<P> The foundation of water fluoridation in the U.S. was the research of the dentist Frederick McKay (1874--1959). McKay spent thirty years investigating the cause of what was then known as the Colorado brown stain, which produced mottled but also cavity - free teeth; with the help of G.V. Black and other researchers, he established that the cause was fluoride . The first report of a statistical association between the stain and lack of tooth decay was made by UK dentist Norman Ainsworth in 1925 . In 1931, an Alcoa chemist, H.V. Churchill, concerned about a possible link between aluminum and staining, analyzed water from several areas where the staining was common and found that fluoride was the common factor . </P> <P> In the 1930s and early 1940s, H. Trendley Dean and colleagues at the newly created U.S. National Institutes of Health published several epidemiological studies suggesting that a fluoride concentration of about 1 mg / L was associated with substantially fewer cavities in temperate climates, and that it increased fluorosis but only to a level that was of no medical or aesthetic concern . Other studies found no other significant adverse effects even in areas with fluoride levels as high as 8 mg / L. To test the hypothesis that adding fluoride would prevent cavities, Dean and his colleagues conducted a controlled experiment by fluoridating the water in Grand Rapids, Michigan, starting January 25, 1945 . The results, published in 1950, showed significant reduction of cavities . Significant reductions in tooth decay were also reported by important early studies outside the U.S., including the Brantford--Sarnia--Stratford study in Canada (1945--1962), the Tiel--Culemborg study in the Netherlands (1953--1969), the Hastings study in New Zealand (1954--1970), and the Department of Health study in the U.K. (1955--1960). By present - day standards these and other pioneering studies were crude, but the large reductions in cavities convinced public health professionals of the benefits of fluoridation . </P> <P> Fluoridation became an official policy of the U.S. Public Health Service by 1951, and by 1960 water fluoridation had become widely used in the U.S., reaching about 50 million people . By 2006, 69.2% of the U.S. population on public water systems were receiving fluoridated water, amounting to 61.5% of the total U.S. population; 3.0% of the population on public water systems were receiving naturally occurring fluoride . In some other countries the pattern was similar . New Zealand, which led the world in per - capita sugar consumption and had the world's worst teeth, began fluoridation in 1953, and by 1968 fluoridation was used by 65% of the population served by a piped water supply . Fluoridation was introduced into Brazil in 1953, was regulated by federal law starting in 1974, and by 2004 was used by 71% of the population . In the Republic of Ireland, fluoridation was legislated in 1960, and after a constitutional challenge the two major cities of Dublin and Cork began it in 1964; fluoridation became required for all sizeable public water systems and by 1996 reached 66% of the population . In other locations, fluoridation was used and then discontinued: in Kuopio, Finland, fluoridation was used for decades but was discontinued because the school dental service provided significant fluoride programs and the cavity risk was low, and in Basel, Switzerland, it was replaced with fluoridated salt . </P> <P> McKay's work had established that fluorosis occurred before tooth eruption . Dean and his colleagues assumed that fluoride's protection against cavities was also pre-eruptive, and this incorrect assumption was accepted for years . By 2000, however, the topical effects of fluoride (in both water and toothpaste) were well understood, and it had become known that a constant low level of fluoride in the mouth works best to prevent cavities . </P>

What type of fluoride is added to water