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Though today brushing your teeth with dental fluoride is nearly universal in developed countries, as recently as 1939, most people knew nothing of its value in preventing tooth decay. The story of dental fluoride begins in 1901, when the dentist Dr. Frederick S. McKay moved to Colorado Springs and noticed that the natives had mottled, discolored teeth with brown stains. After several years of investigation, it was determined that this was due to dental fluorosis, an overdose of fluorine that harms teeth and causes brown stainage and holes. However, it was observed that people with dental fluorosis had a lower incidence of tooth decay from other causes.
McCay's findings on fluoride were eventually read by Dr. H. Trendley Dean, a dental officer of the U.S. Public Health Service. Dean began conducting his fluoride studies in the early 1930s. They conducted tests where lower and lower concentrations of fluoride solution were administered to volunteers until no brown stainage or mottled enamel occurred. In 1936, Dean and his team came upon the "magic number" of 1 part per million (ppm) where people experienced no fluorosis in addition to a lower rate of tooth decay. In 1939, Dean proposed the concept of adding fluorine to water supplies at a part of one per million to promote public health. In 1950, Procter & Gamble conducted a study on whether fluoride could be added to toothpaste for clinical benefit, and successful trials led to their introduction of fluoride toothpaste in 1955.
Dental fluoride is helpful for our teeth for three reasons. First, fluoride promotes the remineralization of teeth, which is especially beneficial for children under the age of 12. Trace amounts of fluorine in the saliva bond to teeth, attracting more minerals and remineralizing them. Another benefit of dental fluoride is that the remineralized teeth are more resistant to decay. New teeth are made of hydroxyapatite and carbonated hydroxyapatite. When teeth mineralize in the presence of fluoride, fluorapatite, a harder material, is formed. Fluorapatite is especially resistant to bacterial acids. The third benefit of fluoride is that it directly inhibits the ability of microbes to produce acids, slowing down their ability to dissolve teeth. It also inhibits the ability of bacteria to metabolize sugars, slowing down their growth rate.
When all three of the benefits of dental fluoride interact and are kept consistent by regular brushing twice a day, the dental benefits are substantial. Daily brushing starting early in life can save the brusher $50,000-$100,000 US Dollars in dental bills over the course of a lifetime. This also translates into a lot of avoided pain and a lighter burden on national medical systems and expenses.