Fluoride in drinking water -- long controversial in the United States when it is deliberately added to strengthen teeth -- can damage bones and teeth, and federal standards fail to guard against this, the National Academy of Sciences reported on Wednesday.

The vast majority of Americans -- including those whose water supply has fluoride added -- drink water that is well below the limit for fluoride levels set by the U.S. Environmental Protection Agency (EPA).

But the academy's expert panel said some 200,000 people in the United States may consume water that is at or above the government's standard because of naturally occurring fluoride.

Children exposed to the government's current maximum fluoride limit "risk developing severe tooth enamel fluorosis, a condition characterized by discoloration, enamel loss and pitting of the teeth," the academy said in a statement.

Children are at particular risk in communities with water at or near the federal limit, where about 10 percent of young people develop severe tooth enamel fluorosis, the report said.

The report does not examine risks or benefits from the purposely fluoridated water that millions of Americans drink, which contains about one-fourth the government's limit, or less.

But its findings were bound to boost the arguments of those who have long opposed addition of the chemical to public water systems for a variety of reasons. Half a century ago, some groups charged that putting fluoride in the water was a communist plot to poison Americans or make them ill. Government officials consistently denied the allegations.

Earlier reviews of health issues associated with fluoride have considered enamel fluorosis to be ugly but not a health hazard. But the new assessment considers it a health hazard as one function of tooth enamel is to protect the teeth and underlying dental tissue from decay and infection.

Over a lifetime, people who drink water with the level near the federal limit of fluoride probably have a higher risk for bone fractures, a majority of the panel concluded.

The EPA allows up to 4 milligrams of fluoride per liter of drinking water -- .000534 ounces per gallon -- but the report found this level did not protect against known risks from chemical.

The Environmental Working Group, a non-profit watchdog organization, applauded the academy's report for raising health concerns about excessive fluoride in drinking water.

"The bottom line from the nation's top voice on science is that you can protect your children's teeth by brushing them and you can protect their bones by getting rid of fluoride in tap water," Tim Kropp, the group's senior scientist, said in an e-mail.