tooth decay

From Wired: 1945: Grand Rapids, Michigan, becomes the first U.S. city to fluoridate its drinking water supply.

Fluoridation, implemented as a means of reducing tooth decay,
involves adding one part per million of fluoride to the water supply.
(The optimum level, according to the Centers for Disease Control, falls
between 0.7 and 1.2 ppm.) Fluorine, of which fluoride is a compound, is
one of Earth's most abundant elements, found in almost everything. In