Even though there are only five cases of Wuhan coronavirus in the United States, the mask hoarding has begun.
Some pharmacies report being entirely sold out of masks. Some popular sellers on Amazon say deliveries will be delayed for weeks.
Although masks actually do little to protect healthy people, the prospect of shortages created by panic buying worries some public health experts.
Masks are thought to slow the spread of disease when they are worn by sick people in crowded places like emergency rooms, offices, subways and buses. By containing coughs and sneezes, masks stop virus-laden droplets from being spewed into the air and onto nearby surfaces.
But hoarding by those who are well means that hospitals, clinics and doctors’ offices could run short. Doctors and nurses treating patients for respiratory infections should wear masks and replace them often — as soon as they become soggy, the Centers for Disease Control and Prevention says.
The C.D.C. is now reaching out to manufacturers to head off the possibility of shortages, especially in hospitals, an agency official said.
“We see panic ordering and buying that doesn’t reflect the actual need,” said Dr. Anita Patel, the senior adviser for pandemic medical care in the agency’s influenza coordination unit. “We’re talking to manufacturers. They understand the situation, and I’m confident that they are being responsible. The health care industry is their biggest customer.”
On Tuesday, Alex M. Azar II, the secretary of health and human services, said it was “unnecessary” for Americans to buy masks now.
“In the U.S., the risk to any individual American is extremely low,” he said.
Some experts want the government to step in and educate the public about the dangers of hoarding.
“I worked through the 2009 H1N1 flu epidemic at Yale Hospital, and we ran out of N-95 masks — and being in a high-risk situation without enough masks is not a good feeling,” said Dr. Peter Rabinowitz, co-director of the University of Washington MetaCenter for Pandemic Preparedness and Global Health Security in Seattle.
“There’s no rational reason why everyone needs to run out and get masks,” he added. “Public health officials should be talking about this.”
During the West African Ebola outbreak in 2014, when a few cases turned up in the United States, some hospitals were unable to get waterproof Tyvek suits “because there was a run on them,” said Dr. Amesh Adalja, a scholar at the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health in Baltimore.
Panicked people engage in irrational impulse buying, he said: “People like to buy duct tape during emergencies — it’s psychologically soothing.”
But artificial shortages can harm those who really need the goods.
“I think public health authorities like the H.H.S. or the C.D.C. or the Surgeon General should be talking about the repercussions of a run on masks,” Dr. Adalja said.
Kristen Nordlund, a C.D.C. spokeswoman, said she would ask Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases to address that issue in her next public briefing.
Masks are part of the medical equipment stored in the National Strategic Stockpile, which is distributed in government-controlled warehouses around the country. But experts said they did not know how many were in storage or how long the supply was projected to last in an epidemic.
On Tuesday, major pharmacy chains said they were seeing spot shortages but were not yet restricting sales. The CVS chain had stores running out of masks and was resupplying them “as quickly as possible,” said Stephanie Cunha, a company spokeswoman.
Walgreens and Duane Reade pharmacies saw greater demand for face masks and hand sanitizer in many stores, said Alexandra Brown, a spokeswoman for the Walgreens Boots Alliance, which owns both chains.
The company is moving supplies “to meet the needs of our customers,” she added.
Masks are not very protective when worn by healthy people, experts say. People often pull them aside for a variety of reasons.
Vapor from breath can leave masks soggy, for example. Users may want to talk on their cellphones, or put their fingers under their masks to scratch their noses. Frequent handwashing is considered more protective.
Yet there is strong evidence that masks protect health workers.
Trials in Canadian hospitals during the 2003 SARS epidemic and during flu season showed that nurses who wore a mask were less likely to get flu.
Nurses who wore N-95 masks — which are thicker, fit tighter to the face and are designed to filter out 95 percent of all particles — were particularly protective for those who did dangerous procedures like intubating SARS patients.
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There is less data proving that masks keep patients from spreading germs. But this common sense notion has led many hospitals to adopt rules saying that any emergency room patient with flu symptoms is immediately handed a mask and asked to put it on.
Dr. Mark Loeb, an infectious disease specialist at McMaster University in Hamilton, Ontario, who led the SARS and flu trials, said one relatively small study of hospitalized flu patients in Beijing found that mask-wearers were less likely to infect their family members. But the effect was so small that it was considered statistically insignificant.
Another obstacle to mask-wearing is psychological.
People who are sick and should be wearing a mask are often reluctant to do so, because it makes them stand out in the crowd. Unless these patients are ordered to wear one — as they might be in a hospital emergency room — people are reluctant to do so.
“When masks aren’t common in a culture, it does raise eyebrows,” Dr. Adalja said.
The exceptions are in some Asian communities, where it is common for people to wear masks to protect themselves against germs and pollution, or because it is considered impolite to not wear a mask if one is coughing or sneezing.
Experts, including Dr. Patel of the C.D.C., said they knew of no studies of the psychology of mask usage or how reluctance could be overcome.