When government scientists reported last spring that rates of childhood tooth decay had risen slightly over the past two decades, some dental professionals proposed a possible explanation: Those children might not be getting enough fluoride, a chemical that binds to tooth enamel and makes it resistant to decay. While the theory remains to be proved, the finding underscores the need for kids to get some fluoride—but not too much.

Tap water in most of the country has been fluoridated for decades, but bottled water often isn’t. So parents who rely on brands like Dasani and Aquafina could be putting their own and their children’s oral health at risk, say experts like Steven Levy, a professor of dental public health at the University of Iowa in Iowa City.

Water isn’t the only source of fluoride. The chemical is also added to many toothpastes, and it’s in foods like raisins and shellfish, as well as teas. But people worried about overexposure may avoid those sources as well. In excess, fluoride can cause white spots on teeth—an unattractive but harmless condition called dental fluorosis. Federal standards require cities to notify residents if fluoride levels in tap water exceed 2 parts per million, and the highest level allowable is 4 ppm.

Nearly a quarter of Americans under age 40 have some fluorosis, according to a 2005 report, but under 1 percent have severe fluorosis, marked by yellow, malformed teeth. Unsupervised kids can swallow toothpaste, so children under 2 shouldn’t use fluoridated pastes unless it’s advised by a dentist or pediatrician, says Howard Pollick, a spokesperson for the American Dental Association and professor of dentistry at the University of California-San Francisco. (The ADA recommends fluoridated pastes for others, though, as fluoride’s benefits last only an hour or two after each exposure.) Pollick emphasizes that 2-to-6-year-olds need only a “pea size” amount of toothpaste and should be supervised while brushing. Moreover, parents who feed their infants only formula should prepare it with unfluoridated water.

“We’re trying to reduce tooth decay, but we’re also trying to reduce dental fluorosis,” says Pollick. “It’s a fine line.”

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