Dean was quick to realize that solving the mystery of mottled enamel, though useful, was of secondary importance compared to the broader public health implications of dental caries. In a letter to the U.S. Surgeon General in 1932, Dean repeated McKay’s earlier observation that “individuals in an endemic [brown-stain] area show a lesser incidence of caries than individuals in some nearby non-endemic areas. Consequently, the study of mottled enamel may disclose some lead applicable to the vastly more important problem, dental caries.”

Once it became clear that fluoride was the cause of brown stain—which Dean would soon label dental fluorosis—Dean shifted the focus of his research, and that of the government’s health bureaucracy, from eliminating fluorosis to combatting caries.

Optimizing Nature

Dental caries was perceived as one of America’s most widespread health problems in the early twentieth century. Since dentists were comparatively few and dental surveys virtually nonexistent, it is difficult to know just how pervasive the condition was and to what extent, if at all, it had gotten worse over time.

Nonetheless, dentists themselves were convinced that it had reached epidemic proportions, a perception that appears to have been borne out by military fitness records. These show that in 1916, for example, one third of potential recruits failed their health exam due to caries-related problems. As a result, funds began to flow toward dental research, both from government sources and from corporate foundations.

Many dentists and medical scientists were convinced that Americans’ diets, particularly their fondness for refined flour and sugar, were largely to blame. But changing people’s dietary habits, then as now, seemed to be an insurmountable obstacle.

No wonder, then, that Dean and others were excited by the discovery of fluoride’s impact on teeth.

During the 1930s, Dean, McKay, and colleagues from the PHS and various university dental schools set about trying to demonstrate fluoride’s connection to both dental fluorosis and reduced rates of caries. Although nobody understood exactly how it worked—and nobody would for a long time—fluoride did indeed seem to change the structure of teeth in a way that offered some protection against the assaults of the 20th century American diet.

Embarking on a succession of epidemiological studies in towns that had fluoride-rich water supplies, Dean was able to gradually zero in on a ratio that appeared to offer considerable protection against caries while causing limited and barely discernable fluorosis. The magic number, he determined, was 1 part per million (1ppm).

A 2009 map depicting areas with groundwater fluoride concentrations above the recommended levels.

As the studies continued, Dean and his colleagues published a series of articles that would become the scientific bedrock of fluoridation. So although water naturally containing 1ppm fluoride existed in very few places, it nonetheless came to be seen as the optimal level, and water containing less was deemed “fluoride deficient.”

Dean himself did not advocate artificially augmenting the level of fluoride in drinking water, at least not during the 1930s and 1940s. A cautious and methodical researcher, he felt that many years of further investigation would be required before such a prospect could be contemplated. Even the American Dental Association, subsequently fluoridation’s most steadfast advocate, was reluctant to endorse the idea. However, some dental researchers were less circumspect.

In the early 1940s, Dean began to explore the possibility of testing artificial fluoridation in a handful of carefully chosen communities. After consulting with colleagues at the University of Michigan, Dean selected the towns of Grand Rapids and Muskegon to participate in a 15-year fluoridation trial. Both cities drew their water, which had virtually no natural fluoride, from Lake Michigan. In January 1945, with the enthusiastic cooperation of city officials, Grand Rapids began adding sodium fluoride—a waste product of aluminum production—to its water supply while Muskegon remained fluoride free.

But not everyone was prepared to wait fifteen years.