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According to the U.S. Centers for Disease Control and Prevention (CDC), more than 70% of the United States has access to fluoridated water. The American Dental Association (ADA) says, “Fluoridation of community water supplies is the single most effective public health measure to prevent tooth decay. To learn more about water fluoridation, click on the questions below.

Community water systems have added fluoride, a naturally occurring element, to their water supplies since 1945 to help prevent tooth decay. Since that time, fluoridation is effective in reducing tooth decay by at least 25% in children and adults according to the American Dental Association.

In the seminal report, Oral Health in America: A Report of the Surgeon General, Surgeon General David Satcher observed a "'silent epidemic' of dental and oral diseases [...] with those suffering the most found among the poor of all ages." The report affirms that community water fluoridation is "an inexpensive means of improving oral health that benefits all residents of a community, young and old, rich and poor alike." Because of its contribution to the dramatic decline in tooth decay, CDC named community water fluoridation 1 of 10 great public health achievements of the 20th century.
Your water system publishes a consumer confidence report each year and makes that report publicly available. The report is often available on the internet, but you may need to contact your water provider to request a copy. Contact your public water system to learn more about fluoride levels in your water. You can usually find contact information for your public water system on your water bill. You can also visit CDC’s My Water’s Fluoride page.

If you get your drinking water from a private well, you should have your water tested by a certified laboratory at least once a year. You can find information on how to sample for fluoride and where to send samples for analysis by contacting your state water laboratory certification officer. Contact information for your state can be found on EPA's drinking water lab certification page. Additional information about well water testing from EPA is available on their private drinking water well FAQ page.
Fluoride's effect is topical. It keeps the tooth enamel strong by preventing the loss of important minerals.

According to the CDC, “Bacteria in the mouth produce acid when a person eats sugary foods. This acid eats away minerals from the tooth’s surface, making the tooth weaker and increasing the chance of developing cavities. Fluoride helps to rebuild and strengthen the tooth’s surface, or enamel. Water fluoridation prevents tooth decay by providing frequent and consistent contact with low levels of fluoride. By keeping the tooth strong and solid, fluoride stops cavities from forming and can even rebuild the tooth’s surface.”
Public health institutions like the CDC and the ADA conclude that extensive research conducted has demonstrated that fluoridation of public water supplies is a safe and effective way to reduce the incidence of tooth decay in a community.

While exposure to high levels of fluoride over a long time can cause dental fluorosis, a condition that leads to mottled tooth enamel, discoloration, and in some cases erosion the gum line, the fluoride content in your drinking water is limited under federal law so the levels are very low.

In April 2015 the U.S. Department of Health and Human Services announced the final U.S. Public Health Service recommendationannounced the final U.S. Public Health Service recommendation for the optimal fluoride level in drinking water to be a single level of 0.7 milligrams of fluoride per liter of water. This recommended level replaced the previous recommended range (0.7 to 1.2 milligrams per liter) issued in 1962.
The CDC reports that it is acceptable to use fluoridated water when preparing a bottle with formula. However, the Center suggests using low-fluoride bottled water some of the time to decrease the chance of a child developing dental fluorosis.
The ADA has excellent advice on this topic. It does recommend the use of fluoride toothpaste in children because it prevents tooth decay. For children younger than three years old, “no more than a smear of the size of a grain of rice” should be used. For children three to six years old, “no more than a pea-sized amount” should be used.