The fluoridation of drinking water often generates both strong support and opposition within communities. The practice is recommended by the U.S. Department of Health and Human Services (HHS) to prevent tooth decay. The decision to fluoridate a public water supply is made by the state or local municipality and is not mandated by any federal agency. Opponents have expressed concern regarding potential adverse health effects of exposure to fluoride, and some view the practice as an undemocratic infringement on individual freedom. The medical and public health communities generally have supported water fluoridation, citing it as a safe, effective, and equitable way to provide dental health protection community-wide. With the increased use of products containing fluoride, such as toothpaste and rinses, questions have emerged as to whether current fluoridation practices and levels are necessary and offer the most appropriate way to provide the beneficial effects of fluoride while avoiding adverse effects (such as tooth mottling or dental fluorosis) that can result from exposure to too much fluoride. Moreover, research gaps regarding the potential health effects of exposure to increased amounts of fluoride and among different age groups continue to add controversy to decisions regarding water fluoridation. Although many communities add fluoride to drinking water to strengthen teeth, others must treat their water to remove excess amounts of fluoride, which often is present naturally in water. The Environmental Protection Agency (EPA) regulates the maximum amount of fluoride that may be present in public drinking water supplies to protect against certain adverse health effects. In 1986, EPA issued a drinking water regulation for fluoride that includes an enforceable standard (a maximum contaminant level, or MCL) and a non-enforceable health-based maximum contaminant level goal (MCLG) of 4 milligrams per liter (mg/L) to protect against adverse effects on bone structure. EPA acknowledged that the standard did not protect infants and young children against dental fluorosis, which EPA considered a cosmetic effect rather than a health effect. To address concerns, EPA included in the regulation a secondary (advisory) standard of 2 mg/L to protect children against dental fluorosis and adverse health effects. As part of its ongoing review of the fluoride regulation, EPA asked the National Research Council (NRC) of the National Academy of Sciences to review the health risk data for fluoride and to assess the adequacy of EPA's standards. On March 22, 2006, NRC released its study and concluded that EPA's 4 mg/L MCLG should be lowered. This report discusses the potential benefits and adverse effects associated with the fluoridation of drinking water supplies. It also discusses the regulation of fluoride in drinking water to protect against adverse health effects from exposure to higher levels of fluoride, and it reviews the status of federal efforts to update the health risk assessment for fluoride and the primary drinking water standard for fluoride. The following review of issues related to fluoride in drinking water presents information from research published in peer-reviewed scientific journals, reports, and statements of federal agencies—including EPA and the Centers for Disease Control and Prevention (CDC) and the U.S. Public Health Service (PHS) within the Department of Health and Human Services (HHS)—and the World Health Organization, studies by the National Research Council, and other sources. Fluoride is a naturally occurring substance and is present in virtually all water, usually at very low levels. Higher concentrations of naturally occurring fluoride often are associated with well water, where fluoride has dissolved from the rock formations into the groundwater. Community water fluoridation began in 1945, after scientists discovered that higher natural levels of fluoride in a community water supply were associated with fewer dental caries (cavities) among the residents. In 2010, CDC reported that more than 204.2 million (73.9%) of the people in the United States who received their water from public water systems received fluoridated water. Of the total U.S. population, 66.2% received fluoridated water. In 2004, more than 170 million (67%) people served by public water systems received fluoridated water. Many public health agencies and professional health organizations have advocated the addition of a small amount of fluoride to drinking water to help strengthen teeth and prevent dental caries. Although this practice has been controversial in various communities, CDC, the American Medical Association, the American Dental Association (ADA), the American Academy of Pediatric Dentistry, and others have recommended fluoridation of public water supplies as an effective way to protect dental health. This approach has been advocated for its ability to provide community-wide benefits, particularly in poorer communities where children may be less likely to receive adequate dental care. CDC considers the reduction in tooth decay from fluoridation one of the top public health achievements of the 20 th Century. In 2002, CDC reported that "During the second half of the 20 th century, a major decline in the prevalence and severity of dental caries resulted from the identification of fluoride as an effective method of preventing caries. Fluoridation of the public water supply is the most equitable, cost-effective, and cost-saving method of delivering fluoride to the community." One of CDC's national health goals is to increase the proportion of the U.S. population served by community water systems with "optimally" fluoridated drinking water to 79.6% by 2020. The optimal fluoridation level that has long been recommended by PHS for decay prevention is in the range of 0.7 to 1.2 milligrams per liter (mg/L). The World Health Organization (WHO) has identified dental caries (cavities) as a worldwide epidemic and recommends adding fluoride to drinking water where naturally occurring levels of fluoride are below optimal levels. The WHO states that the goal of community-based public health programs "should be to implement the most appropriate means of maintaining a constant low level of fluoride in as many mouths as possible." According to the WHO, [w]ater fluoridation in low fluoride-containing water supplies helps to maintain optimal dental tissue development and dental enamel resistance against caries attack during the entire life span.... People of all ages, including the elderly, benefit from community water fluoridation. For example, the prevalence of caries on root surfaces of teeth is inversely related to fluoride levels in the drinking water: in other words, within the non-toxic range for fluoride, the higher the level of fluoride in water, the lower the level of dental decay. This finding is important because with increasing tooth retention and an aging population, the prevalence of dental root caries would be expected to be higher in the absence of fluoridation. The recommended beneficial amount of fluoride can be obtained from a variety of sources other than water (e.g., fluoride toothpastes, rinses, and supplements). However, health officials historically have recommended fluoridation of community water supplies, citing socioeconomic reasons that may vary among countries and communities. The WHO explains this preference as follows: The consensus among dental experts is that fluoridation is the single most important intervention to reduce dental caries, not least because water is an essential part of the diet for everyone in the community, regardless of their motivation to maintain oral hygiene or their willingness to attend or pay for dental treatment. In some developed countries, the health and economic benefits of fluoridation may be small, but particularly important in deprived areas, where water fluoridation may be a key factor in reducing inequalities in dental health. Despite such recommendations, fluoridation remains far from universally practiced. Worldwide, an estimated 370 million people receive artificially fluoridated water, and another 50 million drink water that is naturally fluoridated at or near the optimal level. Overall, some 30 countries practice water fluoridation, and the percentage of populations receiving artificially fluoridated water varies greatly. Countries where fluoridation is practiced (and the percentage of their populations receiving fluoridated water) include Argentina (21%), Australia (61%), Brazil (41%), Canada (43%), Chile (40%), Colombia (80%), Israel (75%), Malaysia (70%), New Zealand (61%), and Singapore (100%). Of the Western European countries, the Republic of Ireland (73%), Spain (10%), and the United Kingdom (10%) fluoridate drinking water. Most other Western European countries have ceased, or never practiced, water fluoridation for various reasons, including the availability of other sources of fluoride (especially toothpaste), the availability of free school-based dental care programs in some countries, broader public skepticism about the safety and efficacy of fluoridation, and greater political opposition. In several Latin American countries, where centralized water supplies are often lacking, fluoridated salt is the chosen method of providing dental protection across disparate communities. Fluoridated salt also is available in some European countries, including Austria, France, Germany, Hungary, and Switzerland. Water fluoridation has generated less opposition in the United States than in Europe. However, notwithstanding recommendations from many governmental and professional health organizations, this practice continues to generate controversy in some U.S. communities. Research gaps regarding the effects of long-term exposure to increased levels of fluoride fuel this debate, and decades into this practice, the safety and efficacy of water fluoridation continues to be questioned, debated, and studied. Some oppose water fluoridation because of a concern that even recommended "optimal" levels of fluoridation may cause some dental fluorosis in children. Dental fluorosis is caused by excessive fluoride intake while teeth are developing, and it is during this period before teeth erupt that dental tissues are very sensitive to fluoride (typically during a child's first eight years). Mild dental fluorosis is characterized by opaque white or stained patches in the dental enamel. More severe fluorosis is characterized by pitting of tooth enamel. In the 1960s, when PHS recommended an "optimal" fluoride concentration in water of 0.7 to 1.2 mg/L, this level was intended to "maximize prevention of caries while limiting the prevalence of dental fluorosis to about 10% of the population, virtually all of it mild to very mild."