Legislation is going through the House of Lords that will shift the decision on whether to add fluoride to drinking water from the water companies to local health authorities. Fearing legal action, water companies have been loath to take responsibility over what is an extremely charged issue.

If fluoride reduces tooth decay, why not just use fluoridated toothpaste?

Good point. But a letter from Hazel Blears, the Health Minister, and Elliot Morley, the Environment Minister, to John Prescott, the Deputy Prime Minister, leaked to The Sunday Times, said: “Experience of oral health promotion projects shows that it is much harder to establish regular toothbrushing in deprived areas because of the costs of toothpaste and, perhaps, because of the less ordered lifestyles lived by families.” The British Dental Association says that fluoridated water would most benefit the young and provide a lifelong improvement in oral health.

So what’s the problem?

Plenty. Fluoride has been linked to cancer, osteoporosis, kidney problems, skeletal fluorosis, Alzheimer's disease and mottled teeth. Critics see this move as mass medication by a nanny state and say the benefits are far from conclusive.

What do other countries do?

More than 150 million people in the US have fluoridated water, as do 70 per cent of people in Ireland and 2 per cent of people in Britain, mainly in the West Midlands and Newcastle. Supporters say if fluoride were harmful, there would be unexplained clusters of disease in these places. But France, Italy, Germany, Sweden, Denmark and the Netherlands have rejected it, as has Basle in Switzerland, and 50 cities in America.

What is fluoride?

A naturally occurring mineral — trace elements are found in some water. Artificial fluoride is added to water in the form of disodium hexafluorosilicate and hexafluorosilicic. Highly toxic, the legal limit is one part per million, but critics says it’s impossible to say how much individuals are exposed to as fluoride is also present in fish and tea (six cups of tea contain about 1mg of fluoride or half of what is considered a safe daily amount).

What happens next?

A study into exposure levels and the effects on dental caries is being funded by the Department of Health. It reports in the autumn.

What do you do if the water companies decide to add it to water anyway?

The leaked letter from ministers said: “Those who remain adamantly opposed would be able to use water filters that remove fluoride or buy bottled drinking water.”

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