How fluoride works Fluoride reduces the incidence of caries in the following ways In the developing tooth, calcium hydroxyapatite in enamel is replaced by calcium fluorapatite which withstands a lower pH of 4.5 rather than the normal 5.5. Teeth that erupt with shallower pits and fissures when systemic fluoride is given are more resistant to acid attacks Topical fluoride plays a part in ionic exchange, acting as a catalyst and helping return more acid-resistant crystals to the tooth. uoride blocks the enzyme systems of plaque bacteria, inhibiting their ability to turn sugars into acids. Fluoride has also been to remineralise early carious lesions when used shown topically.