Adding Fluoride to drinking water is used to prevent and repair tooth decay using processes called demineralisation and remineralisation.

During demineralisation bacteria in plaque feed on sugars and carbohydrates in your mouth and cause saliva to acidify. This acidic saliva weakens the enamel of teeth and the calcium and phosphorus underneath. Fluoride helps control the acidity of saliva, limiting the decay.

Teeth are made out of multiple layers, the enamel being the outer protective layer, covering the dentin and pulp. This enamel is made up of crystals of hydroxyapatite- which is made from calcium, phosphate (phosphorus + oxygen), and hydroxide (oxygen + hydrogen). When this hydroxyapatite is exposed to acidic conditions (pH less than 5.5 for enamel, its 6.5 for dentin) the the phosphate in the enamel gets removed to replace the phosphate levels in saliva. This happens because the phosphate in saliva is joining with the extra Hydrogen ions (acid) in the saliva to make other compounds, which is then balanced out by removing phosphate from the tooth. When there is fluoride available in the saliva, following on from demineralisation, remineralisation occurs. This is when fluoride joins the calcium and phosphate in the enamel to replace the decay with fluoroapatite crystals, leaving the tooth stronger.

In remineralisation, fluoride helps replenish areas of decay by joining with the phosphorus and calcium to restrengthen enamel. This remineralised enamel with fluoride present is less prone to tooth decay, as it is less soluble and more resilient to acid.