MPs are debating whether local health bodies, with public support, should be able to add fluoride to the water supply.

Advocates say fluoride can cut rates of tooth decay, but critics say the chemical can be harmful.

BBC News Online looks at the issues MPs will be discussing on Monday.

What are MPs actually debating?

Monday sees the second reading of the Water Bill in the House of Commons.

An amendment to the Bill would enable fluoride to be added to the water supply if local communities and health bodies agree.

The amendment was backed by the House of Lords in July.

Don't some areas already have fluoride in their water?

Yes. Water companies have had the power to add the chemical to supplies since 1985.

Around five million people in areas including Birmingham and Gateshead already drink fluoridated water.

But many of the private water companies have been deterred from fluoridation because of fears of legal action from those opposed to the move.

This amendment would mean local health authorities would indemnify them against such action.

What is the argument for adding fluoride to water supplies?

Dentists are wholly in favour of the plan. They say it has significant benefits for children's dental health, cutting rates of tooth decay.

They point to Manchester - where fluoride is not added to water has three times the rates of tooth decay in Birmingham - where it is.

They say there is no evidence fluoride is harmful.

If the benefits are so clear cut, why is there any argument?

Critics, such as the Green Party, say there are health concerns about fluoride.

There are fears high levels could be linked to diseases including bone cancer, brittle bone disease and kidney problems.

They say, although levels of fluoride in the water supply would be low, when that was added to fluoride from other sources such as toothpaste and tea, they would be too high.

They also say there is little evidence fluoride actually benefits dental health, adding educating people about the risks of eating a lot of sugary foods would have a far greater influence on tooth decay rates.