A dentist named Frederick S. McKay opened his practice in 1901 in Colorado Springs and soon noticed something odd about his patients: many of them had brown, mottled teeth, very ugly but also highly resistant to decay.

McKay collected water samples from several areas where brown teeth were common and sent them to a chemist who examined them using a new technique, spectrographic analysis. The chemist found concentrations of fluoride in the samples as high as 12 parts per million, and researchers quickly confirmed in epidemiological studies that ...