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~ ~NTRODU=ION Early in this century, researchers observed that people with dental fluorosis, or mottled teeth, had a lower incidence of dental caries than people without dental fluorosis. Later, naturally occurring fluoride in drinking water was identified as being responsible for the reduction in dental caries. Community studies conducted in the 1940s showed an inverse relation between fluoride in drinking water and dental caries. Those findings led to the public-health practice of adding fluoride to fluoride-def~cient water. Since 1962, the "optimal" concentration of fluoride in drinking water for the United States has been set at 0.7-~.2 milligrams per liter (or parts per millions, clepending on the mean temperature of the locality (0.7 mg/L for areas with warm climates, where water consumption is high, and I.2 mg/L for coo] climates, where water consumption is low). That range has been considered optimal be- cause it provides a balance between prevention of dental caries and oc- currence of objectionable dental fluorosis. Objectionable dental fluorosis is a mottling of dental enamel characterized by staining or pitting. In addition to ingesting fluoride in drinking water, people now receive fluoride from a large number of other sources, such as toothpastes, mouth rinses, soft drinks, tea, processed foods, and vegetables. Fluoricie 'The fluoride level in drinking water can be expressed equivalently as parts per million (ppm) or milligrams per liter (mg/L). Fluoride concentrations in tissues such as bone can be expressed equivalently as parts per million or mil- ligrams per kilogram. 15
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16 Health Effects of Ingested Fluoride is also addec! when fluoridates! water is user} during cooking. Therefore, fluoride intake from sources other than drinking water can be substantial. Despite the apparent success of water fluoridation in reducing the incidence of dental caries, water fluoridation remains controversial. Some people believe that fluoride can cause severe dental fluorosis, which may lead to psychological problems. Assertions have been made that fluoride is related to skeletal fluorosis and subsequent fractures, cancer, soiPt-tissue effects, and arthritis. Critical analysis of toxicological data on fluoride has shown, however, that important adverse health effects, such as crippling skeletal fluorosis and kidney ant} other soft- tissue damage, occur in humans only when fluoride concentrations in drinking water exceed ~ mg/L for many years. The controversy esca- lated in 1975 when those opposed to fluoridation cIaimec] that cancer mortality was higher in areas with fluoricIatec! water than in areas with nonfluoridated water. Although that claim was refuter! subsequently by other investigators, the lingering concern over a possible association between fluoridation and cancer prompted the National Toxicology Pro- gram (NTP) of the U.S. Department of Health and Human Services to conduct a carcinogenicity bioassay to determine whether sodium fluoride is carcinogenic in mice and rats. The results of the NTP study were published in 1990 in a report entitled "Toxicology and Carcinogenesis Studies of Sodium Fluoride in F344/N Rats and B6C3F~ Mice (Drinking Water Studies)." The results of this study are that rounder the conditions of the 2-year dosed water studies, there was equivocal evidence of carcinogenic activity of sodium fluoride in male F344/N rats, based on the occurrence of a small number of osteosarcomas in dosed animals. 'Equivocal evidence' is a category for uncertain findings defined as studies that are interpreted as showing a marginal increase of neoplasms that may be related to chemical administration. There was no evidence of carcinogenic activity in female F344/N rats receiving sodium fluoride at concentrations of 25, 100, or 175 mg/L sodium fluoride in drinking water for 2 years. Inhere was no evidence of carcinogenic activity of sodium fluoride in male or female mice receiving sodium fluoride at concentrations of 25, 100, or 175 mg/L in drinking water for 2 years. Dosed rats had lesions typical of fluorosis of the teeth and female rats receiv- ing drinking water containing 175 mg/L sodium fluoride had increased osteo- sclerosis of long bones [NTP, 1990~.
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Introduction 17 The NTP findings have further stimulated the controversy regarding the safety of fluoride. In response to the controversy, especially regarding possible car- cinogenicity of fluoride, the U.S. Environmental Protection Agency (EPA) requested that the National Research Council's Board on Environ- mental Studies and Toxicology (BEST) review the toxicological and exposure data for fluoride and characterize the risk associated with ingested fluoride (e.g., from water and food) and determine whether EPA's current maximum contaminant level (MCL) of 4 mg/L of drinking water is acceptable to protect the public health. In addition, EPA re- quested identification of gaps in fluoride data ant} recommendations for future research. In response to EPA's request, this project was assigneci to BEST's Committee on Toxicology. The Subcommittee on Health Effects of Ingested Fluoride was establisher} to review the current fluo- ride toxicity data and drinking-water standarcis. This report is the result of a detailed evaluation of possible adverse health effects associated with ingested fluoride. The report examines in detail the current data on carcinogenicity of fluoride in humans and animals, dental fluorosis, skeletal effects, effects on the renal, gastro- intestinal, and immune systems, effects on reproduction, ant] genotoxicity from ingested fluoride. In addition, gaps in knowledge on fluoride toxicity are identified, and recommendations are made to minimize the risk to human health from fluoride ingestion. Chapter 2 discusses dental fluorosis. Chapter 3 reviews the relation of fluoride intake to bone strength, hip fractures, and skeletal fluorosis. Chapter 4 considers the reproductive toxicity of fluoride. Chapter 5 clis- cusses the effects of fluoride on the renal, gastrointestinal, and immune systems. Chapter 6 reviews the genotoxicity of fluoride. Chapter 7 pre- sents the results of carcinogenicity studies in humans and animals. Chapter X considers the pharmacokinetics of fluoride.
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