Fluoride is an ion (F–) of the element fluorine. The fluoride ion is very reactive and is found bound to other metals such as sodium and calcium in nature. Fluoride is very abundant and is considered the 13th most abundant element on the plant. This explains the almost universal nature of fluoride in foods consumed by man. Fluoride is most widely known for its addition to drinking water, allegedly based on the premise that fluoride is able to reduce the amount of dental caries in children. However, many European countries have now stopped this practice, or never began it, because there is little solid data to support the practice. It is not known if fluoride is an essential dietary component, but there is no known deficiency disease associated with it. Some researchers have attempted to cause fluoride deficiencies in animals, but only inconsistent results were obtained.
Around 3 to 4 mg/d fluoride is recommended as a safe intake for a typical adult. Water is usually fluoridated at around 1 to 2 parts per million (ppm), which is 1 or 2 µg fluoride per gram water. A litre of such water would supply roughly 1 or 2 mg of fluoride, which suggests that the recommended 3 to 4 mg/d could easily be exceeded by drinking fluoridated water. Fluoride accumulates in bones and organs in humans, and toxicity occurs when levels reach a critical threshold. Acute toxicity of fluoride in humans occurs at intake of around 2 to 9 mg/d in children, and this results in dental fluorosis, characterised by a mottling of the teeth. All pharmacological effects of fluoride are toxic and result in diminished energy production from metabolic pathways. Around 5 to 10 g of sodium fluoride in a single does is enough to cause death.
Some of the toxicity associated with fluoride may be due to its reactivity with calcium. Studies have shown that increasing dietary calcium decreases fluoride absorption considerably, possibly because of competition for absorption. This may explain some of the benefits of a high calcium diet to health. However, because fluoride is present in food, water and the air, it is difficult not to ingest substantial amounts. Plants accumulate fluoride based on the water and air supply they are exposed to. Fluoride is a pollutant and industrial areas can release significant amounts of airborne fluoride. The forms of fluoride in the air can include hydrogen fluoride and silicon tetrafluoride, as well as a particulate form of sodium fluoride. If this fluoride settles on plants it can be absorbed to tissues. Dry periods can increase uptake of fluoride by plants, and wet periods have the opposite effect.
Green leafy vegetables are therefore susceptible to airborne fluoride. Some plants such as tea can accumulate fluoride from 9 to 400 ppm of dry leaf, although consumption does not seem to have detrimental health effect. Green tea contains more fluoride than black tea, and older leaves contain more fluoride that younger leaves. Some reports suggest that around 6 cups of tea are enough to supply 1 mg of fluoride. That fluoride from tea is not detrimental to health may relate to the fact that calcium fluoride, the most abundant form in food, is very poorly absorbed when compared to some synthetic preparations of fluoride such as hexafluorosilicic acid. Uptake of fluoride into the roots of plants is directly related to the concentration of fluoride in the soil and water. Some agricultural soils contain significantly higher concentrations of fluoride compared to others. Cereals, particularly oats, are fluoride poor containing only around 1 ppm.
In animals, most fluoride is found in the Skelton (100 to 1000ppm). Bone meal, bones and bone products are therefore very rich in fluoride. The amount of fluoride present in animals is directly related to the fluoride content of the food and water they consume. Age is also important because older animals have more time to accumulate fluoride. Some fluoride can accumulate in the muscle, organs and skin of animals (0.2 to 10 ppm), but this process is not well understood. Fish can contain large amounts of fluoride (50 to 100 ppm) because water is a rich source of fluoride which can range between 1 and 8.72 ppm. Sea food with a high calcium content, such as crustaceans, can accumulate substantial amounts of fluoride. Milk contains around 0.09 to 0.32 ppm but this is not linked to the fluoride content of the water of the cattle, suggesting that the fluoride in milk may be a contaminant of the processing in the dairy industry.
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