Despite the spread of Covid-19, some nations been slow to embrace face masks, despite calls from public health experts.
The president of UK's Royal Society Venki Ramakrishnan said that wearing a mask "is the right thing to do" and that a refusal to do so should be seen as socially unacceptable as drink-driving or not wearing a seatbelt, reports The Guardian.
The World Health Organization (WHO) recommends people wear cloth masks.
Ramakrishnan said that in the UK, "the message has not been clear enough, so perhaps people do not really understand the benefits or are not convinced".
In England, it is mandatory to wear a mask on public transport and in hospitals, for visitors and outpatients. From Friday, masks will be mandatory on public transport in Northern Ireland, and Scotland is also extending that to shops this week. In Wales masks are recommended, though not obligatory, on public transport.
A mask, says Martin McKee, a professor of European public health at the London School of Hygiene & Tropical Medicine, is a "means of reducing the propensity of someone who has got Covid-19 to spread it to others."
"We are not talking about protecting yourself by wearing one, but about reducing the risk to other people," he said.
Wearing a mask is just one measure, along with handwashing and social distancing, to try to contain Covid-19, and seems particularly useful for stopping people who have unwittingly contracted the virus, but who are not showing symptoms, from spreading.
"What you are doing is catching all the little droplets that are coming out of your mouth before they can get into the atmosphere, when they can dry out and become very small and float around as an aerosol. There is still stuff that is going to get out, but you are reducing that risk," McKee added.
McKee and other experts answered some common questions, with many still coming to terms with this "new normal."
Where should one wear a mask?
"Indoors. The risk of transmitting the virus outside is low. The risk is indoors, in crowded situations, where the air is not being filtered out, and particularly where people are speaking loudly, shouting or singing," Mckee said.
People should wear them at the supermarket and while out shopping, said Maitreyi Shivkumar, a virologist and lecturer in molecular biology at De Montfort University.
"Anywhere you are likely to come into closer contact with people you can't really get away from,"
Should one need to wear one while exercising outdoors? McKee says not, but Shivkumar says possibly. "I have seen pictures of lots of people running alongside the Thames in London, and in that scenario I would suggest you wear one. Outdoors, if you're staying away from people, it's fine, but in large crowds you should wear one."
Which type of mask should be chosen?
"The one that may provide maximum protection for the wearer. But we know that the production of them is more difficult and healthcare workers are not getting access to them, so it is important to reserve those for frontline workers who come into contact with Covid-19 patients," says Shivkumar.
Masks with valves – found on dust masks and antipollution cycling masks, for instance – are not thought to be effective at stopping the spread of Covid-19. A study by researchers at the University of Edinburgh into different mask types found that the valves improved breathability for the wearer, but would not stop infectious matter being breathed out.
"For the public, cloth masks are fine," says Shivkumar.
"There is not a lot of data on the efficacy of cloth masks but they're better than nothing."
How many times can one wear a disposable mask?
Jeremy Howard, a data scientist and cofounder of the campaigning organisation Masks4All, says a mask should really be worn once.
"They're not designed to be worn more than once or cleaned," he said.
"They're not a great choice to use at all," he added.
"Disposable masks are generally surgical masks: they are designed to protect from all the stuff that might be coming out of the surgeon's face during surgery. They have to do so many things that they're not perfect at any of them. A cloth mask, on the other hand, can have a better fit and more absorbent materials, and can be reused as many times as you like. It really is a better approach."
Do expiry dates matter?
Some disposable masks come with an expiry date. If one is planning to wear one for everyday use, it might be something to worry about.
"Do you want the official answer?" said McKee.
"I would have to say, yes. But as long as you are using the mask to nip to the chemists on the bus, rather than caring for Covid-19 patients in hospital, the risk of using ones past their expiry date, providing they actually look physically all right, is probably fine for this sort of circumstance".
Are homemade masks effective?
"Nearly any kind of face-covering is effective at blocking droplets coming out of your mouth," said Howard.
"Shortly after they come out of your mouth, they evaporate and become much harder to block, which means it is more difficult for masks to block droplets coming into your mouth."
There isn't good evidence that wearing a mask will prevent one from getting Covid-19, as opposed to spreading it.
What material should be chosen?
Any kind of tightly woven fabric is a good choice, says Shivkumar.
"The more tightly woven, the better," Howard recommends using cotton of 600-thread-count.
"Things such as high-quality bedsheets, for example. Generally, a better-quality cotton is going to have a higher thread count."
The WHO advises a combination of fabrics, with an inner layer of absorbent fabric (to contain the droplets) and a more waterproof outer layer, such as polyester.
Is a scarf just as good?
"It is better than nothing," said Howard.
The fabric may be too thin or, if it is a chunkier scarf, it can be difficult to get several comfortable layers out of it.
"There was some data suggesting that bandanas and scarves are not very good because the fabric has a lot of holes in it," said Shivkumar.
How many layers should the mask have?
The WHO advises a minimum of three, although four layers can be up to seven times more effective than a single bit of fabric.
According to Howard: "You don't want to go overboard because it reduces breathability".
He suggests two layers of cloth with a filter inserted between them; this should be of a different material, such as a piece of paper towel or silk.
"I use a piece of paper towel – I have got a little pocket for it in my mask," said Howard.
"When I come home after going out, I dispose of that, and put in a new one."
How should it fit?
According to the University of Edinburgh study, masks need to have a tight fit to be really effective, though this needs to be balanced with wearability. It needs to be fairly snug to block droplets coming out of your mouth, and even though it has not been proven that a cloth mask rather than personal protective equipment protects the wearer from Covid-19, this may change.
There is "some evidence that masks might directly benefit the wearer", according to Paul Edelstein, emeritus professor of pathology and laboratory medicine at the University of Pennsylvania and one of the authors of a new report for the Royal Society.
There are three places to check for fit, according to Howard. The first is around the nose.
"It is a good idea to use something with a mouldable nosepiece. If your mask doesn't have one, you can get a paperclip, a pipe cleaner or even just a piece of aluminium foil that you can roll up, attach to the top of your mask and mould to your face."
It should be snug around the chin, and fit well over the cheeks.
"This is where cloth masks can be quite a lot better than surgical masks," said Howard.
"You can get or make a cloth mask that goes further, closer to your ears, and then tie it up around the back of your head. Surgical masks tend not to be as wide."
How clean do the hands need to be before touching the mask?
"Before you put the mask on, wash your hands," sayid Shivkumar.
"Only touch the straps; try to avoid touching the mask area."
Howard is a bit more relaxed: "Surface transmission through cloth is pretty much unheard of for Covid-19. Officially, the medical advice is to take it off from the straps rather than the front, and you may as well do that because it's easier."
How to prevent the glasses from steaming up?
A tip from Howard is to submerge the glasses in soapy water and then let them dry by themselves, creating a thin antifog layer on the lenses.
"Otherwise, you can play around with where your glasses sit – you can wear them a bit lower over your nose. If you use a mouldable nosepiece, you can make a tighter fit [at the top of the mask] so that less air comes out."
Does one have to be careful about storing or carrying a mask?
Even if the chance of picking up an infection from mask is low, one should keep it as clean as possible – wearing a dirty face-covering isn't going to be particularly pleasant.
"Put it in maybe a Ziploc bag that keeps it away from everything else – that's what I tend to do," said Shivkumar.
"It is going to be fine if you put it in your bag and it's wrapped in a scarf or something."
Should one change mask during the day?
Howard thinks that, for most, one mask a day should be sufficient.
"If you are doing extremely hot work and getting super-sweaty, you should probably change it if it gets very wet," he added.
It also depends on the material – a thin mask, or scarf, will become damp quickly.
"Once it becomes damp, it is not going to be as effective," said Shivkumar.
"But I am hoping people are going to continue to be sensible and not spend hours together in a situation where they would have to wear masks."
McKee questions whether a damp mask is going to be less effective, but he adds it can cause other problems, such as skin irritation.
"You wouldn't want to be wearing a damp mask."
How often should one wash it?
Treat your face mask "the same way as you treat your socks or underpants", said Howard, adding "as long as you are the sort of person who only gets one wear out of their pants before washing."
"It is good to have a spare mask so you can have one being washed and wear the other one the next day."
How should one wash it?
In usual laundry load, ideally at a hot temperature, but handwashing every evening should also be acceptable, recommended Howard.
Although the WHO advises boiling your face mask for one minute if it has been handwashed in room-temperature water.
"Anything is fine as long as you use some kind of soap that destroys the lipid layer that protects the virus," he said.
"You don't need to wash it separately [from your other clothes]."
Can household share clean masks?
Howard says they can – as long as the masks have been washed thoroughly.
How to get children to wear one?
"In our house, we present wearing a mask as something that's fun and exciting," said Howard.
"We let our daughter pick out which colour she wants."
Explain to children why masks are a good idea.
"We talked about how coronavirus is a disease that can make people sick, and we could even make her grandmother sick if we weren't wearing a mask."
One must make sure that the mask is child-sized and as comfortable as possible, "and particularly think about breathability".
Should a mask be worn in a restaurant?
"If you are sitting indoors, should you wear a mask to order from the waiter and then take it off to eat? I can't see how this would work," said Shivkumar.
"The thing to remember with masks is: it's not everything – it is important along with washing your hands, not touching your face and social distancing. It's part of the bigger picture."
What about visiting someone's home?
"If it is a well-aired house, it is maybe not necessary, but I would say generally if you're indoors, stay 2 metres away and wear a mask, and that will reduce the risk," said Shivkumar.
"It's about recognising the risks and working towards reducing them."
It depends on the situation, says McKee.
"This is a continuum – trying to reduce it to a yes or no is problematic. If you are going to be close to them, if you are going to be there for a long period of time and it is a very confined space, then you're moving towards a point where you may think about wearing a mask. If you are not going to be close to them, or if it is a large room, then you are towards the end of not needing to wear one."
Who shouldn't wear one?
As well as younger children, there are exemptions for some people with health conditions or disabilities and people who assist them.
For example, if one is travelling with someone who relies on lip-reading, they are not required to wear a mask.
"There may be people for whom it may be very difficult," said McKee.
Asthma UK advises people with asthma to try a few different face coverings to see if they can find one that works for them.
"We need to think about people who lip-read," said McKee.
"There are transparent masks that may help, but we have to recognise it is going to be a problem. And it may be an issue for people with learning disabilities."