What is the primary action of fluoride when applied topically?

The primary action of fluoride when applied topically is to facilitate the remineralization of tooth enamel. Fluoride enhances the natural process of remineralization, which occurs when minerals like calcium and phosphate are redeposited into the tooth enamel after being lost due to acid attack from bacteria in dental plaque. By promoting this remineralization process, fluoride helps to strengthen the enamel, making it more resistant to future demineralization and decay.

This action is crucial because it addresses early stages of tooth decay, where the enamel has started to weaken but has not yet formed a cavity. Regular application of fluoride through toothpaste, mouth rinses, or professional treatments can significantly reduce the risk of caries by reinforcing the enamel.

Other options may suggest benefits that fluoride is known for but are not its primary action. For instance, while fluoride can help reduce sensitivity by improving enamel strength, this is not its main function. Whitening teeth and eliminating bad breath do not pertain to the core protective and restorative roles of fluoride in dental health.