History of water fluoridation
The history of water fluoridation can be divided into three periods. The first (c. 1901–33) was research into the cause of a form of mottled tooth enamel called the Colorado Brown Stain, which later became known as fluorosis. The second (c. 1933–45) focused on the relationship among fluoride concentrations, fluorosis, and tooth decay. The third period, from 1945 on, focused on water fluoridation, which added fluoride to community water supplies.
In the first half of the 19th century, investigators established that fluoride occurs with varying concentrations in teeth, bone, and drinking water. In the second half they speculated that fluoride would protect against tooth decay, proposed supplementing the diet with fluoride, and observed mottled enamel (now called severe dental fluorosis) without knowing the cause. In 1874, the German public health officer Carl Erhardt recommended potassium fluoride supplements to preserve teeth. In 1892 the British physician James Crichton-Browne noted in an address that fluoride's absence from diets had resulted in teeth that were "peculiarly liable to decay", and who proposed "the reintroduction into our diet ... of fluorine in some suitable natural form ... to fortify the teeth of the next generation".
Community water fluoridation in the United States is partly due to the research of Dr. Frederick McKay, who pressed the dental community for an investigation into what was then known as "Colorado Brown Stain." The condition, now known as dental fluorosis, is characterized in its severe form by cracking and pitting of the teeth. Chemical analysis in 1931 revealed the correlation between mottled teeth and high concentrations of fluoride.
H. Trendley Dean, a dental officer at the newly created National Institute of Health was appointed to study dental fluorosis in 1930. Through a series of surveys, Dean analyzed fluoridation levels and mottling of teeth in different communities and determined that at concentrations below 1ppm, fluoride does not stain teeth. Dean and other dentists began investigating the use of fluoride in low concentrations to prevent dental caries. Many of these studies implicated fluoride as an effective prevention measure, and by the early 1940s the NIH began conferences and studies on the possibility of introducing fluoride into the public water supply. 
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. 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 other locations, fluoridation was used and then discontinued; for example, 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.
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.
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