Fluoride ions are found in several minerals, particularly flourite. Flouride salts are used extensively in industrial chemicals, especially to produce hydrogen fluoride for fluorocarbons.
Soluble fluoride salts such as sodium fluoride can be toxic in high concentrations and skin or eye contact can be dangerous. In most cases, a lethal dose of sodium fluoride would be 5 or 10 mg, although a dose that is 15 or 20 times lower can cause gastrointestinal effects if the fluoride is ingested.
When used in high doses to treat osteoporosis, for example, the toxic effects of sodium fluoride can include pain in the legs and even fractures of the long bones due to minor trauma. It can also lead to stomach irritation and gastric ulcers. Fewer complications are seen when sodium fluoride tablets are produced as slow release preparations.
Although fluoride is known to be beneficial to dental health at a low concentration, long-term exposure to large amounts of fluoride can disrupt the bone formation process. Some of the main examples of fluoride leading to toxic effects have arisen due to ground water being rich in fluoride.
Water fluoridation is a routine procedure that is undertaken to reduce and prevent tooth decay in populations. However, exposure to high concentrations of fluoride while the teeth are developing can lead to a condition called dental fluorosis, which refers to a mottling of the dental enamel. In most cases, the effect is mild and the fluorosis is almost unnoticeable, leading to the formation of small white dots or streaks in the tooth enamel. However, in its most severe form, the teeth become strongly discolored and have brown markings. The surface of the enamel may also appear pitted and feel rough. The marks formed on the teeth due to fluorosis are permanent and may gradually become darker as a person ages.
Hydrogen fluoride can be even more toxic than the fluoride salts since it is corrosive, volatile and can lead to death if it is inhaled or even when contact is made with the skin. Calcium gluconate gel is an antidote to hydrogen fluoride poisoning.
Further Reading
Fluoride and sealants win against youth dental caries, a robust review informs US health policy