Nor were her masks designed to protect people from coronaviruses. Brown tells me she’s been emailing with a virologist in the hope of getting expert advice on what—if anything—she can tell people about their effectiveness against transmission of the new outbreak. But he responds with long answers about how it depends on the properties of any given virus, and how much a contagious person is secreting, and how closely the person comes into contact with others. Essentially, no masks are perfect, and their value always depends on the context in which they’re used. In moments of fear, even a small chance of protection can become extremely valuable to people who feel they have few other actions to take.

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This may be why some people in the U.S.—still at very low risk of contracting the new coronavirus—have newly begun wearing masks. Americans are now diving into and depleting global stock that could be needed elsewhere, in order to don masks as they go about their daily life. In the past few days I’ve noticed growing numbers on the streets of Boston and New York and New Haven, and on the T and subway and Amtrak in between, despite a lack of any new recommendation for their use.

Masks have been strongly suggested and even temporarily made mandatory in some places by the Chinese government, but no U.S. authority has done the same. Saad Omer, an infectious-disease epidemiologist and the director of the Yale Institute for Global Health, says that it can’t hurt if healthy people simply choose to wear masks in the U.S., but if there’s a benefit at this point it’s likely psychological: “People want to feel empowered; there’s nothing worse than a lack of feeling of self-efficacy.”

At the same time, any unfounded sense of protection or immunity carries at least some possibility of detracting from effective preventive measures. As the U.S. Centers for Disease Control and Prevention recommends: Wash your hands often, with soap and water, for at least 20 seconds, or use an alcohol-based hand sanitizer. Disinfect frequently touched surfaces. Don’t carry around used tissues, even if they still have some usable real estate. Stay home as much as possible when in the pathogen-spewing phase of any respiratory disease—be it coronavirus, influenza, or other. Bringing a contagious disease to work or onto the subway poses a hazard to others that can’t be totally eliminated with a mask. And, maybe the most difficult recommendation: Avoid touching your eyes, nose, and mouth. Most of us touch our faces several times every hour, mindlessly, and this is often how we catch viral illnesses. Despite never being especially close to anyone sick, we self-infect by rubbing our nose or eye after touching a mucus-misted surface.

The physical barrier of an elective face mask could help protect us from ourselves, by minimizing inadvertent face touching. But most of the value of masks is in situations where they are clearly necessary, even crucial. And in times of shortage, proper and judicious use becomes especially worth considering. In a café yesterday, I sat next to a man wearing a surgical mask and coughing beneath it. But his commitment to the cause seemed low; the mask wasn’t covering his nose. The value of a mask worn incorrectly can plummet to nearly zero. This point was made clear by a widely shared video last week, in which Wing Hong Seto, an infectious-disease specialist at Hong Kong University, demonstrated proper technique. “It works; we’ve done many studies,” he says, but emphasizes that fitting the mask tightly around one’s nose is crucial. (I debated the social value of correcting the man in the café, but did not.)