Fluoride (/ˈflʊəraɪd, ˈflɔːr-/)[3] is an inorganic, monatomic anion of fluorine, with the chemical formula F− (also written [F]−), whose salts are typically white or colorless.

Fluoride ions occur on Earth in several minerals, particularly fluorite, but are present only in trace quantities in bodies of water in nature.

For example, sulfur hexafluoride and carbon tetrafluoride are not sources of fluoride ions under ordinary conditions.

[citation needed] Fluorine is estimated to be the 13th-most abundant element in Earth's crust and is widely dispersed in nature, entirely in the form of fluorides.

[4] Natural weathering of some kinds of rocks,[5][6] as well as human activities, releases fluorides into the biosphere through what is sometimes called the fluorine cycle.

Fluoride is naturally present in groundwater, fresh and saltwater sources, as well as in rainwater, particularly in urban areas.

[9] Groundwater (well water) concentrations vary even more, depending on the presence of local fluoride-containing minerals.

For example, natural levels of under 0.05 mg/L have been detected in parts of Canada but up to 8 mg/L in parts of China; in general levels rarely exceed 10 mg/litre[10] Fluoride can be present in rain, with its concentration increasing significantly upon exposure to volcanic activity[14] or atmospheric pollution derived from burning fossil fuels or other sorts of industry,[15][16] particularly aluminium smelters.

The identity of the solvent can have a dramatic effect on the equilibrium shifting it to the right-hand side, greatly increasing the rate of decomposition.

In terms of its reactivity, fluoride differs significantly from chloride and other halides, and is more strongly solvated in protic solvents due to its smaller radius/charge ratio.

Because of their high basicity, many so-called naked fluoride sources are in fact bifluoride salts.

[25] The sterically demanding imidazolium cation stabilizes the discrete anions and protects them from polymerization.

Fluorine, in the form of fluoride, is considered to be a micronutrient for human health, necessary to prevent dental cavities, and to promote healthy bone growth.

[28] The tea plant (Camellia sinensis L.) is a known accumulator of fluorine compounds, released upon forming infusions such as the common beverage.

The study indicates that tea drinking communities are at an increased risk of dental and skeletal fluorosis, in the case where water fluoridation is in effect.

[42][43] The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for some minerals in 1997.

Where there was not sufficient information to establish EARs and RDAs, an estimate designated Adequate Intake (AI) was used instead.

As for safety, the IOM sets tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient.

[45] The EFSA reviewed safety evidence and set an adult UL at 7.0 mg/day (lower for children).

[46] For U.S. food and dietary supplement labeling purposes, the amount of a vitamin or mineral in a serving is expressed as a percent of Daily Value (%DV).

Although there is information to set Adequate Intake, fluoride does not have a Daily Value and is not required to be shown on food labels.

For older children and adults, who are no longer at risk for dental fluorosis, the upper limit of fluoride is set at 10 mg/day regardless of weight.

[4] The lethal dose for most adult humans is estimated at 5 to 10 g (which is equivalent to 32 to 64 mg elemental fluoride per kg body weight).

[57] For sodium fluorosilicate (Na2SiF6), the median lethal dose (LD50) orally in rats is 125 mg/kg, corresponding to 12.5 g for a 100 kg adult.

[56] Hydrogen fluoride is more dangerous than salts such as NaF because it is corrosive and volatile, and can result in fatal exposure through inhalation or upon contact with the skin; calcium gluconate gel is the usual antidote.

Slow-release and enteric-coated versions of sodium fluoride do not have gastric side effects in any significant way, and have milder and less frequent complications in the bones.

[60] In the lower doses used for water fluoridation, the only clear adverse effect is dental fluorosis, which can alter the appearance of children's teeth during tooth development; this is mostly mild and is unlikely to represent any real effect on aesthetic appearance or on public health.

[62] In areas that have naturally occurring high levels of fluoride in groundwater which is used for drinking water, both dental and skeletal fluorosis can be prevalent and severe.

Of this, about 10%, approximately 300 million people, obtain water from groundwater resources that are heavily contaminated with arsenic or fluoride.