Geochemical and Health Policy Dimensions of the Controversy about Fluorides in Water Resources

Abstract

Fluorine has natural and anthropogenic sources. Its presence in water resources and its addition to drinking water as fluoride to aid dental health have generated scientific controversies and policy disputes in many parts of the world during the past 15 years. The concentration of fluorine in the earth’s crust is about 0.3 g/kg. It is a constituent of rocks ~especially, volcanic rocks!, soils, and living matter. By virtue of its position in the periodic table of chemical elements, it is highly electronegative. Consequently, it tends to react with other substances under prevailing environmental conditions to form fluorides. Fluorides enter surface water and groundwater systems through dissolution and leaching of mineral deposits and rock formations; precipitation and flooding events; and release of inadequately treated or untreated wastes into the environment. The near-surface environment in which country rocks exist is dynamic and promotes the development of physicochemical interactions that have produced rocks that vary in geochemistry from one region to another. Globally, the reported levels of fluoride in groundwater are in the range of 1–35 mg/L. At a specific location, water that is taken from wells that penetrate fluoride-bearing bedrock is likely to have greater concentrations of fluoride than water from wells that terminate in the soil profile. Industrial operations conducted within the last 60 years constitute the main source of anthropogenic fluoride in groundwater. Fluoride is released during phosphate processing for fertilizers, aluminum smelting operations, and production processes of glass, bricks, and steel. In the United States and other technologically developed countries, the deployment of innovative ~and less polluting! technologies, voluntary environmental stewardship by industrial organizations, and enforcement of industrial effluent guidelines by local, regional, and national agencies have made it rare for fluoride concentrations to exceed 3 mg/L in groundwater. For example, in Nebraska, the reported range is 0.1–2.6 mg/L ~Headrick 1996!. In the State of New Hampshire, less than 1.0% of bedrock wells produce water with fluoride concentrations in excess of 4.0 mg/L ~NHDES 2002!. As a continuation of the practice that started in Grand Rapids, Michigan, on January 25, 1945, fluoride is added to drinking water by most municipalities in the United States. The known benefit is that it reduces tooth decay when it is taken at the appropriate dose. Daily consumption of water that has a fluoride concentration of 4 mg/L does not present a health risk. The World Health Organization recommends a fluoride content of 1.5 mg/L in drinking water. In general, in the United States, the optimum fluoride content in drinking water is considered to be 0.7–1.2 mg/L. It is also known that daily drinking of water with fluoride concentrations above 2.0 mg/L can cause dental fluorosis. Fluo-