All fluoridation methods, including water fluoridation, create low levels of fluoride ions in saliva and plaque fluid, thus exerting a topical or surface effect. A person living in an area with fluoridated water may experience rises of fluoride concentration in saliva to about 0.04 mg/L several times during a day. Technically, this fluoride does not prevent cavities but rather controls the rate at which they develop. When fluoride ions are present in plaque fluid along with dissolved hydroxyapatite, and the pH is higher than 4.5, a fluorapatite-like remineralized veneer is formed over the remaining surface of the enamel; this veneer is much more acid-resistant than the original hydroxyapatite, and is formed more quickly than ordinary remineralized enamel would be. The cavity-prevention effect of fluoride is mostly due to these surface effects, which occur during and after tooth eruption. Although some systemic (whole-body) fluoride returns to the saliva via blood plasma, and to unerupted teeth via plasma or crypt fluid, there is little data to determine what percentages of fluoride's anticavity effect comes from these systemic mechanisms. Also, although fluoride affects the physiology of dental bacteria, its effect on bacterial growth does not seem to be relevant to cavity prevention.