Some dentists are concerned about a problem they haven’t worried about as much in decades — whether children are getting enough fluoride.

Fluoride helps strengthen teeth and prevent tooth decay. Mass fluoridating of municipal water supplies, started in the 1940s, has been called the most effective public-health measure ever made to improve oral health. But the growing popularity of bottled water has brought concerns about adequate fluoride levels.

While praised otherwise for its health benefits, bottled water often doesn’t contain enough fluoride to prevent tooth decay, according to the American Dental Association. And it isn’t just water in a bottle or office cooler that presents a problem, dental experts say. Some home water-treatment systems that put tap water through high levels of filtration, such as reverse osmosis, may also take fluoride out, the ADA says. Now, a growing number of bottled-water producers are adding fluoride to brands and packages aimed at kids. Nestlé Waters North America Inc., a subsidiary of Nestlé SA and the largest bottled-water company in the U.S., introduced new fluoridated lunchbox-size versions of its popular spring-water brands this month. The 8-ounce bottles of Poland Spring, Deer Park, Ozarka, and other brands contain levels of added fluoride from 0.17 milligram to 0.21 milligram, says Jane Lazgin, a spokeswoman. That is within the optimal range for fluoride in drinking water of 0.7 milligram to 1.2 milligrams per liter, as set by the U.S. Public Health Service, and within the government’s limits for bottled water. (The differences in fluoride levels vary by local air temperature.)

Dannon’s “Fluoride to Go” spring water, owned by Danone SA and marketed and distributed by Coca-Cola Co., is also marketed in a kid-friendly 8.5-ounce bottle. The brand contains 0.25 milligram of fluoride per bottle, also enough to meet optimal fluoride guidelines, according to a Coke spokesman.

Overall, about 20 bottled-water makers now add fluoride to some of their water brands in bottles and home and office coolers, according to the International Bottled Water Association.

While the number of children and teens with cavities declined from 1994 to 2002, according to the Centers for Disease Control and Prevention, dental experts say they’re still concerned. Despite the drop, two-thirds of 16- to 19-year-olds have had tooth decay or fillings, the CDC says. The agency’s data also show a slight rise in the number of preschoolers with cavities in their baby teeth, although the CDC says the increase isn’t statistically significant.

Those children who do have cavities seem to have more of them, dentists say. “I’m seeing more childhood decay than I did 10 years ago,” says Matthew Messina, a general dentist in Cleveland and a spokesman for the ADA.

While they concede their evidence that children and teens’ dental health is worsening is largely anecdotal, dentists worry that if the trend continues, some of the gains made in dental health since mass fluoridation could be lost. Studies show that fluoridation of community water supplies reduces tooth decay by as much as 40%, according to the ADA.

Fluoride, a naturally occurring compound, helps inhibit and even reverse tooth decay by “remineralizing,” or strengthening weak areas of the teeth that are beginning to form cavities. It is especially critical during childhood, when teeth are forming. Children are normally exposed to fluoride through the drinking water if their community has it, through toothpaste, and fluoride treatments when they visit the dentist.

Bottled water is scarcely the largest perpetrator of cavities in children and teens. A bigger factor is inadequate brushing, along with poor diet, such as heavy consumption of sugared soft drinks.

While most bottled waters don’t contain enough fluoride, it isn’t clear at what point consumption of bottled water over fluoridated tap water would start making an impact, says Scott Presson, a dental officer with the CDC. Little research has been done on the use of bottled water and risk of tooth decay, dental experts concede.

Bottled water has become popular partly because of concerns about the safety of municipal water supplies and partly for the convenience. U.S. consumers drank an average of 26 gallons of all sorts of bottled water per capita last year, more than double the 11.7 gallons per capita they downed in 1995, according to Beverage Marketing Corp., a New York-based consultancy.

Dentists also caution against too much fluoride; ingesting too much toothpaste or taking too many fluoride supplements under the age of 8 years old can leave children with chalky white spots on their teeth — one reason dentists say parents should make sure their kids don’t use too much toothpaste and help them brush their teeth. Dentists say the spots, also known as fluorosis, can be hidden by polishing or sanding the enamel, or bonding tooth-colored filling materials on them. About 32% of children and adolescents 6 to 19 years old have some fluorosis, according to the CDC. But the CDC says it continues to recommend fluoridation, which reaches 67% of people in the U.S.

Company officials and dental experts say the bottled waters with added fluoride have the same amounts that are in tap water. “There hasn’t been a prospective clinical study,” says Joel Berg, chairman of pediatric dentistry at the University of Washington and president of the American Academy of Pediatric Dentistry Foundation, “but there’s no reason to believe you’d get a different result.”

While many big bottled-water brands — such as Coke’s Dasani brand or PepsiCo Inc.’s Aquafina, which are produced by a high-tech purification process — don’t contain fluoride, some bottled-water brands do have plenty without having to add it. One of the springs that is the source of Nestlé’s Arrowhead bottled-water brand has about 1.2 milligrams per liter, Ms. Lazgin says.

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