Body & HealthFact
Fluoride repairs enamel – but only at the surface.
Tooth enamel is hydroxyapatite. Fluoride from toothpaste builds into dissolved spots and forms fluorapatite, which only dissolves at a lower pH (about 4.5 instead of 5.5). Early caries can re-harden this way. The steep decline of caries in industrialised countries since the 1970s is attributed mainly to the widespread use of fluoridated toothpaste.
Useful: That's why you spit after brushing but don't rinse thoroughly – the fluoride should stay.
Source: doi.org