tough for a regular non-healthcare-worker person, to achieve a fit good enough to give a reduction by a factor of over a hundred in the particles. Most of us will struggle to get a very good fit between mask and face. But Aaron Collins on his YouTube channel (see also previous blog post) gets performances of 20 and above, with FFP2 and similar masks. So a reduction in dose of virus of at least a factor of 20 looks possible, but only with an FFP2 or similar mask. Not with a surgical or cotton mask.
So, why are people wearing the masks that perform so poorly?*** Possibly people don’t realise that there is a standard for masks, and that if you want some guarantee that the mask works, you should get one that meets a standard. And even if they do, they don’t know that there are two different standards for masks. And that one of the standards (Type II) is pretty rubbish.
To be fair on people, given that these masks are there to protect us from harm, it is rather strange both that the UK government is not advertising these standards, and that there are two very different standards. For example, I don’t think there are two different standards for car seats for children, one of which is rubbish, with car seat makers able to choose which standard to follow.
* Type I surgical masks are even worse than Type II ones.
** The orange bars are filtration measured over a range of particle sizes, not just the size the material is worst at filtering. In other words inlike the blue bars they are not the worst-case situation. This is why the orange bars can be a bit higher than the blue ones. See Duncan, Bodurtha and Naqvi for details.
*** One disadvantage FFP2 masks have with respect to cotton masks is that FFP2 masks are not washable, but with normal office/shopping use you could get a week or more’s use out of an FFP2 mask. As far as I know the only advantage surgical masks have over FFP2 is that they are about half the price.