Whether wearing a face mask can help prevent the spread of coronavirus has been a divisive subject among health professionals.
So far, advice on containing the spread of Covid-19 has largely been based on knowledge of similar viruses spread via cough droplets, and so covering your nose and mouth when sneezing is strongly advised. However, the jury is still out on face masks.
The mayor of London has called for the government to change its guidance on face masks to make them compulsory for all Londoners, while scientists and politicians met last Thursday (23 Apr) to discuss whether the public should now be advised to wear them.
Here’s what we know so far about the effectiveness of face masks, the current official guidance on who should wear them - and what new research says.
Should I be wearing a mask?
The World Health Organisation (WHO) is currently only advising those who are sick and showing symptoms of coronavirus to wear a mask, such as coughing or sneezing.
If you are healthy, you only need to wear a mask if you are caring for someone with a suspected coronavirus infection.
The WHO emphasises that wearing a mask is only effective if it is combined with frequent hand washing with soap and water, and an alcohol-based hand rub.
Experts from the Scientific Advisory Group on Emergencies (Sage) met on Tuesday (21 Apr) to debate the issue, and recommended the public should wear homemade masks when they venture outdoors.
The scientists have passed on research to the government showing that coverings could help to stop asymptomatic people from passing on the virus, and claimed the UK’s current policy on masks does too little to prevent infections.
Scientists believe that while masks won’t prevent people from catching the virus, it could help in stopping it being spread to others.
The chairman of the Royal College of General Practitioners Professor Martin Marshall said: "If [people] are coughing and spluttering then it makes complete sense to wear masks in order to protect other people.
"I think the guidance that we're expecting to hear is that the wearing of face masks is a voluntary activity not mandated and it certainly makes a lot of sense to focus limited resources that we have at the moment on those who have greatest need and that's the health professionals."
How should I wear and dispose of a mask?
If you are going to wear a mask, the WHO recommends cleaning your hands with an alcohol-based hand rub, or soap and water, before putting it on.
You should cover your nose and mouth with the mask, and make sure there are no gaps between the mask and your face.
Avoid touching the mask while you are using it - and if you do, thoroughly wash your hands after.
Replace the mask with a new one as soon as it is damp, and do not reuse single use masks.
The mask should be removed from behind - do not touch the front of it - and immediately discard it in a closed bin.
Wash your hands again after you have disposed of it.
Should more people be wearing masks?
The WHO is currently advising people to keep a distance of at least two metres away from anyone who is coughing or sneezing to avoid the risk of infection.
This is because evidence has shown that viruses like coronavirus can only be transmitted within droplets of liquid, such as from coughs or sneezes.
It is understood that most of these droplets will either evaporate or fall to the ground close to the person who released them.
However, if people who are displaying symptoms of the virus cover their mouth with a mask, this helps to prevent droplets being released into the air, thereby minimising the virus spreading.
The WHO is considering changing its guidance, although it is concerned that making masks compulsory could lead to complacency about social distancing and handwashing.
The WHO has said that medical masks must first be prioritised for health workers, but can be useful in preventing the spread of the virus for those who have coronavirus symptoms.
Officials in New York City are now urging people to wear masks when they go outside, with Mayor Bill de Blasio stating that coverings for the nose and mouth can be “homegrown”, and made from a scarf or bandana.
It is believed the new US Government guidance would suggest that non-medical masks, such as T-shirts or bandanas, be used to cover the nose and mouth when in public. Professional-grade masks are to be kept for healthcare workers.
Los Angeles mayor Eric Garcetti has also encouraged residents in the city to cover their faces while in public.
Scientists are now recommending the UK takes a similar approach to the US, advising people wear homemade masks outdoors in an effort to prevent the spread, while the British Medical Association (BMA) has said that all essential workers outside of the NHS should be given face masks to wear when outside of their homes.
What does new research say?
New research from the Massachusetts Institute of Technology (MIT) in the US used high-speed cameras and sensors to assess exactly what happens after a person coughs or sneezes.
The study found that after exhaling, a small fast-moving cloud of gas that can contain droplets of various sizes liquid is released, which can be carried over long distances.
Findings showed that coughs can project liquid up to six metres, while sneezes can reach up to eight metres away.
Professor Lydia Bourouiba of MIT, told the BBC: “What we exhale, cough or sneeze is a gas cloud that has high momentum that can go far, traps the drops of all sizes in it and carries them through the room.
"So having this false idea of safety at one to two metres, that somehow drops will just fall to the ground at that distance is not based on what we have quantified, measured and visualised directly."
"Flimsy masks are not going to protect from inhaling the smallest particles in the air because they do not provide filtration.
"But they would potentially divert the cloud that is being emitted with high momentum to the side instead of forward."
However, a spokesperson for Public Health England (PHE) argued there is little evidence that wearing masks outside of clinical settings offers any widespread benefits: "Face masks must be worn correctly, changed frequently, removed properly, disposed of safely and used in combination with good universal hygiene behaviour in order for them to be effective.
"Research also shows that compliance with these recommended behaviours reduces over time when wearing facemasks for prolonged periods."