fluoride
What is fluoride, and how does it interact with other elements?
How is fluoride naturally produced, and where is it commonly found?
What are the biological effects of fluoride?
What are the benefits and controversies surrounding water fluoridation?
What are the effects of fluoride toxicity and deficiency?
fluoride, negatively charged ion (anion) of the chemical element fluorine that is characterized by its high reactivity and ability to form stable compounds with other elements, particularly metals. Because of its strong affinity for positively charged ions (cations), fluoride readily binds to calcium and other minerals, forming inorganic salts such as sodium fluoride (NaF) and calcium fluoride (CaF2, also known as fluorite or fluorspar). These compounds influence biological processes, especially the formation of tooth enamel and the mineralization of bone.
Fluoride has various applications. For example, it is commonly incorporated into toothpaste and mouthwash to improve dental care, and it is used in certain industrial processes, including the manufacture of glass and steel. Fluoride-based acids, such as hydrofluoric acid, are used in etching processes. Hydrogen fluoride is a key component in the making of fluorocarbons, which are used in aerosols and refrigerants.
Natural production and sources
Fluoride is produced naturally through the weathering and erosion of fluoride-containing minerals, including calcium fluoride, apatite, and cryolite, which are found in rocks and soil. As these minerals break down, fluoride is released into the environment. It is released into the atmosphere through volcanic activity and is commonly present in groundwater in concentrations varying widely by geographic region, from being nearly undetectable to exceeding 10 parts per million (ppm; equivalent to mg/L). Areas with naturally high levels of fluoride in groundwater include China, East Africa, India, parts of central Australia, and the western United States, where geologic features contribute to increased water fluoride concentrations.
Plants and animals absorb fluoride from the environment. Humans acquire fluoride mainly through drinking water, particularly fluoridated water, and through the use of dental products with fluoride. Fluoride is also ingested via foods that naturally contain fluoride, foods and beverages made with fluoridated water, and infant formulas (especially liquid concentrate and powder-based formulas).
Effects in biological systems
Fluoride is highly electronegative and is therefore very effective at pulling electrons away from other atoms, which results in the formation of ions. This is especially relevant in biological systems, where fluoride can disrupt hydrogen bonds and alter enzymatic activity, depending on its concentration and the types of cells and tissues with which it interacts.
Fluoride is attracted in particular to tissues that are calcium rich, such as teeth and bones. At the tooth surface, for instance, it facilitates the attachment of calcium and phosphate ions, promoting the formation of fluorapatite crystals, which help remineralize enamel. In addition, fluoride in saliva reduces the adhesion and growth of bacteria that contribute to tooth decay by blocking the activity of specific enzymes, thereby decreasing acid production and further aiding in remineralization. Fluoride affects bone structure by stimulating the proliferation of osteoblasts (cells responsible for bone synthesis and mineralization) and increasing bone density.
- Optimal for dental health: 0.7 ppm (mg/L); prevents tooth decay.
- Secondary standard (nonenforceable guideline): 2.0 ppm; routine exposure may cause cosmetic effects.
- Upper safe limit (according to the U.S. Environmental Protection Agency): 4.0 ppm; routine exposure can lead to bone disease and, in children, mottled teeth.
- Dental fluorosis: >1.5 ppm; chronic exposure produces white streaks or dark spots on teeth.
- Skeletal fluorosis: 8–10 ppm; chronic exposure (10 years or longer) manifests as abnormal bone development and increased bone density.
- Acute toxicity: 5–8 mg/kg; a single dose can produce symptoms of fluoride toxicity, including gastrointestinal distress and paresthesia (commonly described as a “pins and needles” sensation).
- Lethal dose: 32–64 mg/kg.
A lack of fluoride can have a significant impact on oral health, increasing the risk of tooth decay (particularly in children). Thus, in areas with low fluoride concentrations in groundwater, small amounts of fluoride may be added to community water supplies to help optimize dental health, which is achievable at 0.7 ppm (mg/L). In areas where fluoride levels in groundwater are high, individuals who are chronically exposed are at increased risk of dental fluorosis and, at exceptionally high concentrations, skeletal fluorosis. Fluoride can be removed from water by using such processes as distillation, ion exchange, or reverse osmosis filtration.
Water fluoridation: benefits and controversy
Community water fluoridation was initiated in 1945 in the United States and was subsequently introduced in other countries worldwide. It is considered one of the great achievements in modern public health; indeed, research has shown that water fluoridation leads to a 25 percent reduction in dental caries in children and adults. Health organizations such as the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) play a key role in supporting fluoridation, especially in communities where access to dental care is limited.
Nonetheless, the potential risks associated with water fluoridation and concerns about the effects of fluoride on aspects of physical development and bone health are sources of controversy. Opponents of mass fluoridation argue that it may lead to overexposure, increasing the risk of such conditions as dental and skeletal fluorosis. Some also argue that it may affect the function of the thyroid gland and neurological health, though evidence for these claims remains limited or inconclusive. Critics also raise ethical objections, asserting that adding fluoride to drinking water infringes on personal freedoms—for example, by forcing individuals to purchase bottled water in order to avoid the consumption of fluoridated water.