Fluoride – friend or foe?

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Fluoride is a source of great confusion, because on one hand it is added to tap water in hundreds of towns around the world, but on the other hand people are concerned about its adverse effects. One example on this confusion is the discontinuation of fluoridation of water in Kuopio, Finland. Fluoride was added for thirty years (1959-1990) to prevent dental caries which was very prevalent in the region since the water naturally had a very low fluoride content. But a vocal local opposition movement finally succeeded in turning the general opinion on this issue, and the city fathers decided to discontinue fluoridation against expert advice, as they said that even beneficial interventions cannot be done without popular support. After this discontinuation of fluoride supplementation, the incidence of dental caries increased again.

Benefit and risk

Fluoride hardens the outer layer of tooth enamel and protects children's teeth from caries. This is because the fluoroapatite mineral in enamel is denser than the hydroxyapatite formed in the absence of fluoride. It may have beneficial effects also on the heart in decreasing the risk of heart attack. The beneficial concentration in water is about 1 mg/l. It is most effective in children before permanent teeth erupt, but there may be some limited benefits in children of older ages.

We are dealing with a typical benefit-risk assessment. Low (1 mg/l) concentrations of fluoride are beneficial to the body, high concentrations (over 2 mg/l) cause harm. Exactly the same situation holds true for most vitamins and minerals. Additional confusion is caused by the inappropriate use of the word fluorine, which is a highly irritant and caustic vapour. The difference between fluorine and fluoride is as remarkable as between chlorine gas and table salt, sodium chloride.

The safe range of fluoride concentrations in the drinking water is relatively narrow. To some extent, fluoride concentrations leading to useful and adverse effects also overlap and therefore it is best not to strive for maximal benefit. On the other hand, the allegations that fluoride causes cancer or increase mortality commonly argued by grass-root campaigners have not been substantiated in well-executed studies.

Local application of fluoride solutions by dental personnel appears to be effective if dabbed on newly erupted teeth. The results of adding fluoride to toothpastes are conflicting. Dietary fluoride supplements can be used for children whose drinking water contains less than 0.7 mg/l fluoride. If one uses fluoride tablets, then care is required to ensure that children do not take extra tablets, because harmful effects are then possible.

High natural concentrations

It is well known that the natural fluoride concentrations in some geographical areas in many countries are too high. Often the limit value for fluoride in drinking water is 1.5 mg/l, but concentrations of 2–3 mg/l fluoride may already cause some mottling or irregular colouring of tooth enamel, a condition called dental fluorosis, and at 4–6 mg/l the mottling incidence approaches 100%. After chronic exposure to high concentrations of fluoride, also the bones become harder and brittle, at still higher exposures there may be structural changes in bones.

The results on the association of excessive fluoride with fractures are equivocal. Some studies do suggest that hip fractures are increased in elderly women living in high-fluoride regions. In contrast, other studies have suggested fluoride at suitable levels may increase bone mineral density and decrease fractures. It seems to be clear that excessive fluoride intake does more harm than good.

Fluoride is a beneficial trace mineral for tooth development, but the problem is the relatively narrow range of the safe intake level. Excessively high concentrations cause adverse effects in both teeth and bones.

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