BEFORE HIS WORKDAY BEGINS, Arthur Maxwell routinely stops at his favorite bar in downtown Toronto and gets tanked. As morning commuters buzz by outside, Maxwell, 52, the owner of a construction business, sidles up to the white marble counter, orders a drink, then forks out $13 for a 20-minute fix—of fresh air. "I have more energy," he says. "I miss it when I don't get it."

Maxwell is a regular at the O2 Spa Bar, North America's first oxygen bar. Already familiar in smogged-out Asian cities, these so-called air bars—where patrons pay to hook up to oxygen tanks—could be America's next health fad. Since its March opening, O2, which also offers fresh organic juices, has served up low flow-rate (about half that of hospital tanks), 99.9 percent pure oxygen to more than 4,000 customers. And owners Lissa Charron, 31, and Shamila Hunter, 33, longtime pals, are already fielding calls from potential U.S. franchisers. But "we want to make sure others have the same intentions we do," says Charron, an architectural designer. "It comes from the heart."

And goes to the head: Devotees claim their oxygen hits cure everything from hangovers to hot flashes. "I came in stressed-out, and now I feel like I've done an hour of yoga," says advertising copywriter Alex Mohler, 35. "I used to live with a nasal spray up my nose," says the bar's allergy-ridden manager Shireen Odho, 49. But since becoming an air head, she says, "I feel great."

The feel-good feeling isn't always immediate. "It takes time," says Hunter, a TV camerawoman, who came up with the idea after filming a story on a 96-year-old man who says he saved his gangrenous legs by using oxygen. "Sometimes the effects are subtle."

Too subtle, say critics. "Small amounts aren't harmful," says Dr. John Granton, a respirologist at the Toronto Hospital. "By the same token, it's not helpful either." Charron disagrees. "You need oxygen more than you need spring water," she says. "Until now, you couldn't get it unless you were flat on your back in an ambulance."