The element fluorine has long been recognised to have benefits for dental health: low-fluoride intake has been linked to development of dental caries and the use of fluoride toothpastes and mouthwashes is widely advocated in mitigating dental health problems. Fluoridation of water supplies to augment naturally low fluoride concentrations is also undertaken in some countries. However, despite the benefits, optimal doses of fluoride appear to fall within a narrow range. The detrimental effects of ingestion of excessive doses of fluoride are also well documented. Chronic ingestion of high doses has been linked to the development of dental fluorosis, and in extreme cases, skeletal fluorosis. High doses have also been linked to cancer (Marshall, 1990), although the association is not well-established (Hamilton, 1992).
Drinking water is particularly sensitive in this respect because large variations in fluoride concentration exist in water supplies in different areas. Concentrations in natural waters span more than four orders of magnitude, although values typically lie in the 0.1–10 mg/ L range. Where concentrations are high, drinking water can constitute the dominant source of fluorine in the human diet. Concentrations in drinking water of around 1 mg/ L are often taken to be optimal. However, chronic use of drinking water with concentrations above about 1.5 mg/ L is considered to be detrimental to health. The WHO (2011) guideline value for fluoride in drinking water remains at 1.5 mg/ L. Many countries also use this value as a national standard for drinking water, although the standard in China is 1.0 mg/ L (Table 1). The US Environmental Protection Agency (EPA) has set the primary standard (enforceable limit) at 4 mg/ L for fluoride in drinking water, although the secondary standard (non-enforceable) for United States drinking water is 2 mg/ L. In Tanzania, the national standard is as high as 8 mg/ L, reflecting the difficulties with compliance in a country with regionally high fluoride concentrations and problems with water scarcity.
High fluoride concentrations are most often associated with groundwaters as these accumulate fluoride from rock dissolution as well as geothermal sources. Many high-fluoride groundwater provinces have been recognised in various parts of the world, particularly northern China, India, Sri Lanka, Mexico, western USA, Argentina and many countries in Africa. Fluoride removal by water treatment is carried out in some countries. However, as many of the high-groundwater provinces occur in developing countries, fluoride removal practices vary widely and many high-fluoridewater sources are used without treatment. As a result, large populations throughout parts of the developing world suffer the effects of chronic endemic fluorosis. Estimates are not well-established, but more than 200 million people worldwide are thought to be drinking water with fluoride in excess of the WHO guideline value. This includes around 66 million in India (Majumdar, 2011), 45 million people in China (Wuyi et al., 2002) and some 5 million in Mexico (Diaz-Barriga et al., 1997). The population at risk in Africa is unknown but is also likely to be tens of millions.
Despite the clear evidence for health problems related to fluoride in drinking water and the links between fluoride occurrence and geology, there have been few reviews on the hydrogeochemistry of fluoride. This chapter addresses the hydrogeochemical aspects of fluoride in water, particularly groundwater, and outlines the links with health impacts. The chapter characterises the typical ranges of concentrations of fluoride found in water bodies, along with their distribution, speciation and mechanisms of mobilisation. The principles of fluoride behaviour are also illustrated with case studies from Canada, East Africa, Ghana, India, Sri Lanka and the UK.
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CITE: “Fluoride in Natural Waters in Essentials of Medical Geology, Impacts of Natural Environment on Public Health”