The Environmental Protection Agency allows so much fluoride that some children in areas with unusually high natural fluoride levels are developing discolored teeth and weakened tooth enamel, according to the report from the National Academies' National Research Council.

The council notes that municipalities in areas with low or no fluoride in their water add low levels of the compound to drinking water to help prevent tooth decay, but water supplies in some areas have much higher amounts of naturally occurring fluoride. Industrial pollution also can increase the levels.

The EPA's ceiling on fluoride in drinking water is 4 milligrams per liter, or 4 parts per million. That's four times the concentration recommended to fight cavities, which is 0.7 to 1.2 parts per million, the American Dental Association says.

High Natural Fluoride Levels

"Fluoride is nature's cavity fighter," the ADA said in a prepared statement. "Fluoride makes the entire tooth structure more resistant to decay."

Drinking water presents the greatest exposure to fluoride, says John Doull, chairman of the panel and emeritus professor of pharmacology and toxicology at the University of Kansas Medical Center in Kansas City.

States that have regions with levels of natural fluoride at or above the EPA's maximum containment level include Colorado, Indiana, New Mexico, North Dakota, Oklahoma, South Carolina, Texas and Virginia. About 200,000 Americans live in areas where drinking water contains fluoride levels over the EPA standard, the report says.

At those levels, 10 percent of children younger than 8, whose adult teeth are still developing, get severe enamel fluorosis. The condition is characterized by discoloration and pitting of the teeth and loss of enamel, the panel says.

Put It Where It Works

The report notes that infants and young children are exposed to three to four times as much fluoride as adults because of their low body weight. But adults are vulnerable because of fluoride accumulation in bones.

People exposed to water at or above the EPA's upper limit over a lifetime are at increased risk for bone fractures and a rare, crippling bone-and-joint condition called skeletal fluorosis, the panel finds.

Though a few studies appear to show a connection between fluoride and bone cancer, the National Academies committee called the results "tentative and mixed." A large study at the Harvard School of Dental Medicine is expected to shed some light on the subject this summer.

Tim Kropp of the Environmental Working Group, a research organization based in Washington, D.C., says fluoride should be limited to toothpaste. "It really only makes sense to put it where it works and don't put it where it can cause harm," he says.

The study was sponsored by the EPA. The council is the operating arm of the National Academy of Sciences and the National Academy of Engineering, advising the government on science and technology.