Frank Muller of Saarland University, Saarbrucken, Germany, and colleagues said fluoride makes teeth more resistant to decay by changing the hydroxapatite in tooth enamel into fluorapatite. However, the researchers find this fluorapatite layer is only 6 nanometers thick -- much thinner than anyone had expected.
The researchers pointed out 10,000 of these layers would be needed to span the width of a human hair. They have suggested a layer this thin may be quickly worn away by ordinary chewing and question whether a layer this thin can shield teeth from decay or whether fluoride has some other unrecognized effect on tooth enamel.
They say they will be searching for an answer to this question.
The study is published in the American Chemical Society's journal Langmuir.
Fluoride's reputation for cavity-fighting has led to its addition to toothpaste, mouthwash and municipal drinking water.