By Andrew Polmear, our intrepid wine and cheese man – it’s OK kids, he’s a retired doctor

Most of us are pretty good at wearing a mask in a public place, possibly because we’d get thrown out if we don’t. But have a look round on a bus or train and you’ll often see someone whose mask is useless – either below the nose or even round the chin. There are people, of course, who are exempt, but they wouldn’t be wearing one at all. Does it matter? How good is the evidence that wearing a mask saves lives?

Comparing different countries. The first sort of evidence is from countries where almost everybody wears a mask when they leave home. One careful study showed that transmission of SARS-CoV-2 (the virus that causes Covid-19) is 7.5 times higher in countries where mask wearing is not universal compared to those where it is. That’s not proof, of course – the difference may have been due to something else; they may also have been hand-washing, or distancing, or just not shaking hands, for example. But the masks probably help. A study of US states found that states in which mask-wearing was compulsory had slower disease growth rates than states where it wasn’t.

Studies that asked contacts of coronavirus cases whether they wore a mask or not, then checking whether or not they caught the disease. It’s not a reliable technique – it all depends on people’s memory and honesty. Roughly, wearing a mask seemed to result in half the risk of catching coronoavirus disease. But almost all of these cases were SARS or MERS, not Covid-19.

Lab studies. The final sort of study is done in a laboratory where they measure particles the size of the water particles that carry coronavirus. Researchers have looked at the different situations that arise. I’ll focus on where the contagious person is coughing:

• The person coughing is masked: a mask roughly halves the number of particles getting out. (Warning – different studies come up with different figures, according to what size particles were measured and how far away from the person being tested.) A 3-layer homemade cloth mask does as well as a surgical mask. An N95 mask may do a bit better but it’s quite a bit more expensive. Whether it fits properly may be more important than what it’s made of.
• The person with the mask is in the same room as the person coughing (who is not masked): there is benefit but it’s much less.
• If both are masked the benefit is much greater at about 80%. But what really makes a difference is if both wear double masks – that is, a surgical mask with a fabric mask over the top. That reduces particle transmission by 95%. It may be that the double masking works more by ensuring a close fit, than by the ability of the extra mask to filter more particles. Try it, it’s quite comfortable!

Where should we wear those masks? Obviously in public enclosed spaces. Keeping 2 metres apart isn’t enough. Those masking studies I mention above were done 6 feet apart. I think masks should also be worn out of doors. If there are no crowds and a good breeze you would probably be all right, but you can never protect yourself against the panting jogger who pushes past you, or against bumping into someone as you round a blind corner. If you can smell the cigarette smoke of a smoker you are inhaling their breath.

Finally, what about in the home if a family member becomes ill or is a contact. There’s just one study on this, from Beijing. Transmission of Covid-19 was reduced by 79% if everyone in the household was masked. But there’s a catch. Masking has to be all the time, including before the person who becomes ill develops symptoms. That’s partly why Covid-19 is such a problem. The time when a person is most contagious is from 2 days before symptoms begin until 1 day after they start. If the person you live with is a contact you have time – mask up straight away, don’t wait for them to become ill. If they develop symptoms out of the blue you’ve probably missed the boat, but mask up anyway.

PS. You may be wondering whether higher quality masks would do better than the fabric and surgical masks described above. After all, an FFP2 mask is, by EU definition, one that filters out 94% or more particles (roughly the same as an N95 mask (the US equivalent) and a KN95 (the Chinese equivalent)). An FFP3 mask filters out 98% or more particles. The answer is no, not necessarily. They were designed for industrial use, not specifically for viral protection. Also, those EU tests do not attempt to mimic the human situation, with one person coughing, or talking, or even just breathing out, and another person breathing in. They may still be better than fabric or ordinary surgical masks, especially if properly fitted, but don’t expect 94% or 98% reduction in transmission. And, of course, don’t wear a mask with a valve in it – that would allow out any virus you are exhaling.

PPS. Those who wear glasses find masks a problem. Their moist breath fogs up the inside of the lenses. From all the people who have commented on this it is clear there’s only one solution: wear the mask high up on your nose and perch the glasses on top of it. One eye surgeon, who needs to wear glasses while operating, tapes the top of his mask to his nose, so it can’t slip down below his glasses.

This is an area of science where new information is coming out every day. By the time this article is published it may already be out of date. And please bear in mind that it is the opinion of one retired GP, not a leading scientist. Andrew regrets that he cannot enter into correspondence with individual readers.