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Most U.S. communities have fluoride in their tap water supplies. ((Shannon Stapleton/Reuters))

Fluoride in drinking water is credited with dramatically reducing cavities, but has also been linked to a reported increase in spots on some children's teeth, which is one reason the U.S. government announced Friday it plans to lower the recommended levels of the mineral.

The first such change in nearly 50 years comes after a surprising government study recently found that about two out of five adolescents have tooth streaking or spottiness because of too much fluoride. In some extreme cases, teeth can even be pitted by the mineral — though many cases are so mild only dentists notice it.

Health officials note that most communities have fluoride in their water supplies, and toothpaste has it, too. Some kids are even given fluoride supplements.

The U.S. Department of Health and Human Services proposed a recommended fluoride level of 0.7 milligrams per litre of water to replace the current recommended range of 0.7 to 1.2 milligrams per litre.

The U.S. Environmental Protection Agency will review whether the maximum allowable concentration it has set for the mineral — 4.0 milligrams per litre — is too high.

In 2008, a panel of experts convened by Health Canada recommended lowering fluoride levels in drinking water to 0.7 mg/L from the maximum acceptable concentration of 1.5 mg/L.

About 13.5 million Canadians, or about 43 per cent of the population, live in communities with fluoridated tap water, but almost no fluoridation is done in British Columbia or Quebec, according to Health Canada.

The U.S. Centers for Disease Control and Prevention reports that the splotchy tooth condition, fluorosis, is unexpectedly common in children age 12 through 15. And it appears to have grown much more common since the 1980s.

Cavity trade-off

The government also is expected to release two related EPA studies, which look at the ways Americans are exposed to fluoride and the potential health effects. This shift is sure to re-energize groups that still oppose it.

Fluoride is a mineral that exists naturally in water and soil.

About 70 years ago, scientists discovered that people who lived where water supplies naturally had more fluoride also had fewer cavities. Some locales have naturally occurring fluoridation levels above 1.2 mg/L.

Fluoridation has been fought for decades by people who worried about its effects, including conspiracy theorists who feared it was a plot to make people submissive to government power.

According to the CDC, nearly 23 per cent of children age 12-15 had fluorosis in a study done in 1986 and 1987. That rose to 41 per cent in the more recent study, which covered the years 1999 through 2004.

Except in the most severe cases, health officials considered the discolouring of fluorosis to be a welcome trade-off for the protection fluoride provides against cavities.

Generally, the prevalence of tooth decay in at least one tooth among U.S. teens has declined from about 90 per cent to 60 per cent.

Health officials call water fluoridation one of the 10 greatest public health accomplishments of the last century.  

The government is not suggesting people change their brushing or other tooth-care habits.

The American Dental Association on Friday released a statement applauding the government announcement as fulfilling its mission to protect and enhance health.

HHS Secretary Kathleen Sebelius could make a final decision within a few months, the administration official said.

There is no fluoride in most European water supplies, where some countries add it to salt instead.

It is also controversial in Britain, where only about 10 per cent of the population has water with fluoride in it.