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To understand the scale of the public-health challenge vaping presents, consider The Vapor Shop on Sunset Boulevard in Los Angeles.

The sparsely stocked store sells mostly flavored vaping products alongside regular cigarettes, snacks, bongs, and hats that proclaim, “I Vape.” If the U.S. government removes flavored e-cigarette cartridges from the market pending their approval by regulators, as senior health officials threatened on Wednesday, it could be fatal for the shop and others like it, according to 25-year-old manager Steven Hernandez.

“The business would die,” said Hernandez, just after the Trump administration unveiled its plans. “There’s no profit in selling cigarettes.”

But there’s another, bigger risk: Curbing flavored vapes could close off access to what many people claim has been an effective tool in helping them break a cigarette habit. Hernandez puts himself in that group. He says vaping helped him largely quit smoking after his children were born.

Justin Boivin, 39, who owns the Juicy Vape Shop in Abbotsford, British Columbia, was visiting the U.S. and stopped in at The Vapor Shop on Wednesday. He said he used to smoke cigarettes and would get winded while hiking with his dogs. He switched to vaping several years ago.

“I feel way better,” Boivin said. “My clothes don’t smell. It’s cheaper. I don’t have to huff and puff anymore. I haven’t gotten any diseases.”

The hope that vaping could help to curtail tobacco use, which leads to the more than 480,000 deaths a year in the U.S., shaped the Food and Drug Administration’s initial approach to regulating the industry. Tobacco-related illnesses are the leading cause of preventable death in the world. But an explosion in the use of vaping products by teenagers—many of whom said that they had never smoked cigarettes—caused the agency to change tack. According to a U.S. survey last year, about a quarter of high-school seniors reported vaping in the past 30 days.

Limiting access to flavored e-cigarettes is a good strategy to stop some kids from getting hooked on nicotine, said Eric Lindblom, director for Tobacco Control and Food and Drug Law at Georgetown University’s O’Neill Institute for National and Global Health Law. But it raises the question of what will move into that vacuum, he said.

“The good part is it will stop a lot of kids from becoming addicted e-cigarette users,” he said. “On the negative side, it could push a lot of current flavored e-cigarette users to move on to flavored smoked tobacco products.”

A number of young adults visiting a vape shop in New York this week said they hadn’t smoked before they started vaping, or had started after experimenting with traditional cigarettes. Many said they used flavors like mint or mango that would be pulled from the market for a time under the restrictions proposed by U.S. officials this week.

Miho Common, a 22-year-old financial analyst, said she had used watermelon and mint pods with a device called RELX but has since stopped vaping. She said she didn’t start in an attempt to quit smoking. She thought vaping looked fun and was relatively harmless, though the recent outbreak of a mysterious lung disease in hundreds of people who reported vaping had given her pause. Health officials haven’t yet settled on the precise cause of the illness, which so far has killed six people nationwide.

Still, Common wondered about the unintended consequences of new curbs. “It’s already illegal for kids,” she said. “It will probably create a black market with dangerous, unregulated stuff.”

Following the emergence of the lung illness, several states have taken steps of their own to rein in vaping. In New York, Governor Andrew Cuomo ordered health officials to require retailers to post warnings about vaping risks. And in Michigan, Governor Gretchen Whitmer last week ordered a ban on the sale of flavored nicotine products in stores and online for six months. Stores will have 30 days to stop sales once the emergency rules are written, likely later this month, Whitmer said.

Michael R. Bloomberg, the founder and majority owner of Bloomberg News parent Bloomberg LP, has campaigned and given money in support of a ban on flavored e-cigarettes and tobacco.

Christopher Bacho, 34, is the chief executive of the Vapor Shoppe, which operates nine retail operations in suburban communities in and around Troy, Mich., about 20 miles north of Detroit. The company also sells 140 flavors of its own vaping juice under the brand name TVS. Bacho says regulation is good for the vaping industry, as long as it’s fair.

Tobacco-flavored vape juice, which federal regulators said Wednesday they will allow to stay on the market, accounts for about a quarter of Vapor Shoppe’s sales, Bacho said. Fruit flavors make up another 25% and menthol and mint account for the rest. Bacho said many customers are vaping as an alternative way of getting nicotine. Others vape instead of smoking a hookah, which is popular with the large Arabic population in the Detroit area, he said.

Users of online forums such as a Reddit message board dedicated to e-cigarettes have said the uproar over vaping is misplaced. They blame illegal cartridges modified to vape synthetic THC, the compound in marijuana that creates its high. Last week, the U.S. Centers for Disease Control and Prevention cited THC as a possible factor in some cases of the disease, while noting others had said they used THC and nicotine products, or nicotine by itself.

“A lot of the stuff has nothing to do with our industry,” Bacho said. “But if an industry is getting as big as ours, and it’s not regulated, I think it needs to be looked at, at least, and make it better for the consumers. But it’s got to be a fair process.”

Others point out that even if flavored vapes are taken out of stores, dangerous flavored tobacco products would remain on the market.

Smoking opponents have been trying to get menthol removed from cigarettes since 2009, when the Family Smoking Prevention and Tobacco Control Act banned flavored cigarettes in the U.S. but exempted menthol, said Delmonte Jefferson, executive director of the National African-American Tobacco Prevention Network. The law also didn’t ban candy-flavored cigars and cigarillos, which along with menthol, attract minority teens who can’t afford vaping products, he said.

“I’ve heard this before, we’ve heard this since 2009, and nothing happened,” Jefferson said. “Let’s not forget these products that vulnerable populations are consuming. Your tackling e-cigarettes is all well and good, but vulnerable populations can’t afford e-cigarettes.”

Juul Labs, the maker of the biggest-selling vaping device, has positioned itself as a tool to help smokers quit—though regulators are looking into whether it crossed the line in any of its marketing with unapproved claims of effectiveness as a smoking-cessation tool. Still, Juul has said it doesn’t want consumers to pick up a nicotine habit from its products. Chief Executive Officer Kevin Burns said in an interview with CBS last month that if you don’t already use nicotine, “Don’t vape. Don’t use Juul.”

The company has also responded to pressure from public-health officials and parents. Following criticism of its flavors and its marketing last year, Juul closed down much of its social-media presence and stopped selling all but its mint, menthol, and tobacco pods in retail stores, limiting sales of other flavors to its online shop. But the dwindling inventories of those flavors at some stores remain popular.

Jorge Piedrahita, manager of the Stogz Smoke Shop on La Cienega Boulevard in Los Angeles, said the store’s big seller is mango Juul.

“That’s what people go crazy about,” he said. “They did a great job on that product.”

Juul has signaled that it will cooperate with the FDA and other regulators as they change their approach. On Wednesday, company spokesman Matt David said Juul “strongly agreed with the need for aggressive category-wide action on flavored products.”

Users meanwhile are sticking with Juul. Elliot Lynch, a 38-year-old from Brooklyn who works as a restaurant host, is a former smoker who says using Juul’s device, which is small and sleek, resembling a USB stick, helped him stop using cigarettes.

“Before I tried Juul, I tried every other type of vaping and it was not working,” Lynch said. “The convenience of Juul is what helped me stop.”

Lynch said that potential restrictions on flavors aren’t an issue for him, though he said right now he uses mint. “If all they had was tobacco-flavored, I would be fine with that,” he said, adding he didn’t think banning flavors would deter other Juul users, either.

“They’re saying the flavors are getting the kids to come in, but you’re still selling the Juul, so what difference does it make?” Lynch said. “They’re already addicted.”

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