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€4m annual cost of treating water we'll soon be paying for

fluoridation costs up to €4m a year for 200 public water supplies serving about 70pc of the population.

The chemical – hydrofluorosilicic acid – is bought by the HSE from a Spanish company and distributed to local authorities who add it to the public water supplies.

An HSE spokesperson said "the supplies or water plants are tested on a monthly basis" to make sure the fluoride levels are not breached.

Fluoridation is mandatory in Ireland under the Health (Fluoridation of Water Supplies) Act 1960. Regulations from 2007 require treatment plants to test daily in addition to the monthly tests by the HSE.

Since 2007 the levels of fluoride in drinking water here have been set at between 0.6 and 0.8 parts per million, which is about half the safe level of 1.5 parts per million stipulated in a Council of Europe directive. Fluoridation of water supplies began in Dublin in 1964 after a national dental survey highlighted the high levels of decay at the time.

The then Chief Justice and later President Cearbhal O Dalaigh (inset) said: "The effect on the teeth (of fluoridation) is demonstrably beneficial."

Last November the Government opposed a private members' bill from Sinn Fein proposing that fluoridation of water supplies be halted.

Independent TD Catherine Murphy said when people were going to have to pay for water, many would want to be able to choose not to have it fluoridated.

Junior Minister for Primary Care Alex White, however, said the Government could not take a risk by temporarily halting the fluoridation of water. He said there were no risks to public health from fluoridating tap water.