Public Health Service's
The original rationale for artificial water fluoridation was based on a survey in the 1930s of 21 cities in four states by H. Trendley Dean, a PHS dentist. Based on his "hunch" that fluoride prevented dental caries, Dean presented data suggesting that fluoride naturally in local water supplies apparently correlated with fewer cavities in children.
Dean's findings were much criticized for their scientific method but became the foundation for justifying the implementation of artificial water fluoridation in 1945. Dean later admitted under oath that his studies in those 21 cities did not even meet his own criteria. [Bryson C. The Fluoride Deception. Seven Stories Press;2004:43. Chapter 3:Notes;74,75.]
In 1981, the Austrian statistician Rudolph Ziegelbecker analyzed all of Dean's studies (Dean had omitted data from 26 states) plus all other published studies involving rates of dental caries in areas where the drinking water contained natural fluoride. The research involved more than 48,000 examined children in North America and Europe. The data showed that fluoride in water was not associated with less cavities:
Today there is no evidence of any better oral health in the eight million Americans the CDC said are served by community water systems that have "sufficient naturally occurring fluoride concentrations." [CDC 2008]
On the other hand, Dean was correct about increased dental fluorosis, as Ziegelbecker's analysis confirmed:
"It is evident that the incidence of 'mottled enamel' is positively correlated with the concentration of natural fluoride in drinking water."
From 1986 to 2004, the prevalence of mild dental fluorosis in US adolescents (aged 1215) more than doubled, from 4.1% to 8.6%. Moderate and severe dental fluorosis nearly tripled. [CDC 2010]
"Dental fluorosis, no matter how slight, is an irreversible pathological condition recognized by authorities around the world as the first readily detectable clinical symptom of previous chronic fluoride poisoning. To suggest we should ignore such a sign is as irrational as saying that the blue-black line which appears on the gums due to chronic lead poisoning is of no significance because it doesn't cause any pain or discomfort." New Scientist (1983)
More Fluoride is Absorbed from Artificially Fluoridated Water
Ultimately, the fluoride ion is the culprit.
Public Health Service Agency for Toxic Substances and Disease Registry
"The fluoride ion is the toxicologically active agent." (page 29)
Fluoridated water's multiple correlations with life decay
make a far more compelling case to halt fluoridation, than its
single correlation with tooth decay did to start fluoridation in 1945.