A group that advises the government wants tighter limits on the amount of fluoride allowed in drinking water.
The National Academies' National Research Council has issued a report saying the Environmental Protection Agency's current fluoride limit of four parts per million is too high. The report says about 10 percent of children exposed to levels near the limit will develop teeth that are damaged for life.
"Ongoing exposure to water fluoride at levels of four parts per million puts children at risk of developing severe enamel fluorosis," said Dr. John Doull of the University of Kansas, who chaired the report committee. Enamel fluorosis, he said, "is characterized by discoloration, by enamel loss and by pitting of the teeth."
This sometimes happens in places like Colorado and Texas, where the water naturally contains high amounts of fluoride. The problem is not associated with the low levels of fluoride added to water systems to protect teeth.
In 1993, another National Academies panel dismissed enamel fluorosis as a cosmetic problem. But the new committee found that severe fluorosis can actually weaken teeth.
Charles Poole, a committee member from the University of North Carolina, says the Environmental Protection Agency (EPA) could virtually eliminate severe enamel fluorosis by using a fluoride limit lower than four parts per million.
"Once it gets below two," he says, "the prevalence would drop from 10 percent to essentially zero."
Poole says the committee also had concerns about the effect of high levels of fluoride on bones.
"This would be a problem mostly for elderly people who have perhaps been drinking water at a certain level their entire lives," he says. "There is evidence that fluoride can increase the risk of bone fractures.
The backdrop to the report is years of wrangling about the benefits and dangers of fluoridation. Dental groups endorse a level of about one part per million. Some environmental groups don't want any. Both sides found something to like in the new report.
Dr. John Stamm, a spokesman for the American Dental Association, says dentists have known for nearly a century that too much fluoride can harm teeth.
"A young dentist from Northwestern University was working in the Colorado area and he noticed small populations that had abnormally colored teeth, and he called that 'Colorado brown stain,'" Stamm said.
But dentists realized that people exposed to fluoride also had fewer cavities. Stamm says that led to deliberately adding small amounts of fluoride to water supplies starting in the 1940s.
"The country has benefited hugely from the application of water fluoridation as a public health measure," Stamm said. "In fact, not too long ago, the [Centers for Disease Control and Prevention] held up water fluoridation as one of the 10 most important public health measures in the United States."
Environmental groups say people get plenty of fluoride in toothpaste these days and don't need more from their water.
Richard Wiles of the Environmental Working Group says the report supports his group's contention that fluoride can be dangerous. But, unlike the panel, Wiles would like to see fluoride removed from water altogether.
"The committee talked about harm down to levels that are very close to the levels that are currently found in tap water, which leaves us with no margin of safety," Wiles says.
Members of the committee say their report isn't intended to settle the debate about adding fluoride to water for dental health.