Calcium fluoroapatite is chemically stable than calcium hydroxyapatite in acid environment of the mouth
Calcium fluoroapatite dissolves at pH of 4.5
Calcium hydroxyapatite dissolves at pH of 5.5.
This means that a higher concentration of calcium fluoroapatite in tooth enamel decreases tooth dissolution, and therefore can decrease the incidence of tooth decay.
Note: low-dose fluoride supplementation is also being used, on an experimental basis, to reduce the incidence of bone fractures in people affected by osteoporosis (fluoride strengthen bones for the same reason as in teeth).