As May began, Tidal Creek, a co-op marketplace in Wilmington, North Carolina, put a new rule into place: No mask, no service. The policy, which many other retailers across the country have also adopted, is meant to curb the spread of COVID-19, adding a level of protection for workers and customers alike. But some Americans are unwilling to wear a mask, saying the face coverings are uncomfortable or impinge upon their civil liberties. It only took a few days for Tidal Creek’s rule to spark an incident involving an unruly shopper who didn’t want to cover up.

“The gentleman was attempting to escalate the situation, just loudly and acting threateningly for sure,” says Anthony Garguilo, the store’s co-general manager. Garguilo said the customer called the rule “socialism” and that it was going to “precipitate social unrest.” “That was assuredly meant as some kind of vague threat,” says Garguilo. Garguilo leaned on his deescalation training, calmly and repeatedly stating the store’s rules without leaving an opening for debate. The customer eventually left without further incident.

Many shoppers are currently required by state, city or local regulations to wear masks or other facial coverings. But it’s largely been left to grocery store employees and other workers to enforce those rules, despite having little or no training on how to do so. “People are anxious about being in contact with others,” says Bill Hallan, president and CEO of the Michigan Retailers Association. “They’re nervous, and you don’t want a retail employee having to enforce the laws. It’s just not what they’re paid to do, or trained to do for that matter.” While many people have been willing to wear a mask to protect themselves and others, a small minority are not — leading in some cases to fraught, even violent confrontations across the country.

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Earlier this month, a Target security guard in Los Angeles broke his arm in a fight with two men being escorted from the store for allegedly not wearing masks. A man in Holly, Michigan was arrested and charged with assault for allegedly wiping his face on a Dollar Tree employee’s shirt on May 2 after being told he needed to wear a mask in the store. Some of these confrontations have turned deadly: In Flint, Michigan, four people were charged in connection to a fatal May 1 shooting of a dollar store security guard after an argument over mask rules. On May 18, a man was arrested for attempted murder after opening fire on an Aurora, Colorado Waffle House employee who repeatedly turned him away for not wearing a mask.

“It’s a very difficult position for us to put our staff in, whether it’s an 18, 19-year-old college student working, or a single mom, a dad,” says Bryan Neiman, who runs a small chain of Michigan grocery stores called Neiman’s Family Market. He has put up signs asking customers to wear facial coverings, and shoppers can get masks at customer service. But he’s telling his employees not to confront mask-less customers, for fear of triggering a confrontation. “People are on such edge sometimes that it becomes very nerve-wracking. And that’s why we’ve told our staff, ‘do not engage, just let them shop, get them out and follow all of our procedures that we had in place and go from there.’”

Debby Kavulish, a 62-year-old who’s been a cashier service manager at Neiman’s St. Clair location for 13 years, says she feels a little bit safer knowing the people who regularly shop there. “The group of customers that we have here, they’re like family,” she says. “They’ve been shopping here forever.” Still, the spat of violent incidents — especially the nearby Flint shooting — has alarmed her and the rest of the staff. “We all worry about it,” says Kavulish. “I’m not going to say that we don’t.”

Workers at Tidal Creek are following similar guidelines to those at Neiman’s. “We’ve been told to not confront the situation if anybody does decide to cause a problem, just because we don’t want any violence in the store,” says Matthew Plourde, a produce manager at the co-op.

Marissa Baker, an assistant professor of occupational health sciences at the University of Washington, says the federal government could help by setting universal standards for mask-wearing requirements across states and cities, rather than the current patchwork approach. “We’re really lacking that federal presence to say, ‘here’s what you should be doing, here’s how you should be doing it,’” says Baker. Such guidance may not completely stop in-store confrontations, but they could give workers something else to lean on. “It might give a little more power to the workers to feel like they are being protected and that there is a plan to protect them,” Baker says.

Baker also recommends reframing mask-wearing rules as a way to protect one another, rather than an outright requirement to enter a store. “How can we change the messaging more to a, ‘we’re all in this together,’ as opposed to ‘you can’t come into the store because you don’t meet the requirements that we’ve laid out?,’” says Baker.

At Tidal Creek, Garguilo says the vast majority of their customers and co-owners have been supportive of the mask rules, and they’ve gained more business than they have lost after requiring masks. Still, the possibility of another confrontation with an angry customer isn’t far from mind.

“We have a lot of really young folks that work here [and] several older folks,” says Garguilo. “For the pay grade of some of these folks, that is not something that they should have to deal with, just whether it’s psychological impacts or, god forbid, something physical.”

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Write to Alejandro de la Garza at alejandro.delagarza@time.com.