Jane Rosen began yelling sometime in April. By May, it had become routine. The incidents usually occur near her minivan, which she parks alongside?Central Park in New York City.

As she attempts to enter or exit the vehicle, a cyclist or a runner will whiz by, so close she can practically smell them. ?I scream, ?Where is your mask??? said Ms. Rosen, 73.

Her daughter warned her that these confrontations could end badly. But it feels worth it, she said, because lives are at stake. She?s had about 18 such confrontations. The figure would be higher, she said, if she ventured out more often.

Melissa Mayen, a high school senior in Washington, D.C., had also been avoiding going outside. Then in mid-May, she set out for a ride for the first time in nearly a month.

She was startled when a man, walking across the street, yelled something about a mask. ?I almost fell off my bike,? she said. She owns one mask, which her father brought her from a construction site where he works. Aside from the fact that it?s so thick that she can barely breathe in it, she tries to preserve it for higher-risk situations. ?If you are yelling at someone to wear a mask, then give them a mask,? she said.

Several months?into the coronavirus pandemic, battles over when to wear masks?have heated up, sometimes with?political alliances?replacing health guidelines.

And few activities seem to have incited more debate than exercise: walkers, cyclists, runners, skaters ? everyone seems to have contradicting interpretations of the science and etiquette around how to behave outside.

Not necessarily. When cities and states started urging people to wear masks to reduce transmission of the coronavirus, some made exceptions for exercising. Carry a mask, many seemed to say, but if you?re by yourself on an empty street, you don?t have to wear it.

New York City?explicitly states?that face coverings are not required while walking, running or biking, if you can keep your distance. Likewise, San Francisco?has urged?runners to carry a mask and put it on when they are within 30 feet of other people.

Since mid-May, Los Angeles has required residents to put on a face covering upon leaving home. But masks are not required while running and biking so long as distance is maintained ? though they should be carried, the county and city later clarified.

In Boston, an elevated heart rate is no excuse not to cover your nose and mouth. ?You need to be wearing a face covering when you?re out exercising,? Mayor Marty Walsh?said?in April.

The Centers for Disease Control and Prevention?recommends wearing?cloth face coverings in public settings ?where other social distancing measures are difficult to maintain,? but offers no specific guidance on exercising.

A sign at the entrance to Valentino Park in the Red Hook neighborhood of Brooklyn.

It can be really hard to run in a mask.

Many runners are put off by how challenging it is to inhale as their heart rate rises. It can be much more difficult than walking in a mask.

?It?s harder to breathe, and it?s a lot more clammy,? said Gaston Ly, a store manager at Running Room in Honolulu.

Others forgo one because, even as the virus spreads, masks have not been widely adopted in their communities.

?Oh gosh no!? said Larry Holt, the owner of Ken Combs Running Store in Louisville, Ky., when asked if runners there wore masks. ?That?s the craziest thing I?ve ever heard of in my life.?

(In Kentucky, Gov. Andy Beshear asked residents to begin wearing masks in public on May 11. Like officials elsewhere,?he made an exception?for people exercising alone.)

Even in Hong Kong, a city so committed to face coverings in public that it has been widely praised as a model, there is little expectation that runners will wear masks, said Brian Woo, a founder of a running group there. ?I assume it?s just understood that running is not a time for wearing masks,? he said.

There is no scientific consensus around the importance of wearing a mask while exercising, primarily because so little relevant research has been completed.

Researchers do agree that masks slow the spread of the virus. They also agree that it?s best to avoid exercising within six feet of anyone beyond your immediate household and that working out is less risky?outside?than inside.

Donald Milton, a professor of environmental health at the University of Maryland School of Public Health who has studied masks? ability to block respiratory droplets, suggests their value depends on location. ?Outdoors is relatively safe, and masks would only be important if you are exercising in crowded areas or indoors in space shared with other people,? he said.

It would most likely occur while you were stopped talking to them, said Julian Tang, a virologist and a professor at the University of Leicester in England. He thinks the risk of infection from quickly passing someone is low, because the ?massive air volume will dilute any exhaled virus and the wind may carry it away.?

In general, researchers?agree that air circulation outdoors?seems to strongly inhibit transmission of the coronavirus. In a study of more than?7,300 coronavirus cases in China, just one was connected to outdoor transmission.

In April, a draft of a scientific?study?by Belgian and Dutch engineers indicating that runners, brisk walkers and cyclists create a wake of air behind them that could carry exhaled respiratory droplets much farther than six feet began to circulate online. A widely shared?Medium post?referring to the research recommended keeping a distance of 32 feet when running or slowly cycling and at least 65 feet ? four car-lengths ? when cycling quickly.

For a few days, every social media platform seemed to be oozing with the same terrifying graphic: two runners, one spewing a colorful cloud ? many interpreted it to be coronavirus ? on a man behind him.

The study?s authors soon?published a?follow-up, noting that their research was just an engineering wind-flow model, which found that when we walk or run, the air moves differently around us than when we are still. Despite telling people not to draw conclusions from their research about how the virus infects people, it had?taken?on a life of its own.

One useful takeaway, both the study?s authors and several researchers not involved in it said: It?s best to avoid running or biking directly behind someone for a prolonged period.

Stranger sweat is disgusting. But it?s not among the bodily fluids that the C.D.C. warns transmits the coronavirus.

Spitting is not only disgusting but also dangerous, as saliva can contain viral droplets. Runners, cyclists, skaters, walkers ? don?t do it! (Or at least not around others.)

Avoid popular routes and times, suggests Douglas Nicaragua, the owner of Go Run in Miami. He advises taking a mask, even if you don?t expect to cross paths with anyone. If you see someone, put it on.

?Over time, you?ll get used to it,? said Joey Ta, a competitive endurance athlete in Los Angeles who recently started wearing a mask.

People exercising have used several kinds of masks, some with drawbacks. A surgical mask can easily grow damp and heavy with sweat; so can a cloth one. A bandanna tied around the head may slip more easily when running. Some?may even consider a face shield.

Others have used a face gaiter or?Buff, a tube of cloth that extends from the collarbone to the ridge of the nose. That?s what Colin Klein, owner of?Fleet Feet?in Burbank, Calif., recommends.

And whether you wear a mask or not, pay attention to the position of people around you. Dr. Benjamin D. Levine, a professor of internal medicine at UT Southwestern Medical Center and Texas Health Presbyterian Dallas, is advising the U.S. track and field team on how to train safely. He urges focusing on what he calls the four Ds: ?double the distance? from six to 12 feet and ?don?t draft,? meaning ?don?t run or cycle directly behind someone so you are continually running into and breathing their expired air.?

No. The idea that wearing a mask mimics high-altitude conditions is a myth, Dr. Levine said.

?I don?t understand how people can?t understand that this is about more than just a mask,? said Ms. Rosen, the New York woman who has taken to yelling at runners.

She said her confrontations were driven by a sense of duty to protect not only herself, but also her neighbors.

But is yelling ? which may also?expel more?viral droplets than?talking?? likely to change behavior? Possibly, said Alexandra Brewis, a professor in the School of Human Evolution and Social Change at Arizona State University and the author of?a book on stigma and global health. But she has found that most people are far more likely to take advice from friends and family than from a stranger and to incorporate feedback delivered with empathy, not shame.


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