Fluoride

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For the addition of fluoride ions to water supplies, see Water fluoridation.

Overview

Fluoride is the ionic form of fluorine. Fluorides are organic and inorganic compounds containing the element fluorine. As a halogen, fluorine forms a monovalent ion (−1 charge). Fluoride forms a binary compound with another element or radical. Examples of fluoride compounds include hydrofluoric acid (HF), sodium fluoride (NaF) and calcium fluoride (CaF₂), and uranium hexafluoride (UF₆).

Occurrence

Fluoride compounds, usually calcium fluoride, are naturally found in low concentration in drinking water and some foods, such as tea, seaweed, and fish bones (as in fish soup).

Water with underground sources are more likely to have higher levels of fluoride, while the total concentration in seawater has an average concentration of 1.3 parts per million(ppm). Fresh water supplies generally contain between 0.01-0.3 ppm, while the ocean contains between 1.2 and 1.5 ppm.

Applications

Hydrofluoric acid is used in the etching of glass and other industrial applications, including integrated circuit manufacturing.

Fluoride, as a concentrated gel, foam, or varnish, is used as a prescription drug. Fluorine is also part of certain drug molecules to resist detoxification in the liver by the Cytochrome P450 oxidase because the strong C-F bonds are not easily broken. This is to ensure that orally administered medication are not inactivated before reaching the blood stream.

Fluoride ion has a very significant use in synthetic organic chemistry. The silicon-fluorine chemical bond is quite strong. Silyl ether protecting groups can be easily removed by the addition of fluoride ion. Sodium fluoride or tetra-n-butylammonium fluoride (TBAF) are the most common reagents used.

In biochemistry, fluoride salts are commonly used to inhibit the activity of serine/threonine phosphatases.

Sulfur hexafluoride is a nearly-inert, non-toxic propellant. Uranium hexafluoride is used in the separation of isotopes of uranium between the fissile isotope U-235 and the non-fissile isotope U-238 in preparation of nuclear reactor fuel and atomic bombs.

Water fluoridation

Main article: Water fluoridation

Fluoride containing compounds such as sodium fluoride, calcium fluoride, and sodium monofluorophosphate are commonly added to toothpaste, drinking water, prescribed treatments, and other commercially available oral hygiene products because fluoride increases the resistance of the enamel to decay. Originally, sodium fluoride was used to fluoridate water; however, hexafluorosilicic acid (H₂SiF₆) and its salt sodium hexafluorosilicate (Na₂SiF₆) are more commonly used, especially in the United States.

Some studies suggest that fluoridation is associated with a median decline in the number of children with cavities of 12.5%, and a median decline of 2.25 teeth with cavities. The fluoridation of water is not without critics, however (see Water fluoridation controversy).

Salt fluoridation

In countries where large, centralized water systems are uncommon, salt fluoridation is sometimes used as an alternative to water fluoridation. In countries where salt fluoridation is common, such as Switzerland, France, and Jamaica, among many others, caries experience has also decreased in groups using fluoridated salt compared to groups using salt without fluoride.

Toxicology

Acute

In high concentrations, fluoride compounds are toxic and can cause death. In mice, the LD₅₀ is estimated to be 184 milligrams of stannous fluoride per kilogram of body mass. An individual report involving fatality following the accidental administration of fluoride ion to a child at 5 mg/kg was cited by G. M. Whitford in 1987. While experimenting on himself in 1899, Herbert H Baldwin reported that symptoms of acute toxicity (e.g. gastrointestinal upset) occurred at doses as low as 0.1-0.3 mg/kg.

When ingested directly, fluoride compounds are readily absorbed by the intestines. Over time, the compound is excreted through the urine, and the half life for concentration of fluorine compounds is on an order of hours. It is thought that fluoride is taken out of circulation by the body and trace amounts become bound in bone. Urine tests are a good indication of high exposure to fluoride compounds in the recent past.

Skin or eye contact with many fluoride compounds in high concentrations is dangerous. In case of accidental swallowing, milk, calcium carbonate, or milk of magnesia is given to slow absorption. Eye or skin contact is treated by removing any contaminated clothing and flushing with water.

Chronic

Fluoride ions replace hydroxide ions in calcium hydroxyapatite, Ca₅[(PO₄)₃OH], in teeth, forming calcium fluoroapatite, Ca₅[(PO₄)₃F], which is more chemically stable and dissolves at a pH of 4.5, compared to 5.5 pH for calcium hydroxyapatite. This is generally believed to lead to fewer cavities, since stronger acids are needed to attack the tooth enamel. In 1951, Joseph C. Muhler and Harry G. Day of Indiana University reported their research results on stannous fluoride as a tooth decay preventive and the university first sold the technology to Procter & Gamble to use in Crest toothpaste. Groups that have evaluated available studies and support water fluoridation include The American Dental Association (ADA), World Health Organization (WHO), and some other health organizations which recommend raising the fluoride level of municipal water supplies to a level between 0.7 and 1.2 ppm.

The most widely accepted adverse effect of excessive fluorine intake at this time is fluorosis. It is a condition caused by 'excessive' intake of fluorine compounds over an extended period of time, and can cause yellowing of teeth. The definition of 'excessive' in the context of fluorosis falls on the order of parts per million and is generally accepted to mean significantly higher than the 0.7 to 1.2 ppm amounts recommended for fluoridated water. However, some believe that any water fluoridation is detrimental to human health. Dosage is crucial to adverse effects, and therefore, what concentration is problematic will depend on the amount of fluoride ingested, how much is absorbed, and the weight of the person ingesting it. The ADA currently recommends using low- or non-fluoridated water to make up formula for infants.

Contested claims

Main article: Water fluoridation controversy

Some opponents of fluoridation have expressed concern that fluoride damages body function. One of the key concerns is that fluoride can weaken bone strength, leading to an increase in hip and wrist fracture. Additional concerns of fluoridation opponents include the potential for fluoride to damage the brain, reduce thyroid function, and cause bone cancer in adolescent boys. While a recent review from the US National Research Council supports concerns that fluoride may cause some of these effects, at least at high doses, more than 100 national and international health service agencies and professional organizations continue to accept that there are benefits of community water fluoridation in preventing tooth decay.

See also

• Chloride
• Dental fluorosis
• Fluoride deficiency
• Fluoride poisoning
• Fluoride therapy to prevent tooth decay
• Halide
• PTFE