The Earth Report   ed. E Goldsmith, N Hildyard.  London: Mitchell Beazley 1988  p. 149

Fluoridation    Technique of adding fluorides to public water supplies, with the intention of reducing tooth decay.

        Fluoridation transformed a liability of the aluminium industry (bulk sodium fluoride) into a commercial product.  Some fluoridation is done with fluosilicic acid, a waste from manufacture of superphosphate fertiliser.
       Tooth decay turns out to have declined by about two-thirds since the 1950s in the wealthy countries that gather dental statistics.  This welcome trend is not understood; possible causes may include improved diet and brushing, widespread antibiotics secreted in saliva, and, recently, fluoride in toothpaste, a concentrated direct application that may well be effective.  But thorough surveys have corrected the earlier claim that natural fluoride in water is correlated with relative freedom from tooth decay.  Trials claimed to demonstrate benefit from fluoridation have been severely criticized for lack of controls, and other major defects.

        In some still-controversial studies, fluoridation has, on the other hand, been rather closely correlated with cancer.  Various other types of harm have been suspected; the one established beyond dispute is dental fluorosis  -  bilaterally symmetrical diffuse white mottling of the teeth, a form of damage commonly observed among children drinking water fluoridated to 1 ppm.  The margin, if any, is uncomfortably slim between 1 ppm and levels known to cause serious damage to bones (skeletal fluorosis).  In addition, there is the possibility of adverse synergistic reactions between the fluoride added to water and the thousands of other chemicals to which modern industrial systems expose us.

        [as the original author I've taken the opportunity to correct a few editing errors  -  RM]