Masks when you are breathing: A hostile environment for viruses?

Early on in the pandemic there were all sorts of schemes going around to allow FFP2 and N95 masks to be reused; in hospitals they are traditionally worn only once (for say a single shift) and discarded but then there was a shortage so people wanted to extend the period a mask could be used. Were these schemes to try and clean a mask needed?

Above is a plot from work published last year by Oswin and coworkers (it is their figure S5). It shows the results of them studying the virus that causes COVID-19, in droplets exposed to varying humidity. The dashed blue line shows the humidity as a function of time over an experiment that lasted a bit over 2 minutes, and the humidity scale is shown in blue on the right. The boxes show the remaining percentage of infectious virus. Note that it drops in each of the two cycles where the humidity was decreased, and then increased again, and is at around 10% after two cycles.

Masks filter out (potentially virus-containing) droplets when these droplets stick to tiny fibres inside the mask fabric. The droplet is then stuck inside the mask. Droplets stuck to the fibres inside a mask you are wearing are subject to a humidity that cycles down and up again every time you breath in and out. So any virus stuck inside a mask you are wearing experiences a humidity that cycles many times a minute (around 16 breaths a minute is typical). Even a modest decrease in infectivity on each cycle would result in your breathing rapidly destroying any virus inside the mask. So cleaning it after use, or leaving it overnight etc (both things recommended at various times) would then be pointless, your breathing would already have killed any virus inside the mask.

It may sound weird that your breathing is so destructive but perhaps that is just because breathing is so familiar with us that there is even an expression: “as natural as breathing.” Because we breathe in and out constantly we don’t even think about it. But as we breathe in and out the temperature, humidity, oxygen and carbon dioxide concentrations in the coming in and out all vary pretty violently. The concentration of carbon dioxide in your breath when you breathe out is a hundred times that when we breathe in fresh air.

Over the course of a lifetime we breathe in and out about half a billion times, how our throat and lungs survive half a billion wild swings in humidity, carbon dioxide etc, is an open question, but they do. Viruses are less tough, the work of Oswin and coworkers suggests that perhaps just a handful of swings destroys them.

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