White and pink pastel blossoms frosted the trees beneath a blue sky Tuesday afternoon in Old City, the kind of spring day that makes long sleeves optional. For many, though, masks were not.

“I do it because it’s the right thing to do,” said Ellen Stroman, as she walked by the colonial columns of the Shambles near Second and Pine Streets with her husband, their daughter, and dog.

There is ample evidence that masks help prevent COVID-19′s spread, and their value indoors, where transmission is almost 19 times more likely than outside, isn’t disputed. The risk of infection outside, especially through passing contact, appears much lower. Researchers have found COVID-19 spreads primarily through aerosols expelled by activities like talking, singing, sneezing, or coughing, and those disperse quickly in open air. Sunlight and humidity also play roles in reducing the risk of outdoor transmission. A letter to the German government from the Association for Aerosol Research this month stated, “Transmission outdoors is extremely rare and never leads to cluster infections as can be observed indoors,” according to Germany’s international broadcaster, Deutsche Welle.

The mass protests in summer 2020 that followed George Floyd’s death didn’t appear to cause coronavirus case surges in Philadelphia, and evidence is uneven about what role, if any, protests played in transmission nationwide.

The Atlantic published an article recently asking whether it was time to consider lifting outdoor-masking mandates, noting confirmed cases of outdoor transmission almost always include close conversation or yelling. Once a person is vaccinated, the risk of being infected outdoors is “microscopic” to “nonexistent,” the magazine reported.

Almost 40% of eligible Philadelphians have been vaccinated, and 7.3% of COVID-19 tests were positive as of last week, according to city data, numbers that shouldn’t yet make people comfortable taking their masks off outside in all circumstances. Several health experts said it is responsible and important to keep wearing masks while near others, even if people are already vaccinated.

“I would say that if you are close enough that you could smell the smoke from a smoker, then you should still wear a mask outdoors,” said Charles Haas, a professor of environmental engineering at Drexel University. “We need to get to a much higher level of vaccination and a much lower level of positive cases before I believe it would make sense to remove a mandate on outdoor masking.”

Wearing a mask outdoors is still important when in a crowded situation such as a busy sidewalk, stadium, or waiting for food at an outdoor restaurant, he said.

Michael LeVasseur, a Drexel University epidemiologist, agreed the risk changes in crowds, but a person walking alone outside would likely not have any impact on transmission. Then he added a less empirical observation.

“It’s probably still polite to wear a mask,” he said.

That highlights the unusual intersection of biology and social science that health experts and the public have navigated over the last year. Masks have moved beyond a public health precaution to become variously a courtesy, an indicator of solidarity, a symbol of respect for science -- or a sign of reluctant acquiescence to government control. Some are so adamant about refusing to wear them they won’t enter places where they’re required.

“In America it’s been politicized,” said Eric Zillmer, a professor of neuropsychology at Drexel. “If you’re wearing a mask, you’ve kind of bought into the idea that there is danger.”

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That politicization, which Zillmer said was fed by the Trump administration’s early, dismissive responses to the coronavirus, helped shape the debate about reasonable precautions. People who obeyed restrictions often did so to protect others, as well as themselves, and still feel deep resentment toward those who flouted them. Wearing a mask says something about their values.

“It reminds me a little bit of the ‘I voted’ sticker,” said Mercedes Carnethon, vice chair of preventative medicine at Northwestern University Feinberg School of Medicine. A mask shows “your civic engagement, the fact that you’re committed. You’re sending a signal that you’re behaving in socially appropriate ways.”

Suggesting mask-wearing might not be necessary outdoors can prompt a visceral reaction.

At a news conference Tuesday, Mayor Jim Kenney said he would prefer to see Americans approach the pandemic as they did World War II, with everyone making sacrifices for a common purpose.

“And wearing a mask isn’t even close to jumping out of an airplane with a parachute into Normandy,” he said, “so I will never understand why this is such a hard thing.”

Asking people to mask up to protect themselves against an invisible foe is in itself a psychological challenge for human beings, Zillmer said.

“The idea of putting something on your face, it reduces your identity,” he said. “You’re dealing with a really complicated behavior.”

A year ago, as Americans faced enormous uncertainty, stress, and information from scientists and politicians that sometimes conflicted, people made a decision about mask-wearing and stuck with it.

Even as the science on COVID-19 evolves, and the understanding of risks becomes more nuanced, people who have invested in mask-wearing may be loath to give them up even as their risk recedes.

“I think they’re going to be difficult to change,” Zillmer said. “I’ve internalized them for a year. I have my own rule pattern.”

Thomas Farley, Philadelphia’s health commissioner, said going maskless outdoors is OK as long as people aren’t congregating too closely. City restrictions, which require masks indoors all the time and outdoors when not distanced from others, will remain in effect.

In Montgomery County, Val Arkoosh, the chair of the county Board of Commissioners and a physician, said she agreed that if people are near each other outdoors, masks should still be kept on.

“We are just not in a place right now where we can relax these measures,” she said.

Carnethon acknowledged the very low risk of a vaccinated person transmitting the virus outdoors but said the nuances of setting different policy for different people carried logistical challenges.

» READ MORE: Pa. imposes a broader mask mandate as officials try to stem a surge in virus hospitalizations

“We really can’t ask people their vaccination status, and it does become awkward,” she said. “I try to engage with people with the expectation that they are not vaccinated at this point. Which is why if I’m approaching someone or even speaking with them outside, I do keep my mask on if I’m around them.”

Farley doesn’t like the approach of having different rules for vaccinated people, because it would “erode the social norm.”

“If you want people to follow rules it’s really important that they be as simple as possible and as clear as possible,” he said.

On the streets of Philadelphia, people make their own subtle adjustments. Some were masked up on a warm afternoon this week, while others went entirely without. Some kept masks ready to quickly put on if another person came nearby.

Mary Keefe, an Old City architect, sat on a business’ stoop with her sister, Martha Dowd, who was visiting from Boston. Both had their masks down. Both are vaccinated. Keefe said she always has a mask on when walking around, and pulls it over her face if she nears people.

“I’m putting it up,” she joked. “I’m taking it down.”

Zillmer believed mask-wearing, for some, would become a habit regardless of COVID-19′s presence. He cited Asian nations where large percentages of the population wear masks not out of concern for themselves but out of consideration for others.

“Ultimately I think it’ll be a really personal expression of us,” he said.

Jack Stroman, Ellen’s husband, dismissed evidence that there was a low risk of transmission outdoors. What did it matter, he said, when the difference between low risk and virtually no risk was a simple piece of cloth?

“If you weigh it all out, that minor inconvenience and the safety of people’s lives,” he said, “it’s not even close.”

Staff writer Justine McDaniel contributed to this report.