To mask or not to mask?

For the past few months, public health officials have been unyielding in their stance that healthy people should not wear masks as a way to protect themselves from coronavirus.

But with new information about how the virus is spread — potentially through the air and by people with no symptoms — the Centers for Disease Control and Prevention on Friday recommended that everyone wear nonmedical face coverings in public settings. President Trump said the guidelines were voluntary, leaving the decision about wearing masks up to individuals. Top health officials, including Dr. Deborah Birx, the White House’s coronavirus response coordinator, warned that masks should not replace social distancing and hand washing.

While longtime advocates for mask wearing applaud the shift, they said it should also include a plan for providing masks to the public. For now, commercially made masks are virtually impossible to find. Many people have hoarded masks in recent months, and everyone agrees that any available supply of medical masks should be reserved for hospitals and emergency workers. That means if you want a mask, you probably have to make it yourself.

“We have a $3 trillion stimulus package and a mask costs very little — they should be free,” said Dr. Siddhartha Mukherjee, assistant professor of medicine at Columbia University.

Last week, the German Medical Association suggested that citizens find or make a simple fabric mask when out in public and leave medical-grade masks for front-line workers. In Austria, grocery store shoppers are now required to wear masks. In New York City, officials advised residents to shield their faces with a scarf, bandanna or other covering when leaving their homes.

“Cover your face with cloth — however you want to do that,” said Shan Soe-Lin, a lecturer at the Yale Jackson Institute for Global Affairs who was a co-author of a widely shared article about the need to cover your face. “Cover your face pretty thoroughly from your mouth to your nose to prevent large aerosol droplets coming out or going in.”

Dr. Soe-Lin said she didn’t understand why public health officials have been so reluctant to recommend nonmedical fabric masks for the public. Even if they aren’t as protective as a medical mask, they are better than nothing, she said.

“We are in the upswing of a pandemic,” Dr. Soe-Lin said. “These cloth masks are protective. It’s a really important complement to the social-distance and hand-washing instructions.”

Experts say it’s important for people to understand that a simple face covering offers enough protection for someone who is practicing social distancing and has only limited exposure to others during brief time outside for exercise or groceries.

The highest-quality, most expensive medical masks — called N95 respirator masks — should be reserved for hospital workers and emergency responders who are regularly exposed to high viral loads from infected patients, both from frequent contact as well as medical procedures that can spew tiny viral particles into air.

“The potential for exposure is so much lower in a grocery store compared to working in a hospital close to patients,” said Linsey Marr, a Virginia Tech scientist and an expert in the transmission of viruses in the air.

If you’re not a health care worker and you have a stash of N95 masks or standard surgical masks, consider donating it to a hospital.

If you’re staying home and nobody in your family is infected, you don’t need a mask most of the time. Studies of mask use to prevent the spread of respiratory illnesses, including SARS, another form of coronavirus, show a simple mask can lower risk of infection. The effect is greatest when masks are used along with hand hygiene and social distancing.

“I think the vast amount of data would suggest that the coronavirus is an airborne infection carried by respiratory droplets, and it also can be passed on by direct contact,” said Dr. Mukherjee, who recently wrote an article about how the coronavirus behaves inside patients. “The mask works two ways — not only to protect you from me, but me from you.”

If you decide to start wearing a mask, you should know that it takes some getting used to. A mask can be hot and uncomfortable and fog your glasses if you wear them. But pulling it up and down defeats the purpose of wearing it.

While we don’t have a lot of research on the effectiveness of homemade masks in preventing the spread of infection, scientists who study airborne diseases can offer some guidance. A mask sewn from a pattern or an improvised face covering made with a T-shirt probably offers some protection. The thicker the fabric, the better: think heavy cotton T-shirt or a thick, felt-like fabric, said Dr. Marr, the Virginia Tech aerosol scientist.

While some people have suggested using a bandanna, the fabric is typically so thin and flimsy that it would likely offer little protection. Double or triple the bandanna fabric if that’s all you have.

“I’ve been saying some protection is better than none,” said Dr. Marr, who noted that local health departments had been asking aerosol scientists for guidance on potential mask materials to deal with supply shortages. She said her team would have results soon with more specific recommendations for materials to use in masks.

Dr. Soe-Lin said she believed an added benefit of a mask was that it serves as a constant reminder against touching your face, a major way that the virus is spread. But no face covering, whether it’s homemade or a medical mask, makes you invincible. Pulling a mask on and off or fidgeting with it will lessen its effectiveness. And in theory, fiddling with your mask could contaminate it. Always remove a mask by the ear loops or the tie — never the part that covers your face.

Dr. Soe-Lin said she had used cloth masks for three weeks and washed and dried them regularly. Someone with only one mask can hand wash at night and let it air dry. If a mask gets wet or damp while you are wearing it, it’s less effective, she said.

Other experts said worries that fabric masks won’t offer enough protection were misguided.

“I don’t think there is any evidence that this is going to make things worse, but there is evidence that it provides some additional good,” said Robert Hecht, professor at the Yale School of Public Health, who was the co-author of the face mask article with Dr. Soe-Lin. “Under this emergency situation we’re in, it seems, in our view, hard to argue against covering your face. We have large numbers of infections occurring which don’t need to happen if people were to use the masks.”

Abby Goodnough and Margot Sanger-Katz contributed reporting.