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2 . 3 Detailed response 2 . 3. 1 Early scientific findings The scientific discovery that fluoride in drinking water prevents dental caries has been described as a " classic epidemiological study" .4 It began with studies of a completely different dental condition , now known as dental fluorosis, and which was first described in the early 20th century as " mottled " teeth and "Colorado brown stain " . By the 1920s, dentist in the U . S . Public Health Service observed that dental caries occurred infrequently, if at all , in children with mottled teeth . In contrast, at that time , dental caries was rampant in children unaffected by mottling . With the aid of the new photospectrographic method of measuring fluoride concentration , new epidemiologic studies investigated the association between concentration of fluoride in drinking water and dental caries . 2 . 3. 2 U. S. Public Health Service studies conducted in the first half of the 20th century The first observational study of an association between exposure to fluoride in drinking water and dental caries was reported by Dean in 1938 . 31 It compared dental caries of nine -year olds exposed to 0 . 6 - 1 . 5 mg/ L F or 1 . 7 - 2 . 5 mg/ L F in drinking water, with the analysis limited to continuous residents of the six cities studied . In the primary dentition , the prevalence of dental caries ( i . e . , children having 1 or more teeth with caries experience ) was 69 percent among those exposed to the higher concentration compared to 89 percent among children exposed to the lower concentration . In the permanent dentition , prevalence was 51 percent and 74 percent, respectively . By 1942 Dean and co - investigators reported associations between dental caries and fluoride using data from 21 cities where the concentration of fluoride in drinking water ranged from undetectable amounts to 2 . 6 mg/ L F . 32 Dental examiners also assessed dental fluorosis . When examined ecologically ( i . e . , by plotting community- average levels of caries and community-average levels of fluorosis ) , the findings identified a range of 1 . 0- 1 . 2 mg/ L F as the " optimal " concentration at which the occurrence of dental caries was lessened , but without elevated prevalence of dental fluorosis . Chapter 23 of Burt et a14 summarizes this extensive set of studies, commenting on their strengths, limitations and implications . Those association studies provided the rationale for subsequent intervention studies designed to determine if dental caries could be prevented by adding fluoride to drinking water . The first such study was conducted in Grand Rapids, MI , where fluoride was added to the public water supply to a concentration of 1mg/ L in 1945 . In that year, epidemiologists conducted dental examinations of 28, 614 school children , virtually the entire child population of the city at the time . After fluoridation , they repeated examinations annually for 10 years, selecting samples of children from five school grades in each year ( i . e . , not necessarily the same individuals from one year to the next) . Five years after fluoridating the water, mean deft index in the primary dentition reduced by one third in 5 year olds who were continuous residents ( and who represented 73 % of the population ) . 33 After 10 years, the mean DMFT index in the permanent dentition had halved in 12 -year-olds . 34 In contrast, over the same period , dental decay levels reduced by less than 20% in samples of continuous residents of non -fluoridated Muskegon , MI , where Dean and colleagues likewise conducted annual dental