Fluoride, a negatively charged ion (anionic) form of the halide element fluorine, is the 17th most abundant element in the Earth's crust. This stable form of fluorine binds with the positively charged calcium ion (cation) to form hard minerals resistant to weathering; such fluorapatite makes teeth and bones stronger and less likely to break. Most calcium and fluoride-based minerals are insoluble in water. (They don't dissolve in water.) Thus, naturally occurring concentrations of fluoride in water are typically less than 1 mg/L (part per million). However, hot spring waters with alkaline pH and low-dissolved calcium have exhibited concentrations as high as 32 mg/L. Most surface waters that range from mildly alkaline to acidic pH values usually have enough calcium to keep free fluoride concentrations in check. It is unusual, therefore, to find fluoride concentrations in rivers or oceans above 1 mg/L. Volcanic rock, ash, and ash interbedded in sedimentary rock, account for the most prevalent sources of fluoride in groundwater. An unusual source of fluoride in groundwater was reported in a coastal aquifer in South Carolina where fossil shark’s teeth were dissolving within the aquifer, releasing fluoride to the groundwater.
In addition to naturally occurring fluoride, some communities add fluoride to the public water supply, a process known as fluoridation. Nationally, about two-thirds of Americans served by public water systems receive fluoridated water, according to the American Dental Association.
The levels of fluoride commonly found in drinking water cannot be detected by taste, sight or smell. Testing is the only way to determine the fluoride concentration.
As with many substances, potential health effects are directly related to the concentration present. The U.S. Public Health Service, Centers for Disease Control and Prevention and the American Dental Association recommend an optimum level of fluoride in drinking water of 1.0 mg/l to ensure potential benefits while minimizing or eliminating potential risks.
At optimum levels, fluoride protects against tooth decay. The American Dental Association has stated that fluoride benefits people of all ages. When children are young and their teeth are forming, fluoride makes tooth enamel harder and more resistant to decay-causing acid. Studies indicate that people who drink optimally fluoridated water from birth will experience up to 40 percent less decay over their lifetimes. For adults, fluoride helps repair the early stages of tooth decay even before it becomes visible, a process known as remineralization. For older adults, fluoride can decrease problems with root caries (decay along the gumline).
Fluoride in drinking water above optimum levels may produce fluorosis (mottling of teeth). Dental fluorosis appears during tooth formation. The effects can be mild to severe, ranging from barely perceptible white striations or specks on teeth to permanent brown to brownish gray stains on teeth and/or severe pitting. The National Research Council (NRC) conducted a scientific review of research on various health effects, including dental effects, from consuming water with fluoride at the maximum level allowed in public water supplies (four times greater than the recommended optimum level.) They reaffirmed that continued consumption of water containing increasingly higher concentrations of fluoride above optimum levels will generally produce more severe dental fluorosis. While the effect of mild to moderate fluorosis is believed to be cosmetic only, 10 of the 12 committee members concluded that enamel loss and pitting associated with severe fluorosis may increase the risk of tooth decay and infection.
Ingesting excessive quantities of fluoride can result in acute toxicity. Consuming 300 to 750 milligrams (depending on body weight) in a single dose can produce nausea or vomiting. At the optimum fluoridation level of 1.0 mg/l, an individual would need to ingest 80 to 200 gallons of water in a few hours to reach the acute toxicity level, an amount impossible to drink in that time period.
The possibility of chronic health effects from continuous consumption of drinking water with fluoride above optimum levels has also been studied. The NRC scientific review included research on possible musculoskeletal, reproductive and developmental, neurotoxicity and neurobehavioral, endocrine, and carcinogenicity effects from exposure to fluoride in drinking water at the maximum level allowed in public water supplies (four times greater than the recommended optimum level.) The committee concluded that scientific evidence suggested adverse effects were unlikely, or that evidence is tentative and mixed, for adverse health effects studied, with the exception of bone fractures. While the report’s authoring committee was not in total agreement, the majority concluded that chronic health effects included a likely increase for bone fractures, particularly in those prone to accumulate fluoride into their bones. The level of risk was not quantified.
Fluoride guidelines for dental health benefits were established by the U.S. Public Health Service in the 1960s. The recommended level of fluoride in drinking water for dental benefits is 0.7 to 1.2 mg/l.
The quality of water supplied by public water systems is regulated by U.S. Environmental Protection Agency (EPA) under the federal Safe Drinking Water Act. EPA standards restrict the total amount of a substance allowed in drinking water and are designed to prevent undesirable effects that could result from exposure to a substance at concentrations above those allowed. Drinking water standards fall into two categories: secondary standards and primary standards. Secondary standards are based on aesthetic factors such as taste, odor, color, corrosivity, foaming and staining properties of water that may affect water’s suitability for drinking and other domestic uses. Primary standards are based on health considerations and are designed to protect human health.
The EPA established a recommended secondary standard and an enforceable primary standard for fluoride in 1986. The Secondary Maximum Contaminant Level (SMCL) for fluoride is 2.0 mg/l, which is equal to 2.0 parts per million (ppm). Water with a fluoride concentration at or below 2.0 mg/l does not present a health risk and should not cause appreciable fluorosis. The Primary Maximum Contaminant Level (MCL) for fluoride in drinking water is 4.0 mg/l, which is equivalent to 4.0 ppm. Daily consumption of water with a fluoride concentration at 4.0 mg/l was not believed to present a health risk based on best available science at the time of the MCL enactment. A National Research Council report (2006) concluded that exposure at the current MCL puts children at risk for developing severe fluorosis that may compromise tooth function, and could put adults at increased risk of bone fracture, as bones can become brittle due to excessive levels of fluoride.
All water sources contain some naturally occurring fluoride. If users want to know the concentration of naturally occurring fluoride in a private water supply, they will need to have the water tested by a state-certified laboratory.
Some public water supplies add fluoride to achieve an optimal concentration for dental health. It is not practical to fluorinate private drinking water supplies. If the fluoride level is less than desired, bottled water may be a viable option. The U. S. Food and Drug Administration regulates bottled water and allows fluoride in bottled water either from naturally occurring sources or fluoridation. Therefore, some, but not all, bottled water may contain fluoride. Unfortunately, bottled water regulations in the U.S. do not require a detailed list of minerals on the label (except for sodium). Contact the manufacturer for information on fluoride concentration in the product selected.
If naturally occurring fluoride or the presence of fluoride through fluoridation is not desired, an alternative drinking water source or water treatment are viable options. Four treatment methods are suitable for reducing the levels of fluoride from drinking water, including activated alumina filters, distillation, reverse osmosis, and anion exchange.
Fluoride Facts; American Dental Association; 2005; (http://www.ada.org/public/topics/fluoride/facts/index/asp)
Oral Health in America: A Report of The Surgeon General; 2000; (http://www.nidcr.nih.gov/sgr/sgrohweb/home/htm)
Review of Fluoride: Benefits and Risks; U.S. Public Health Service: February 1991; Report of the Ad Hoc Subcommittee to Coordinate Environmental Health and Related Programs; Washington, D.C. (http://health.gov/environment/ReviewofFluoride/)
Fluoride in Drinking Water: A Scientific Review of EPA’s Standard; National Research Council; March 2006; (http://www.nap.edu)