The high levels of fluoride that occur naturally in some drinking water can cause tooth and bone damage and should be reduced.

The study did not analyze the benefits or risks of adding fluoride to drinking water. Instead it looked at the current maximum limit of 4 milligrams per liter. Approximately 200,000 people live in communities where that level occurs naturally in water.

The Council suggested further studies to establish a new maximum level, but noted that the problems associated with exposure to fluoride are very small at 2 milligrams per liter and less. Approximately 1.4 million people have drinking water with natural fluoride levels of 2.0 to 3.9 milligrams per liter, said the Council, an arm of the National Academy of Sciences.

More than 160 million Americans live in communities with artificially fluoridated water, which contains between 0.7 and 1.2 milligrams of fluoride per liter.

Fluoride is added to water to help strengthen the teeth.

Drinking water with levels above the maximum can cause tooth discoloration and weaken the enamel, and long-term accumulation in the bones can result in an increase in fractures, the Council reported.

The National Academy of Sciences is an independent organization chartered by Congress to advise the government on scientific matters. The study was requested by the Environmental Protection Agency.