Fluoride

This article is about the chemical ion F ⁻. For the addition of fluoride ions to water supplies, see Water fluoridation.

A fluoride ion 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 common fluoride compounds include hydrofluoric acid (H F), and sodium fluoride (Na F).

Examples

See category for a longer list.

Uses

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

Fluoride in a concentrated form is also a prescription medication.

Fluoride containing compounds are added to toothpaste, drinking water, prescribed treatments, and other commercially available oral hygiene products because they are believed to help strengthen the tooth enamel. Sodium fluoride and sodium monofluorophosphate (MFP, SMFP) are common additives.

Many local water municipalities fluoridate their water supplies by adding fluoride in concentrations of less than 4 ppm. Originally, sodium fluoride was used to fluoridate water; however, hexafluorosilicic acid (H₂SiF₆) and its salt (Na₂SiF₆) are more commonly used, especially in the United States.

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.

Toxicity and Health Concerns for Fluoride

Higher concentrations

In high concentrations, fluoride compounds are toxic and can cause death. In mice, a "semilethal dose" is estimated to be 184 milligrams of stannous fluoride per kilogram of body mass. Fatalities have been reported in some human individuals at doses as low as 5 mg/kg of fluoride ion while symptoms of acute toxicity (e.g. gastrointestinal upset) have been reported to occur 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. Implied is that fluoride is taken out of circulation by the body and trace amounts 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.

Low concentrations

Fluoride is best known for its use in small quantities in an effort to help reduce dental caries (cavity) frequency in teeth. Some studies suggests that fluoridation is associated with a median decline in the number of children with caries of 12.5%, and a median decline of 2.25 teeth with caries ^[1]

Fluoride compounds, usually calcium fluoride, are naturally found in low concentration in drinking water and some foods, such as tea. The ocean itself has an average concentration of 1.3 ppm (parts per million). 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.

The most widely accepted adverse effect of low concentration fluoridation at this time is fluorosis ^[2]. It is a condition caused by 'excessive' intake of fluorine compounds over an extended period of time, and can cause yellowing of teeth, hypothyroidism, or brittling of bones and 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, 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. For this reason, many doctors have advised against using fluoridated water to make up formula for infants. However, this may be more so due to the chemical interaction of fluoride with the calcium in the formula producing CaF₂ which will precipitate out of the body within the stool of the infant, thereby decreasing its absorption. People with kidney problems, or those on dialysis, are also advised not to ingest fluoridated water.

Other hypothesised adverse effects of low concentration fluoridation concern harms that are more systemic in nature.

For example, recent research found a significant correlation between degree of pineal gland calcification, and degree of fluoride deposition in this gland. Research by Jennifer Luke from the University of Surrey showed that "fluoride readily accumulates in the human pineal gland"[3][4] and that "the human pineal gland contains the highest concentration of fluoride in the body"[5].

Others believe these low concentrations are causally associated with widespread thyroid disorders ^{[citation needed]}. Some people believe that studies have shown that no adverse effects have occurred in an increase in Down's Syndrome^[6], bone fractures^[7]^[8], and cancer^[9].

References

^ McDonagh M S, Whiting P F, Wilson P M, Sutton A J, Chestnutt I, Cooper J, Misso K, Bradley M, Treasure E, & Kleijnen J. (2000) Systematic review of water fluoridation. British Medical Journal, 321(7265): 855-859.
^ Whiting P, McDonagh M, & Kleijnen J. (2001) Association of Down's syndrome and water fluoride level: a systematic review of the evidence

BMC Public Health 1: Art. No. 6.

^ Demos L L, Kazda H, Cicuttini F M, Sinclair M I, Fairley C K. (2001). Water fluoridation, osteoporosis, fractures - recent developments. Australian Dental Journal, 46 (2): 80-87.
^ Harrison, P T C. (2005) Fluoride in water: A UK perspective. Journal of Fluorine Chemistry, 126 (11-12): 1448-1456.