Subsequently, many towns and cities in the US added fluoride to their water, and in the UK most local health authorities have requested it. However, water companies are not obliged to fluoridate water and have done so in only a few areas, such as North-east England, West Cumbria, Lincolnshire, South Cheshire and the West Midlands. Water is treated by adding fluorosilicate, a fluoride-rich by-product of phosphoric acid manufacture.
Fluoridation is Government policy and seen as the cheapest way of reducing tooth decay. In 1993 the Secretary of State for Health, Virginia Bottomley, urged all areas to press ahead, but little progress has been made. Despite its benefits, some environmentalists oppose it because they want all drinking water to be free of "chemicals". Others are against it on ethical grounds, seeing it as enforced medication; some doubt its efficacy; and a few think it is harmful. Such people cite epidemiological studies that show marginally more cases of bone and liver cancer in fluoridated areas, and they claim it damages the immune system.
Fluoride is an atom of the element fluorine with an extra electron. This makes it negatively charged and more stable. It is chemically akin to chloride, which we come across daily as sodium chloride, or common salt. But while we can sprinkle a few grams of this on our food with impunity, the same amount of sodium fluoride would kill us. It is an effective insecticide for cockroaches and ants.
Dangerous though fluoride is, we each have about two grams in our body and we take in about two milligrams a day. A litre of fluoridated tap water has one milligram, but most people get their fluoride from chicken, pork, eggs, potatoes, butter, cheese and tea. Cod, mackerel, sardines, salmon and sea salt are particularly rich in this element because seawater contains 1ppm of fluoride.
Laboratory animals on fluoride-free diets failed to grow properly, were anaemic and infertile, and the implication is that fluoride is also essential for humans. It certainly promotes healthier teeth and stronger bones by infiltrating the calcium phosphate of which they are made. Fluoridated teeth resist the action of oral bacteria which convert sugar to corroding acids.
Fluoride has a powerful effect on enzymes, bringing many of these natural catalysts to a dead stop. This is useful in biochemical research and in cleaning fermentation vats in breweries and distilleries. In living organisms the effects are less dramatic because levels of fluoride are very low, but 20ppm of fluoride will inhibit most of the acid phosphatase enzyme of human saliva. On the other hand, some enzymes are boosted by fluoride, such as the lipase in liver.
If our diet contains too much fluoride, we may suffer from fluorosis, the first signs of which are mottled teeth. Later there may be osteosclerosis - hardening of the bones which leads to a deformed skeleton. In parts of India, such as the Punjab, the condition is endemic, especially where villagers drink water from wells with levels of fluoride up to 15ppm. About 25 million Indians suffer mild fluorosis, and thousands show skeletal deformities. In some villages one child in six is affected, but this isimproving with defluoridation schemes.
The only cases of fluorosis in the UK were among cattle grazing near brickworks in Bedfordshire, which once spewed out fluoride-laden fumes. These closed in the early Eighties.
Fluoride toothpaste generally has 0.1 per cent fluoride (1,000ppm), with the fluoride in the form of the less toxic sodium monofluorophosphate. Such toothpaste is not recommended for young children because they tend to swallow it - parents should buy toothpaste with 0.05 per cent (500ppm) fluoride.
Fluoride toothpaste became popular in the Seventies and is credited with reducing dental caries in the UK, according to Professor Andrew Rugg-Gunn, of Newcastle University dental school. He has researched the effects of fluoridation on children's teeth for almost 20 years: "Our findings ... show that fluoridation cuts decay by half."
Dr Rugg-Gunn's research showed a rapid decline in caries between 1976 and 1981, but after that a levelling off. He believes further improvements will come through more use of fluoride toothpaste and less eating of sugary sweets and drinks.
Fluoride is one of the few minerals of which Britain has abundant natural reserves as fluor-spar (calcium fluoride), mined in Derbyshire and currently producing 50,000 tons a year. A further 10,000 tons are imported. According to Dr Alan Comyns, a consultant based in Chester, most fluorspar is converted to hydrofluoric acid (HF).
The chemicals industry needs HF to make organofluorine polymers such as Teflon and CFC-replacements, while the petroleum industry uses it as a catalyst for making unleaded petrol. The largest users are the metallurgical industries: aluminium is produced by electrolysis of aluminium fluoride and refined by using titanium and boron fluorides. Steel-making requires fluorspar as a flux, and HF is used to treat the finished metal.
John Emsley is science writer in residence at Imperial College, London, and author of `The Consumer's Good Chemical Guide', published by WH Freeman at £18.99.