Dr. Frederick McKay (1874-1959), American dentist. Upon setting up a dental practice in Colorado, USA, at the beginning of the 20th century, McKay noticed that a lot of his clients had brown stains on their teeth. The cause was unknown, but they referred to this as Colorado brown teeth. McKay also noted that those with the brown stains exhibited resistance to tooth decay. He isolated the cause of the stains as small amounts of fluoride in the water supply. His research also showed that levels less than one part per million did not cause the staining and significantly lessened the risk of cavities. As a result of his work, fluoride is routinely added to tap water throughout the world.