It seemed to have turned on a dime.
Just a few weeks ago, the official word from many public health officials was that face masks won’t protect you from the coronavirus, so don’t wear them.
Now, it seems everyone from the Centers for Disease Control and Prevention to Boston Mayor Martin J. Walsh is urging people to cover their nose and mouth with a cloth mask when in public places. Boston officials want people to stay masked even when merely outdoors.
What changed?
It wasn’t a sudden new understanding of how masks work. Instead, scary new knowledge emerged about how the coronavirus spreads.
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Growing evidence indicates that it can be transmitted by people who are feeling well — people who may be about to get sick, or who may never feel the effects of the virus inside them. Additionally, there’s new evidence that a person can spread coronavirus just by speaking, as tiny droplets spew out on the breath.
In short, you can carry the virus without knowing it, and you don’t have to be hacking and sneezing to infect someone.
But if everyone wears a mask, experts say, such transmissions are much less likely to occur.
“The number one benefit [of a mask] is that it prevents someone who is sick from infecting others,” said Joseph G. Allen, an assistant professor of exposure and assessment science at Harvard University’s T.H. Chan School of Public Health. “There are other benefits. It does offer some level of protection to the wearer. It’s one more barrier for an infectious particle to enter your nose or mouth.”
Earlier, some experts were saying that evidence is scant that a mask would provide significant protection to the wearer. That’s still true, but it’s a truth seen in a new light as the pandemic bears down on us.
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“We’ve seen the scale and scope of this virus,” Allen said. “It’s unprecedented, and we’re realizing that we have to throw everything we can at it.”
Allen said that he has supported mask use all along, but initially hesitated to be vocal about it for fear of sparking a run on medical masks desperately needed by health care professionals. Since then word has gotten out about the dire shortage of protective equipment. And the public health messages have urged people to make their own face coverings, out of T-shirts, bandannas, scarves, or cloth napkins folded a certain way.
The CDC’s guidance on mask wearing states, “A cloth face covering should be worn whenever people are in a community setting, especially in situations where you may be near people. These settings include grocery stores and pharmacies.”
Asked whether people need to wear a face covering when outdoors, a CDC spokesman said in an e-mail, “If people are outside in a situation where they are unable to maintain social distancing measures, a face mask is recommended.”
Allen provides more specific advice: “If you’re going out for a run or a hike, you don’t need to wear a mask. If you’re going for a walk in your neighborhood, you don’t need to wear a mask.”
But if you think your walk will lead you into a gaggle of neighbors, then keep your face covered. Definitely wear one on public transportation, in the pharmacy or grocery store, or when visiting your doctor, he said.
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But you don’t need a mask driving your own car, gardening in your yard, or playing outside with members of your household all quarantined together, Allen said.
“I wear a mask when I’m going to be around other people,” he said. “You want to minimize how many times you have to do that. When you are out and about you need to take these precautions.”
Marty Martinez, Boston’s chief of health and human services, said the city wants every Bostonian to have a face covering on whenever they leave their home — and ideally, to keep it on.
Even so, he acknowledged some limitations.
“When I go for a hike, if I’m going for a jog, I know it might be difficult to have it over my mouth,” Martinez said. “I would pull it under my chin, but I’d still have it on [because] when I stop, I might encounter some people.”
But Caitlin McLaughlin, spokeswoman for the Boston Public Health Commission, said she kept a bandanna over her nose and mouth when she went for a run Monday morning, and the commission wants others to do the same.
“Yes, these are strict measures,” she said, adding that they’re not mandates. The commission urges people to wear a face covering whenever they’re outside, including for walks, runs, and bike rides, she said, because people can’t control when they’ll encounter others.
“We’re encouraging people to continue to get exercise, but to take precautions. This is in the best interest of keeping everybody healthy,” McLaughlin said.
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Allen, the Harvard professor, emphasized that face coverings are an extra protection — but not a substitute for staying at least six feet away from other people and washing hands often.
He doesn’t buy the argument by some skeptics that wearing a mask may encourage people to touch their faces more, introducing additional germs. He likens that to saying hand-washing doesn’t work because some people do it wrong. Instead, he said, “Give guidance on the proper way to do it.”
The mask, he said, is “a signal to others that you care about them.” It establishes a new social norm. “Two or three weeks ago,” he said, “having a conversation with a neighbor six feet apart felt funny.” Now, it feels uncomfortable if someone breaches the six-foot separation.
“The same will happen with masks,” Allen predicted. “If I see someone without it, I think, ‘Why aren’t you protecting us?’”
Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her @felicejfreyer.