Too much of a good thing on tap?

On Wednesday, a panel of experts recommended the Environmental Protection Agency lower the maximum safety limit of fluoride in water from its current 4 milligrams per liter. An estimated 200,000 people across the U.S. have drinking water that naturally contains this amount or higher. The National Research Council panel cited damage to children’s teeth and a possible heightened risk of bone fractures in the elderly.

Janet Cromley

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Fluoride is a naturally occurring substance found in rocks, soil and water. It leaches into the water system through the natural action of water running over rocks. Additional fluoride is sometimes added to the public water supply to bring it up to the U.S. Public Health Service’s recommended standards of 0.7 to 1.2 milligrams per liter, to fight tooth decay.

Every day, minerals are lost from the tooth’s surface when acids in the mouth attack the enamel. Fluoride binds with calcium and phosphates in the saliva and helps remineralize the enamel. Once in the enamel, fluoride makes the tooth more resistant to decay.

When fluoridated water is ingested, the fluoride travels through the bloodstream to bones, and enters saliva, providing a continual bath of fluoride in the mouth. Applied directly to the tooth, it sticks to plaque, providing a thin protective coating.

Fluoride is particularly helpful as aging gums recede, exposing the decay-prone root surface area, says Vladimir Spolsky, Associate Professor of Public Health and Community Dentistry at UCLA. But too much fluoride in early childhood, when teeth are forming, disrupts remineralization, causing surfaces to be rougher, more porous and more prone to staining than smooth enamel. In severe cases, it causes serious pitting, says Boston University epidemiologist Thomas Webster, a member of the advisory panel.

Damage occurs on a continuum. Children exposed to less than 2 milligrams of fluoride per liter of water don’t have tooth problems. Those exposed to 2 milligrams per liter might experience minor yellowing, and 10% of those exposed to 4 milligrams per liter or more have more serious problems.

Fluoride also takes up residence in bone. Some scientists believe that small doses of fluoride have little or no effect on bone strength -- or maybe even confer a slight benefit. But some studies suggest that, over time, large doses of fluoride might cause the bone to become more brittle and more prone to fracture.