Why is fluoride good for your teeth?

The main component of tooth enamel is calcium hydroxyapatite. I know fluoride ions can replace hydroxide ions in the enamel matrix, slowing the rate of enamel demineralization. Why does hydrofluoric acid dissolved in water have no adverse health effects on other areas of the mouth? I suspect that its because the fluoride is in low enough concentrations to help your teeth and not harm the gums. 


