In the early 1900s, Frederick S. McKay, a Colorado dentist, discovered that an unknown substance in the local drinking water caused a mottling or staining of the teeth and that these teeth also showed fewer cavities. In 1931 the substance was identified as a fluoride. Later, in the 1930s, it was found that a fluoride level in drinking water of about one part per million was high enough to reduce tooth decay but low enough to prevent teeth from becoming mottled.
In some communities fluorides are a natural constituent of the water supply; other communities have added fluorides to their reservoirs. Such action has the support of the American Dental Association, the American Medical Association, and other scientific organizations. Although studies have proven that fluoridation at levels of one part per million is safe, attempts at fluoridation have met with resistance and controversy. Its opponents say that it constitutes compulsory medication, that the amount of fluorine taken into the body cannot be controlled, and that those who wish to prevent tooth decay through fluorides can do so individually by adding the compound to their beverages or by using toothpaste and other dental substances to which fluorides have been added. Despite such resistance, many Americans drink artificially fluoridated water, and fluoridation programs have been started in other countries as well.