As the nation edges away from lockdown and people once again share public spaces in the middle of a pandemic, wearing a face mask — or refusing to — has become a flash point in a moment when civic rules are being rewritten, seemingly on the fly.

The result has been dirty looks, angry words, raw emotions and, at times, confrontations that have escalated into violence.

In Flint, Michigan, a security guard at a Family Dollar store was fatally shot on Friday afternoon after an altercation that the guard’s wife told The New York Times had occurred over a customer refusing to wear a face covering, which is required in Michigan in any enclosed public space.

Police officials declined to comment on details of the case but are still looking for the suspect, who fled. The family of the victim held a vigil on Sunday night with cars lining up around the store where the shooting took place. A news conference was planned later.

In Stillwater, Oklahoma, an emergency proclamation mandating face coverings led to so much verbal abuse in its first three hours on Friday — and a threat involving a gun — that officials swiftly amended it. Masks became encouraged, not required.

“The City of Stillwater has attempted to keep people safe by the simple requirement to wear a face covering to protect others,” Norman McNickle, Stillwater’s city manager, said in a statement posted on the city’s website. “It is unfortunate and distressing that those who refuse and threaten violence are so self-absorbed as to not follow what is a simple show of respect and kindness to others.”

The decision not to wear a mask has, for some, become a rebellion against what they regard as an incursion on their personal liberties. For many others, the choice is a casual one more about convenience than politics. The choice can also be a reflection of vanity, or of not understanding when or where to wear one.

Some people said they found masks uncomfortable, and thus a nuisance they were unwilling to tolerate. Others were sceptical how much difference they made outside on a sunny day.

“I hate it,” groused Ammiel Richards, 27, who said that he had twice been ejected from New York City buses for not wearing a mask.

But public health experts have reacted in horror both at the sight of public places where people have crowded without masks, and at demonstrations, like those in Michigan and California, where protesters without masks have been jammed together and at times yelled in the faces of police officers. Experts described wearing a face covering as a considerate act meant more to protect others than the person wearing it.

Dr Deborah Birx, the White House coordinator for the coronavirus task force, said she was alarmed at the sight of protesters huddled together without masks, such as those at the Michigan State Capitol this week.

“It’s devastatingly worrisome to me personally,” Dr Birx said on Fox News Sunday, “because if they go home and infect their grandmother or their grandfather who has a co-morbid condition, and they have a serious or unfortunate outcome, they will feel guilty for the rest of our lives”.

“We need to protect each other,” she went on, “at the same time we’re voicing our discontent.”

Even as governors imposed orders and public health experts dispensed their professional guidance, the effort to thwart the coronavirus nevertheless amounted to a grand national experiment in cooperation that hinged on the individual decisions of millions.

Governor Mike DeWine of Ohio had included a face-covering mandate as part of plans to reopen businesses, but he changed course. “It became clear to me that that was just a bridge too far,” DeWine, a Republican, said Sunday on ABC’s This Week. “People were not going to accept the government telling them what to do.”

As many states have moved toward reopening but have also set their own pace in doing so, an elaborate patchwork of orders and restrictions has emerged that differs from state to state, even municipality to municipality.

Those guidelines vary just as much with masks, which public health officials have encouraged people to wear, along with adhering to social distancing measures. The Centers for Disease Control and Prevention advises the “use of simple cloth face coverings to slow the spread of the virus and help people who may have the virus and do not know it from transmitting it to others”.

The shift towards reopening businesses reflects a larger recalibration in the balance that officials have tried to strike between aggressively curbing the virus and avoiding further economic devastation.

“We have a public health crisis in this country, there’s no doubt about it,” governor Tate Reeves of Mississippi said in an appearance this week on Fox News Sunday. “But we also have an economic crisis.”

Polls have found broad public support for stay-at-home measures, with many Americans willing to accept a trade-off of lost wages, disrupted routines and an incalculable series of inconveniences to curb the virus. Public health experts expressed concern that easing restrictions could lead to a second, worse wave of the pandemic and weaken the nation’s vigilance.

Indeed, as many spilled outside, officials acknowledged a sense of restlessness as the measures dragged on.

“It’s painful,” governor Philip D. Murphy of New Jersey, who also appeared on Fox News Sunday, said of the frustration over the isolation, noting it as a factor in his decision to allow parks to open statewide.

“This stay-at-home reality has been with us for many, many weeks.”

Rafael Palma, 43, does not wear a mask, and he and his wife, a health worker, have been going out in public and to church. They also participated in a protest this weekend in Sacramento to push California’s governor, Gavin Newsom, to reopen more of the state.

“It’s not like I’m hugging people and kissing them or anything like that,” he said.

He said guidelines were confusing, pointing to earlier guidance that masks should be reserved for health care workers and could only protect those who had the virus from spreading it.

“People argue that we’re careless and not thinking about others — that we’re spreading the disease,” Palma said. “But in order to do that, you have to have it.”

Public health experts note that people can be infected and spread the disease without showing any symptoms or being aware they have it.

Elsa Aldeguer waved a Trump flag as she gathered with others protesting in Huntington Beach, California. The economy, she said, was the genuine crisis.

She said the pandemic had claimed her job as a home health worker.

“I’m sure the virus is real,” Aldeguer, 46, said, but she also contended that the danger it posed had been overblown. “Why do we have the beaches closed?” she said. “Why the parks? This is like a punishment to us. We the people need these things to keep our sanity.”

The allure of pleasant weather in much of the country on Sunday drew many people from their isolation to beaches, parks, walking trails or anywhere else that was not their home.

In Brooklyn, most of the people spending the afternoon in Prospect Park wore masks. Others had them — but they were pulled down to their chin, or in their hands for strategic deployment.

Along a segment of Atlanta’s BeltLine, a walking and biking trail cutting through the city, David Johnson wore a colourful masked covering pulled up to his nose as he rode his bike.

He noticed that it looked like more people had their faces covered.

“People give you the stink eye if you don’t have a mask on,” he said.

Still, that was not enough of a deterrent, it seemed, for the many others without one.

Jay Sokloski sipped beer with a friend on the outskirts of the BeltLine and said he did not feel comfortable in a mask, nor entirely see the need for one.

“I just don’t see the big thing about it,” he said. “If I were riding an elevator for 12 hours a day with people coughing and sneezing, I’d probably feel different. But we’re outside, so it’s fine with me.”

In Huntington Beach, Colin Abbo said he would put on a mask in the grocery store or in a crowded space. But his effort, he admitted, had not been consistent.

“I shared my joint the other day with a buddy, so I haven’t been the best at it,” Abbo said. “I guess I’ll just give all my friends the benefit of the doubt.”

Rosana Lashlay, 60, was much more vigilant. “It’s very important to use the mask,” Lashlay, a teacher who lives in the Clinton Hill neighbourhood of Brooklyn, said on Sunday afternoon.

It was only in “special” moments when she left her face bare, she said, such as that one as she sat in Prospect Park safely separated from others and tried to clear her mind and meditate. “Sometimes,” she said, “I need to take it off to feel the air.”