To mask or not to mask?

For the past few months, public health officials have advised that healthy people should not wear masks as a way to protect themselves from coronavirus. But as we learn more about the virus, more experts are challenging the official guidance and saying that there is probably some benefit to covering our faces in public.

But 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.

“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.”

Last week, the German Medical Association suggested 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, Governor Andrew M. Cuomo has advised anyone over 70 to wear a mask.

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 rest of us don’t need that level of protection.

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. But more experts now say that wearing a nonmedical or homemade mask to go the grocery store or pharmacy may be a good idea. 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. Siddhartha Mukherjee, assistant professor of medicine at Columbia University, 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.”

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 offer some protection. The thicker the fabric, the better: think heavy cotton T-shirt or a thick, felt-like fabric, said Linsey Marr, a Virginia Tech scientist and an expert in the transmission of viruses in the air. 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 have been asking aerosol scientists for guidance on potential mask materials to deal with supply shortages. She said her team will have results soon with more specific recommendations for materials to use in masks.

Dr. Marr emphasized that most people do not need the high level of protection offered by a medical mask. “The potential for exposure is so much lower in a grocery store compared to working in a hospital close to patients,” she said.

Dr. Soe-Lin said she believes an added benefit of a mask is 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 has used cloth masks for three weeks and washes and dries 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.

“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.”

Dr. Mukherjee said he is hopeful that large companies and the government will produce and distribute inexpensive masks for essential workers like grocery clerks and delivery drivers, as well as the general public. Questions about durability, reuse and sanitizing masks, as well as the best fabric to use in a homemade mask, still need to be answered.

“My strong preference is for people to not cobble together homemade masks,” Dr. Mukherjee said. “People are using them because they are desperate, and they don’t know what to do.”

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.

“I still believe that masks are primarily for health care workers and for those who are sick to help prevent spreading droplets to others,” said Dr. Adit Ginde, a professor of emergency medicine at the University of Colorado School of Medicine. “However, I do believe that for limited circumstances when individuals must be in close quarters with others, a correctly positioned mask or other face cover for a short duration could be helpful.”