The fluoride in water strengthens teeth enamel, which prevents cavities.

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Many consider it to be one of the greatest public health achievements of our time: fluoridated water. Fluoride, a group of natural compounds found in water or added to it at water treatment plants, helps prevent cavities and the eventual decay and loss of teeth. It does this by helping to protect and even rebuild the enamel, or hard covering, on your teeth. When sugars and bacteria wash over your teeth, they decay the enamel, which can lead to cavities.

Grand Rapids, Mich., was the first city to implement community-wide water fluoridation in 1945, after a study released in the early 1940s showed that people who lived in areas with high levels of naturally fluoridated water had fewer incidences of cavities than those who didn't [source: NCI]. As of 2008, 64.3 percent of the U.S. population, or about 195 million people, received fluoride in their drinking water [source: CDC].

There's solid evidence that fluoridating the water is a good thing for your teeth: For one thing, fluoridated water helps reduce tooth decay over a person's lifetime by 25 percent [source: CDC]. And for another, providing fluoride via the municipal water system means that some level of preventative dental care is available to everyone, regardless of socio-economic status and whether they can afford a visit to the dentist. Also, there's an interesting cost-savings benefit: Every dollar invested in adding fluoride to water saves about $38 in potential dental treatment costs [source: CDC].

Currently, the U.S. Public Health Service recommends a range of 0.7 to 0.12 milligrams of fluoride per liter of drinking water [source: CDC]. But recently, after review, the Department of Health and Human Services (HHS) recommended those levels be dropped to 0.7 milligrams per liter, based on the fact that back when water was first being fluoridated, fluoride toothpaste, fluoride rinses and professional fluoride treatments didn't exist. Today's population receives fluoride from so many sources that perhaps less is needed in the water supply.

The new recommendations have prompted people to question whether too much fluoride may be bad for you. According to the Centers for Disease Control and Prevention, children are more likely to be affected by too much fluoride because their teeth are still developing. In children age 8 and younger, overexposure to fluoride can cause pitting in tooth enamel. Adults, on the other hand, have fully formed teeth. But if they're overexposed to fluoride during their lifetimes, they can develop increased potential for bone fractures, or pain and tenderness in the bone [source: CDC].

Fluoridated water is not without controversy. Keep reading to find out why some people oppose fluoride in the water supply.