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The credit for the identification of fluoride as an effective means of caries prevention can be largely accredited to two American dentists, Frederick McKay and H. Trendley Dean. Interestingly, this knowledge came about by first noting the detrimental effect of excessive fluoride on tooth enamel. This is a condition called dental fluorosis, in which teeth become speckled with white flecks. In more severe cases of excessive fluoride ingestion, teeth can become mottled with brown stains and pieces of surface enamel might easily break off, though these types of effects are limited to the most severe cases and are not generally seen in the United States.
As a practicing dentist in Colorado Springs, Colorado, in 1901, McKay noticed many of his patients had what was locally called “Colorado brown stain”. He moved out of the area, but returned in 1908 to study the phenomenon in more detail, and found that as many as 90% of children were affected.2 In addition, after conferring with other dentists worldwide, he found similar occurrences of mottled or brown enamel in other towns in the United States, England, and Italy.1,3 Because the phenomenon was isolated to specific geographical areas, McKay thought that the water supply might be an important factor. He put this theory to the test in Oakley, Idaho, where mottled enamel was common, by having a pipeline with an alternative water source pumped into the town. After 10 years of the new water supply, new cases of “brown stain” had disappeared.4 To add another piece to the puzzle, analysis of water in another American town plagued by mottled enamel, Bauxite, Arkansas, uncovered an unexpectedly high level of fluoride—and these high levels were confirmed in the water supply of other towns with rampant dental fluorosis.5,6
The discovery of high concentrations of fluoride was a concern because it was known that high doses of the mineral could be poisonous. This is when H. Trendley Dean, who worked with the US Public Health service, came on the scene. He took up the investigation, mapping areas where mottled enamel was present and relating the severity of mottled enamel to fluoride concentrations, noting that a certain range of fluoride concentrations in drinking water that was not very high or very low was linked with a reduced caries risk.
In a publication in 1942, Dean published his findings of his landmark “21 city study” (actually a series of studies) where he examined the association between the fluoride levels in drinking water and caries levels in children, and developed the first classification system for recording the severity of mottled enamel, using the terms questionable, very mild, mild, moderate, and severe.7-9
These findings from the first half of the 1900s led to a greater understanding of fluoride’s effects on enamel development, how dental fluorosis develops, and advances in the delivery of beneficial amounts of fluoride to reduce caries.