Since even before the 1930's scientists have well understood the amazing properties of fluoride in repairing and delaying the demineralisation of tooth enamel caused by acidic foodstuffs. Just a tiny, regular amount in drinking water - 1 part per million - is effective. It occurs naturally in some district water sources and it was an investigation into the common factors as to why some areas had much less decay rates than others that led investigators to discovering the link. In the 1950's and 60's several controlled experiments were done around the world and the findings that these areas dramatically reduced their rates of dental decay within a few years of fluoride being added to the water supply convinced many public policy makers to recommend adding it where it was needed. Though fluoridation is patchy, it is practiced by many countries and has saved the teeth of millions of people.
However; not everyone agrees. The addition of fluoride has been seen as an infringement of civil liberties and a widespread campaign by the usual conspiracy mongers about "chemicals in the water" has been waged since it started being added. Indeed, in the 1950's it was seen as a commie plot to poison the pure 'mercan water. There are also a number of scientists who claim the benefits are overestimated or simply wrong, but the FDI World Dental Federation supports water fluoridation as safe and effective; as does the European Academy of Paediatric Dentistry, and the national dental associations of Australia, Canada, and the U.S. The American Dental Association calls water fluoridation "one of the safest and most beneficial, cost-effective public health measures for preventing, controlling, and in some cases reversing, tooth decay.
There are some genuine issues. Overexposure to fluoride can cause fluorosis, which is a discolouration of the tooth and in severe cases can cause damage, though levels to do this are far far higher than the amount added to water supplies. Many other claims are made such as an effect on IQ, increases in cancer rates, damage to liver and kidneys and to bone damage. However the studies pointed to are often poorly controlled rat studies and are dismissed by proponents of fluoridation because the claimed effects are not seen in the real world. Scottish Water does not add fluoride to the water supplies despite us having poorer dental health than the rest of the UK. Their response to why not is: "To do so would require the NHS health board to undertake a full public consultation exercise, before applying to Scottish Water, to add fluoride to any drinking water supply. This consultation involves publicising the proposal in local newspapers and giving notice to the Scottish Government and any affected Local Authorities.The onus is on the NHS Health Board to demonstrate that a significant majority of the population are in favour of the proposal. NHS Health Boards then would be responsible for all capital and operating costs, for the addition of fluoride to water supplies".