Water fluoridation programs re-emerge as a controversial topic from time to time. Just last year it was a hotly debated topic in Portland, Oregon.  Compound Interest covers the benefits of water (and other) fluoridation programs:

The enamel that coats your teeth is made up primarily of the compound hydroxyapatite. This ionic compound consists of calcium ions, phosphate ions and hydroxide ions, and is also a major component of your bones. Enamel is well known for being pretty strong, but it can be slowly broken down and lose ions from its structure under acidic conditions. This is known as demineralisation. Our body has a built-in countermeasure for this, and can replace the ions lost with ions from our saliva, in a process known as remineralisation. However, sometimes the rate at which this replacement occurs is below that at which the ions are being lost. When this happens the pores in the tooth can become enlarged, and cavities and tooth decay can result.

Fluoride ions can help arrest this process. They can be incorporated into the hydroxyapatite structure, replacing the hydroxide ions and forming fluorapatite. Fluorapatite is stronger than hydroxyapatite, and is also more resistant to acidic conditions. This means it can greatly delay the onset of cavities and tooth decay, and this is the reason why there’s a clamour to add it to water supplies.

Much more at his blog, including a nice infographic.