"Most water supplies contain some fluoride and around half a million people in the UK receive naturally fluoridated water.
Another 5.5 million people receive water that has had the fluoride concentration raised to around one part per million (1ppm). This level of fluoridation has been shown to have the most benefit for reducing decay levels.
The maximum amount of fluoride permitted in drinking water is 1.5 mg/l (milligrammes per litre)." taken from: http://www.nhs.uk/conditions/fluoride/Pages/Introduction.aspx