A 2007 meta-analysis by CDC researchers found that water fluoridation prevented an estimated 27% of cavities in adults, about the same fraction as prevented by exposure to any delivery method of fluoride (29% average). At the commonly recommended dosage, the only clear adverse effect is dental fluorosis, which can alter the appearance of children's teeth during tooth development; this is mostly mild and is unlikely to represent any real effect on aesthetic appearance or on public health.For example, in Finland and Germany, tooth decay rates remained stable or continued to decline after water fluoridation stopped. The critical period of exposure is between ages one and four years, with the risk ending around age eight.
A 2015 Cochrane systematic review estimated a reduction in cavities when water fluoridation was used by children who had no access to other sources of fluoride to be 35% in baby teeth and 26% in permanent teeth. does not have school-based dental care, many children do not visit a dentist regularly, and for many U. children water fluoridation is the prime source of exposure to fluoride.
Fluoride may also prevent cavities in adults of all ages. Fluoride's adverse effects depend on total fluoride dosage from all sources.
The review found that the evidence was of moderate quality: few studies attempted to reduce observer bias, control for confounding factors, report variance measures, or use appropriate analysis.
Although no major differences between natural and artificial fluoridation were apparent, the evidence was inadequate for a conclusion about any differences.
Higher concentrations of fluorine are found in alkaline volcanic, hydrothermal, sedimentary, and other rocks derived from highly evolved magmas and hydrothermal solutions, and this fluorine dissolves into nearby water as fluoride.