Fluoride is the reduced form of fluorine, which has no known biological essentiality in man. It can occur as fluoroacetate, the fluorinated calcium phosphate common in bedrock silicates. Depending on the rock's mineral content as on well as the water temperature, rainwater filtering through these strata can dissolve fluoroacetate and result in unsafe levels of naturally occurring fluoride in aquifers used for drinking water supplies . However, the majority of ground water reserves do not contain high levels of naturally occurring fluoride.
Ingested fluoride is sequestered into bone as well as teeth  and the long-term effects on bone of low-level exposure to fluoride are unknown. A study in Finland that examined hip fractures in elderly women who drank well water with high concentrations of fluoride found that higher fluoride levels were associated with increased risk for hip fractures in women aged 50-64 years . The condition of skeletal fluorosis, characterized by joint pain, dense bone and limited joint movement, has been reported as occurring in individuals drinking water with a fluoride content some thirty times the level recommended for fluoridation; this condition is more common in subjects suffering from nutritional deficiencies. A WHO working group concluded that skeletal fluorosis and an increased risk of bone fractures occur at a total intake of 14 mg fluoride per day, with evidence suggestive of an increased risk of bone effects at intakes above about 6 mg fluoride per day .
With drinking water fluoride levels above 5 ppm, fluoride can also cause the development of brittle bone disease. Some studies have also found a higher risk of simple bone fractures in older men and women drinking water with fluoride levels around 1.0 ppm, the level recommended for fluoridation . Despite forceful argument to the contrary, the case for water fluoridation is by no means clear  and fluoride ingestion at these low levels has been blamed for a variety of conditions, including many types of cancer (e.g. bladder cancer)  and impaired intelligence in children . In addition, mild fluorosis (hypomineralization of the dental enamel) is found in more than one quarter of US children . In communities with high natural fluoride levels in drinking water, the prevalence of dental fluorosis is directly related to the level of fluoride in the water source .
Fluoride in urine should be measured in subjects with unexplained bone disease, including a tendency to fracture. It may also be of importance in a range of other conditions, including abnormalities of thyroid function. Dental fluorosis in children may be caused by excessive fluoride ingestion. Fluoride in drinking water should be measured to help identify the potential source of excessive fluoride ingestion.
1. British Fluoridation Society. Technical and scientific aspects of water fluoridation. http://www.bfsweb.org/One%20in%20a%20million/6%20technical.pdf. Accessed 2012.
2. WHO Expert Committee on Oral Health Status and Fluoride Use. Fluorides and oral health. WHO technical report series 846,1994. http://www.whqlibdoc.who.int/trs/WHO_TRS_846.pdf.
3. US Department of Health and Human Services. Proposed HHS recommendation for fluoride concentration in drinking water for prevention of dental caries, January 2011. http://www.hhs.gov/news/press/2011pres/01/pre_pub_frn_fluoride.html. Accessed 2012.
4. DeSimone LA, Hamilton PA, Gilliom RJ. Quality of water from domestic wells in the United States. United States Geological Survey. 1 June 2009. http://water.usgs.gov/nawqa/studies/domestic_wells/ Accessed 2012.
5. Chachra D, Limeback H, Willett TL, Grynpas MD. The long-term effects of water fluoridation on the human skeleton. J Dent Res. 2010;89:1219-1223.
6. Kurttio P, Gustavsson N, Vartiainen T, Pekkanen J, Exposure to natural fluoride in well water and hip fracture: a cohort analysis in Finland. Am J Epidemiol. 1999;150:817-824.
7. WHO. Fluoride.http://www.who.int/water_sanitation_health/GDWQ/draftchemicals/fluoride2003.pdf. Geneva: 2003.
8. Chachra D, Viera A, Grynpas MD. Fluoride and mineralized tissues. Crit Rev Biomed Eng. 2008;36:183-223.
9. Cheng KK, Chalmers I, Sheldon TA. Adding fluoride to water supplies. BMJ 2007;335:699-703.
10. Yang CY, Cheng MF, Tsai SS, Hung CF. Fluoride in drinking water and cancer mortality in Taiwan. Environ Res 2000;82:189-93.
11. Lu Y, Sun ZR, Wu LN, Wang X, Lu W, Liu SS. Effect of high-fluoride water on intelligence in children. Fluoride. 2000;33:74-78.
12. Beltrán-Aguilar ED, Barker LK, Canto MT, et al; Centers for Disease Control and Prevention (CDC). Surveillance for dental caries, dental sealants, tooth retention, edentulism, and enamel fluorosis-- United States, 1988-1994 and 1999-2002. MMWR Morb Mortal Wkly Rep. 2005;54:1-44.
13. Beltrán-Aguilar ED, Griffin SO, Lockwood SA. Prevalence and trends in enamel fluorosis in the United States from the 1930s to the 1980s. J Am Dent Assoc. 2002;133:157-165.