Fluoride occurs naturally in most water supplies, though the actual amount present varies depending on the source. Some water supplies can contain fluoride if they originate from sources underground where there are rocks that contain fluoride-rich minerals.

Water supplies in the North West are naturally low in fluoride - and normally contain less than 0.2mg fluoride per litre.

Some water supplies have fluoride added to raise the concentration to 1.0mg per litre. This figure includes the fluoride that was already there naturally.