Many people are wearing surgical masks, even though SARS-CoV-2 is airborne and so a better fitting mask, eg FFP2, would offer a lot more protection. But surgical, and cloth, masks are not useless, if you are infected they are good at stopping you breathing virus-laden particles directly onto to anyone you are talking directly at, and are close to. This is nicely illustrated in a recent work by Bourrianne and coworkers. The basic point is simple, we need to breathe in and out, about once every couple of seconds. When we breathe out we push out a litre or two of air in about a second. This goes through some combination of our nostrils, and a typically mostly closed mouth. This is quite a small area, perhaps a few square centimetres. Basic geometry says that to expel 1 litre through holes say 10 cm2 in area, you need a speed of 1 m/s. This is quite fast and results in a cone of air shooting out from your mouth/nose towards anyone you are facing*. The cone rapidly dissipates in the room air but still you are imparting momentum to your breath which carries it and any nasties it may be carrying towards anyone you are facing.
However, if you place anything over the mouth that allows air through but only with some resistance, then as the jet of air leaves your mouth and nose, it will hit this resistance and spread out along the inside face of the obstruction. Any mask will provide this obstruction. Now, you still need to breathe out a litre of air, but if the material with the resistance to air flow covers the front of your face, with an area of about 100 cm2, then immediately outside the 100 cm2 of material you have a litre of air moving at 0.1 m/s not 1 m/s. Again this is just geometry, expelling 1 litre in one second through an area of 100 cm2, requires a speed of only 0.1 m/s. The resistance spreads out the cone of air and reduces its speed by a factor of about ten, as Bourrianne and coworkers observed
This dramatically reduces the tendency of your breath to spray stuff around, which may be why people sometimes talk about surgical masks as “source control”. It may also be one of the original reasons surgical masks were introduce a hundred plus years ago, so surgeons etc didn’t directly cough germs onto their patients. This effect is also independent of any filtering of your breath by the material of the mask.
It is also not all good. When I went shopping last night, my glasses steamed up as soon as I put my mask** on as I approached Tescos. When you wear a mask, instead of the hot humid air of your breath jetting out away from your glasses, it moves slowly out of the mask. At which point because it is lot hotter than the air outside in the UK in December, it rises (this is convection). Then hot humid air hits cold glasses, which is a recipe for condensation. Annoying, but it is evidence that the mask is doing one of its jobs.
* See the movies in the supplemental material of Bourrianne et al., they’re very nice.
** An FFP2 mask not a surgical mask. FFP2 masks also offer resistance, plus less leakage round the edges so more protection, both for me, and for others if I am infected.