The evidence so far is that the coronavirus causing Covid-19 spreads predominantly in respiratory droplets — spatters of liquid expelled when an infected person coughs, sneezes or even speaks. Transmission can occur if the droplets reach the mouth, nose or eye of someone nearby, either directly or from an unwashed hand that’s touched a contaminated surface. When an infected person wears a face cover, the droplets can be caught by the mask rather than being expelled. This could be especially useful against Covid-19, because evidence suggests that a third to half of transmissions occur from people who have not yet developed symptoms and so may remain out and about. In theory, wearing a face covering could also help protect people who are uninfected.

2. Why did the WHO change its guidance?

A review of 172 studies funded by the WHO and published June 1 concluded that mask-wearing protects the general public against infection by the coronaviruses responsible for the Covid-19 pandemic, severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS). More backing came out in July. (Previous evidence suggesting that widespread use of face masks may have reduced transmission in outbreaks of SARS and influenza had not generally been considered conclusive.) Before changing its tune in June, the WHO had cautioned that use of masks by healthy members of the public may create a false sense of security, leading people to neglect essential measures such as hand-washing and keeping at a distance from other people. It also said wearing a mask can encourage people to touch their faces, which can increase the chances of becoming infected with the coronavirus.

3. What happened to those concerns?

They’re still around. Health-care professionals emphasize that if you’re going to use a mask, it’s vital to do so correctly or you could become infected through contact with it: Put it on with clean hands, replace it as soon as it becomes damp, remove it from behind without touching the front, wash your hands afterward, and dispose of the mask properly. The WHO counsels against reusing masks that are made for a single use.

4. Who has come around on masks?

U.K. Prime Minister Boris Johnson had resisted making masks mandatory until July 13, when his government announced that face coverings will be compulsory in stores as of July 24. Anybody shopping without a mask could be fined 100 pounds ($125). Australia’s second-most-populous state, Victoria, has started to encourage mask-wearing even though the national government still doesn’t. The biggest U.S. retailer, Walmart Inc., said on July 15 that it would begin requiring customers to wear masks in all its stores on July 20.

5. Where are masks facing resistance?

In Brazil, which has the second-highest number of coronavirus cases and deaths in the world, President Jair Bolsanaro has been criticized for using homophobic language to mock the use of face masks (he tested positive for the virus twice in July). U.S. President Donald Trump downplayed the importance of masks for months before wearing one in public for the first time on July 11, and a recent Pew Research Center poll found U.S. Democrats were more likely to say they wear masks than Republicans. (Trump’s presumptive Democratic challenger in the November election, Joe Biden, said if he were in the White House he would “do everything possible to make it required that people have to wear masks in public.”)

6. What are the different types of masks?

The WHO and the U.S. recommend that most people use non-medical masks so as not to aggravate a shortage of medical-grade versions among health-care workers. Companies have begun offering innumerable styles, and some people are hand-sewing them from fabric or improvising them using bandannas or scarves. Medical masks are regulated devices that come in two types. Surgical masks are the loose-fitting, one-size-fits-all kind that are rectangular when flat. These are the type usually given to patients infected with the virus. Health-care providers who deal with such patients are directed to wear a more sophisticated mask called a respirator, which is designed to protect the user.

7. How are respirators different?

They come in different sizes so they can be fit to the face to provide a tight seal. That forces the user to pull air through the device’s filter rather than through gaps on the sides. They are designed to keep out not only respiratory droplets but also smaller aerosolized particles that can carry infectious agents and float for a time through the air. (In July, the WHO said it doesn’t rule out the possibility of airborne transmission of the coronavirus in crowded indoor spaces with poor ventilation but that more research is needed. Previously it had cast doubt on airborne transmission for Covid-19, except possibly during certain medical procedures.) Respirators are uncomfortable to wear for a long period of time. They retain heat and exert pressure on the face, and some people find they make it difficult to breathe, which can make them unsuitable for those with cardiac and respiratory conditions.

8. What happens when medical masks run short?

Scarcity among health-care providers, U.S. Surgeon General Jerome Adams has said, raises the overall risk to the community: Doctors and nurses who get infected can spread the virus to other people and, once sick, can’t look after patients. The argument is that everyone is better off if medical masks are reserved for those who most certainly need them and who can’t avoid exposure to people who are infected or might be.

9. How are professionals dealing with the shortages?

The U.S. Centers for Disease Control and Prevention issued guidance on how to conserve limited gear and what to do if it runs short. The measures, which don’t meet usual standards, include using respirators beyond their designated shelf life and wearing the same one to attend to several patients without removing it. The agency said that limited reuse of respirators might become necessary but counseled caution. It said a last resort would be to turn to untested versions, including homemade masks. Some U.S. hospitals have already adopted these options, with several recruiting the public to sew masks.

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