Many studies conducted prior to the availability of fluoride-containing dental products demonstrated that dietary fluoride exposure is beneficial, owing to its ability to inhibit the development of dental caries in both children and adults (Russell and Elvove, 1951). The results of most of these studies showed that the prevalence of dental caries in communities with optimal water fluoride concentrations (range 0.7 to 1.2 mg/liter, depending on average regional temperature) was 40 to 60 percent lower than in areas with low water fluoride concentrations. The lower concentrations within the optimal range are recommended for warm climates where water intake tends to be greater than in cooler climates. Other studies have shown that the earlier children are exposed to fluoridated water or dietary fluoride supplements, the greater the reduction in dental caries in both the primary and permanent teeth (Hargreaves et al., 1988; Lewis, 1976; Stephen et al., 1987). The lack of exposure to fluoride or the ingestion of inadequate amounts of fluoride at any age places the individual at increased risk for dental caries.
Both the inter-community transport of foods and beverages and the use of fluoridated dental products have blurred the historical difference in the prevalence of dental caries between communities with and without water fluoridation. Brunelle and Carlos (1990) summarized the results of the 1986–1987 national survey conducted by the National Caries Program of the National Institute of Dental Research. The overall difference in caries prevalence between fluoridated and nonfluoridated regions in the United States was 18 percent whereas the majority of earlier studies reported differences of approximately 50 percent. After children with a reported history of exposure to dietary fluoride supplements or topical fluorides were excluded from the analysis, the difference increased to 25 percent. Further, the differences in the prevalence of caries in the seven geographic regions of the United States were inversely proportional to the percentages of water supplies that were fluoridated. In region VII (Pacific), where only 19 percent of the population is served with fluoridated water, the difference in caries scores between fluoridated and nonfluoridated areas was 61 percent. In region III (Mid-west), where 74 percent of the population is served with fluoridated water, the difference was only 6 percent. These findings suggested an important role for the halo or diffusion effect. The results of this survey, like that of the 1979–1980 national survey conducted by the National Caries Program of the National Institute of Dental Re-