CHAPTER 15 WATER FLUORIDATION Thomas G Reeves, P.E National Fluoridation Engineer U.S Public Health Service Centers for Disease Control and Prevention Atlanta, Georgia Fluoridation of public water supplies has been practiced since 1945 Few public health measures have been accorded greater clinical and laboratory research, epidemiological study, clinical trials, and public attention than water fluoridation This chapter will present an overview of the history of fluoridation, as well as the public health and engineering aspects Fluoridation is the deliberate adjustment of the fluoride concentration of a public water supply in accordance with scientific and medical guidelines Fluoride, a natural trace element, is present in small but widely varying amounts in practically all soils, water supplies, plants, and animals, and is a normal constituent of all diets (Hodges and Smith, 1965) The highest concentrations in mammals are found in bones and teeth Virtually all public water supplies in the United States contain at least trace amounts of fluoride from natural sources HISTORY The study of the relationship between fluoride in drinking water and dental health has an interesting and intriguing history The series of studies that led to a demonstration that fluoridated water had caries-inhibiting properties was one of the most extensive programs carried out in the epidemiology of chronic disease It began in 1901, when a U.S Public Health Service (USPHS) physician stationed in Naples, Italy wrote that black teeth observed in emigrants from a nearby region were popularly believed to have been caused by using water charged with volcanic fumes It was later determined that the water supply contained an extremely high amount of fluoride and that everyone drinking it was afflicted with discolored (or “mottled”) teeth, a condition referred to as dental fluorosis In its mildest form, dental fluorosis is characterized by very slight opaque, whitish areas on some posterior teeth As the defect becomes more severe, discoloration is more widespread, and changes in color range from shades of gray to black In the 15.1 15.2 CHAPTER FIFTEEN most severe cases, gross calcification defects occur, resulting in pitting of the enamel In some of the latter cases, teeth are subject to such severe attrition that they wear down to the gum line, and complete dentures must be obtained In 1916 Dr Frederick S McKay, a practicing dentist, reported that many of his patients in Colorado Springs, Colorado had this defect (McClure, 1970) After further study, McKay concluded that the condition was caused by an undetermined substance in the drinking water McKay recommended that the water supply of Oakley, Idaho be changed because of the high incidence of such dental defects among the children there The supply was changed in 1925 to a nearby spring that had been used by a few other children whose teeth were not discolored The cause of dental fluorosis was discovered almost simultaneously in the late 1920s by two different groups of scientists working independently with different tools and methods A W Petrey, a chemist and head of the testing division of the laboratory at the Aluminum Company of America at Pittsburgh, noticed the calcium fluoride band in a spectroscopic examination for aluminum in a water sample from Bauxite, Arkansas The chief chemist of these laboratories, H V Churchill, reported in 1931 that similar examinations of water samples from areas where dental fluorosis was endemic invariably showed the presence of fluoride (McNeil, 1957) In 1931 a paper by Churchill describing the relationship between fluoride and dental fluorosis appeared in the Journal of the American Water Works Association (Churchill, 1931) Churchill reported that endemic regions of mottling had waters containing mg/L or more fluoride, while areas without mottling had water supplies with less than 1.0 mg/L Also in 1931, H V Smith, M C Smith, and E M Lantz at the University of Arizona helped establish the cause of mottling by duplicating the condition in rats by feeding concentrated, naturally fluoridated water and comparing the results with the mottling observed when a diet high in fluorides was used (McNeil, 1957) The Smiths also confirmed Churchill’s findings by reporting that water sources from areas with no endemic mottling contained less than 0.72 mg/L fluoride (Smith et al., 1931) The work by Churchill and the University of Arizona investigators was followed by epidemiological studies by H T Dean of the USPHS Dean confirmed that many localities have water supplies containing fluoride Areas with the largest number of such supplies containing the highest levels of fluorides include those states running from North Dakota to Texas, those along the Mexican border, and Illinois, Indiana, Ohio, and Virginia Similar supplies have been found in the British West Indies, China, Holland, Italy, Mexico, North Africa, South America, Spain, and India Through observation of thousands of children in communities with varying fluoride levels, Dean developed what he termed a mottled enamel index—a numerical method for measuring the severity of fluorosis (Dean et al., 1936) Using this index, Dean established the fluoride level below which the use of such water contributed no significant discoloration This level in the latitude of Chicago was about 1.0 mg/L Many investigators, including McKay, observed during the 1920s that less decay occurred in children whose teeth were afflicted with mottling To confirm this, Dean examined 7257 children in 21 cities with water supplies containing varying fluoride levels Results of this study, some of which are shown in Figure 15.1, revealed a remarkable relationship between waterborne fluorides, fluorosis, and caries incidence (McClure, 1943) Three conclusions were drawn from Dean’s study: When the fluoride concentration exceeds about 1.5 mg/L, any further increase does not significantly decrease the decayed, missing, and filled (DMF) tooth incidence, but does increase the occurrence and severity of mottling WATER FLUORIDATION 15.3 FIGURE 15.1 Relation between amount of dental caries (permanent teeth) observed in 7257 selected 12- to 14-year-old white schoolchildren in 21 cities in four states and the fluoride content of public water supplies (Source: McClure, 1943.) At a fluoride concentration of about 1.0 mg/L, the optimum occurs—maximum reduction in caries with no aesthetically significant mottling At this level, DMF tooth rates were reduced by 60 percent among the 12- to 14-year-old children At fluoride concentrations below 1.0 mg/L, some benefits occur, but caries reduction is not as great and decreases as the fluoride level decreases until, as zero fluoride is approached, no observable improvement occurs Studies on fluoride were interrupted by World War II, but in 1945 and 1947, four classic studies were initiated with the intent to demonstrate conclusively the benefits of adding fluoride to community drinking water (Dean et al., 1950; Ast et al., 1956; Brown and Poplove, 1963; Hill et al., 1961) Fluoridation began in January 1945 in Grand Rapids, Michigan; in May 1945 in Newburgh, New York; in June 1945 in Brantford, Ontario; and in February 1947 in Evanston, Illinois When caries rates in these localities were compared to those in a nonfluoridated “control city,” a 50 to 65 percent reduction in dental caries was found in the fluoridated cities without evidence of any adverse effects These initial studies established fluoridation as a practical and effective public health measure that would prevent dental caries Once the safety and effectiveness of fluoridation had been established, engineering aspects needed to be developed before community water fluoridation could be implemented In the 1950s and 1960s, Franz J Maier, a sanitary engineer, and Ervin Bellack, a chemist, both with the USPHS, made major contributions to the engineering aspects of water fluoridation Maier and Bellack helped determine which chemicals were the most practical to use in water fluoridation, the best mechanical 15.4 CHAPTER FIFTEEN equipment to use, and the best process controls Bellack also contributed to major advances in fluoride testing In 1963, Maier published the first comprehensive book on the technical aspects of fluoridation (Maier, 1963) and, in 1972, Bellack, then with the U.S Environmental Protection Agency (USEPA), published an engineering manual (Bellack, 1972) that was replaced in 1986 by the manual entitled Water Fluoridation—A Training Manual for Engineers and Technicians (Reeves, 1986) Over the past 40 years, fluoride and fluoridation have been the subject of numerous studies undertaken by the USPHS, state health departments, and nongovernmental research organizations Since 1970, over 3700 such studies have been conducted (Michigan Department of Public Health, 1979; Newbrun, 1989; Ripa, 1993) These studies have strongly supported the beneficial effect of water fluoridation Water fluoridation has been supported for approximately 50 years as a public policy by the U.S Government Both the U.S Environmental Protection Agency and the U.S Public Health Service continue to recommend water fluoridation as an effective means to help prevent dental caries (USPHS, 1992, 1995) High levels of fluoride in drinking water have been found to cause adverse health effects As a result, USEPA has established regulatory limits on the fluoride content of drinking water On the basis of a detailed review of health effects studies on fluoride, (National Research Council, 1993) USEPA set a maximum contaminant level of mg/L in water systems to prevent crippling skeletal fluorosis (Federal Register, National Primary and Secondary Drinking Water Regulations, 1986) A secondary level of mg/L was established by USEPA to protect against objectionable dental fluorosis These limits, to be reviewed by USEPA every three years, or as new health data becomes available, primarily impact systems that have naturally high fluoride levels RECENT STUDIES Two different studies reviewed the complete health effects of water fluoridation The Review of Fluoride—Benefits and Risks, a report of the Ad Hoc Subcommittee on Fluoride of the Committee to Coordinate Environmental Health and Related Programs, was published in February 1991 (Public Health Service, 1991) This report is a comprehensive review and evaluation of the public health benefits and risks of fluoride in drinking water and other sources It was prompted by a study of the National Toxicology Program that found “equivocal evidence” of carcinogenicity based on the occurrence of a small number of malignant bone tumors in male rats This report recommends the continued use of fluoride to prevent dental caries and the continued support of optimal fluoridation of drinking water It also recommends scientific conferences to determine an optimal level of fluoride exposure from all sources combined, not only from drinking water The CDC in Atlanta is involved with initiating these recommendations A second study, Health Effects of Ingested Fluoride, was the report of the Subcommittee of Health Effects of Ingested Fluoride of the National Research Council (National Research Council, 1993) Members of the National Research Council are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine The report was the result of a request by the U.S Environmental Protection Agency (EPA) to review the health effects of ingested fluoride and determine whether EPA’s maximum contaminant level (MCL) of mg/L was appropriate The subcommittee conducted a detailed examination of data for the intake, metabolism, and disposition of fluoride; dental WATER FLUORIDATION 15.5 fluorosis; bone strength and risk of bone fracture; effects on the renal, gastrointestinal, and immune systems; reproductive effects in animals; genotoxicity; and carcinogenicity in animals and humans There was no evidence that fluoride affected hypersensitive individuals, but the report suggested that more research is needed in this area The report concluded that EPA’s current MCL of mg/L for fluoride in drinking water was appropriate The subcommittee did recommend additional research in various areas, as it felt the fluoride data was incomplete Concerns have recently been expressed that increases in the prevalence of mild or very mild fluorosis are occurring in communities with negligible and optimal water fluoride concentrations, because of increased total fluoride consumption from various sources (Leverett, 1986) It is true, as Corbin (1989) has written, that it is “virtually impossible to find ‘non-fluoride’ communities due to the many opportunities for alternative exposures to fluoride,” i.e., fluoridated toothpaste, mouth rinses, beverages, and others Ripa (1993) states that “communities in the U.S still may be classified as being optimal fluoridated or fluoride-deficient based upon the concentration of fluoride in the drinking water However, because fluoride is ubiquitous in food and dental health products, practically no American today is unexposed to fluoride.”The goal in water fluoridation, as it always has been, is to obtain the lowest rate of tooth decay with the least amount of dental fluorosis PRESENT STATUS OF FLUORIDATION The Centers for Disease Control and Prevention (CDC) estimated that approximately 145 million Americans, or about 62 percent of those served by public water supplies, consumed fluoridated water daily as of January 1, 1993 (CDC, 1993) Some 10.5 million of these people are served by naturally fluoridated supplies Over the years the USPHS has been setting health objectives to be met for each decade The fluoridation objective for the year 2000 states: “to increase by at least 75% the proportion of people served by community water systems providing optimal levels of fluoride.” Twenty-one states and the District of Columbia provide fluoridated water to 75 percent or more of their populations.As of 1996, nine states require fluoridation, at least for cities above a minimum population The increase in the U.S population served by fluoridated drinking water systems is shown in Figure 15.2 (CDC, 1985b) Data about water fluoridation in other countries is not readily available But it is known that other countries have large populations consuming fluoridated water: Australia (67 percent), Canada (40 percent), Ireland (62 percent), Israel (42 percent), Malaysia (60 percent), New Zealand (64 percent), and the United Kingdom (10 percent) (Hammer, 1996) The city-states of Hong Kong and Singapore are totally fluoridated Israel is planning to double the population drinking fluoridated water by the year 2000 Some progress has been made toward achieving community fluoridation in Central and South America, especially in Brazil Brazil requires fluoridation for all communities with populations over 50,000 There also are strong fluoridation efforts in Argentina and Chile The Pan American Health Organization (a branch of the World Health Organization) has been very active in the promotion of fluoridation in Latin America (World Health Organization, 1984) Japan and South Korea are planning major efforts in water fluoridation Although community water fluoridation has been shown to be both safe and the most cost-effective method of preventing dental caries, a small percentage of the 15.6 CHAPTER FIFTEEN FIGURE 15.2 Fluoridation growth in the United States, 1945–1984 (Source: CDC, 1985b.) population continues to oppose its introduction into community water systems When fluoridation is being considered for adoption by a community, persons opposed to fluoridation often attempt to refute the benefits, safety, and efficacy of this effective public health measure Charges against fluoridation and the corresponding truths have been discussed elsewhere (AWWA, 1988) Assistance in responding to false charges against fluoridation may be obtained from the Oral Health Program of the CDC, Atlanta, Georgia The National Institute of Dental Research in Bethesda, Maryland, a branch of the National Institutes of Health, and the American Dental Association are other sources of information concerning water fluoridation Alternative means of providing the benefits of fluoride besides the fluoridation of municipal water supply systems are available.The six most important methods are listed in Table 15.1 Public health authorities, however, typically recommend that these methods only be considered in situations where municipal water fluoridation is not possible Municipal water fluoridation is the most cost-effective means available for reducing the incidence of caries in a community Topical fluoride methods can be used in conjunction with water fluoridation (optimally fluoridated water in community or school water systems or naturally fluoridated water) Systemic fluoride methods are sufficient alone in preventing tooth decay, and other methods should not be used in conjunction with them The cost and effectiveness of alternatives to municipal water fluoridation are shown in Table 15.2 (Gish, 1978) TABLE 15.1 Alternative Means of Fluoride Supply Topical methods Systemic methods Fluoride gels Fluoride mouth rinses Fluoride dentifrices Fluoride tablets Fluoride drops Salt fluoridation 15.7 WATER FLUORIDATION TABLE 15.2 Comparison of Effectiveness of Different Types of Fluoride Applications Procedure Water fluoridation Municipal School Topical fluorides Supervised application of paste or rinse in school Professional application of topical fluoride Systemic fluorides Supervised distribution of fluoride tablets in school Individually prescribed fluoride tablets or drops Cavities prevented per $100,000 spent Cost/cavities prevented 500,000 111,100 $0.20 0.90 55,500 25,600 1.82 3.90 16,542 10,000 6.06 10.00 Source: Gish (1978) THEORY Causes of Dental Caries Tooth decay is a complex process, and all factors involved are not entirely understood It is usually characterized by loss of tooth structure (enamel, dentin, and cementum) as a result of destruction of these tissues by acids Evidence indicates that acids are produced by the action of oral bacteria and enzymes on sugars and carbohydrates entering the mouth This takes place beneath the plaque, an invisible film composed of gummy masses of microorganisms that adhere to the teeth Oral bacteria are capable of converting some of the simpler sugars into acids, and the bacteria and enzymes acting in combination are capable of converting carbohydrates and more complex sugars into acids The production of acids is a result of the natural existence of bacteria and enzymes in the mouth Until the middle or late 20th century, a very high prevalence of dental caries existed in all developed countries (Burt et al., 1992) Until water fluoridation became widespread, almost 98 out of 100 Americans experienced some tooth decay by the time they reach adulthood The highest tooth decay activity is found in schoolchildren Tooth decay begins in early childhood, reaches a peak in adolescence, and diminishes during adulthood (National Institute of Dental Research, 1981) Dental Benefits of Fluoride in Drinking Water When water containing fluoride is consumed, some fluoride (about 50 percent) is retained by fluids in the mouth and is incorporated onto the teeth by surface uptake (topical effect) The rest (about 50 percent) enters the stomach, where it is rapidly adsorbed by diffusion through the stomach walls and intestine Fluoride enters the blood plasma and is rapidly distributed to all parts of the body, including the teeth (systemic effect) Because of the systemic effect, the fluoride ion is able to pass freely through all cell walls and is available to all organs and tissues of the body Distributed in this fashion, the fluoride ion is available to all skeletal structures of the body in which it may be retained and stored in proportions that generally increase with age and intake Bones, teeth, and other parts of the skeleton tend to attract and retain fluoride Soft tissues not retain fluoride Fluoride is a “bone seeker,” with about 96 percent of the fluoride found in the body deposited in the skeleton Because teeth are part of the 15.8 CHAPTER FIFTEEN skeletal system, incorporation of fluoride in teeth is basically similar to that in other bones It is most rapid during the time of the child’s formation and growth.The fluoride ion is actually incorporated into the apatite crystal of the tooth enamel During formation of the tooth, the fluoride ion, F−, replaces the hydroxyl ion, OH−, in the crystal lattice Thus, the enamel of the tooth is greatly strengthened so that it is more resistant to bacterial acids and inhibits the growth of certain kinds of bacteria that produce acids (Keyes, 1969; Whitford, 1996) In addition, fluoride appears to actually aid in the remineralization of teeth (Mellberg and Mallon, 1984; Silverstone, 1984) Erupted teeth differ from other parts of the skeleton in that once they are formed, with the exception of the dentin (inner part of the tooth) and the root, cellular activity virtually ceases.As a result, very little change occurs in the fluoride level in teeth after they are formed Children must drink the proper amount of fluoridated water during early development of permanent teeth, preferably before they start school, in order to realize full benefits The relationship between dental caries, dental fluorosis, and fluoride level is shown in the classic chart in Figure 15.3 (Dunning, 1986) The beneficial effect of optimally fluoridated water ingested during the years of tooth development has been amply demonstrated At the optimal concentration in potable water, fluoride will reduce dental caries from 20 to 40 percent among children who ingest this water from birth (Newbrun, 1989) Evidence that water fluoridation is effective in preventing caries has been repeatedly demonstrated, starting with the initial community trials in the United States and Canada in the 1940s In recent years, however, the relative impact of water fluoridation appears to have diminished as other sources of fluoride supplementation (toothpastes, food, etc.) have increased Continuation of benefits into adult life is inevitable Stronger teeth result in fewer caries that require fewer and less extensive fillings, fewer extractions, and fewer artificial teeth Fluoridated water helps prevent cavities on exposed roots as a result of receding gums in adults who develop periodontal disease Early evidence indicated that higher levels of fluoride would strengthen bones of older people, thereby reducing the incidence of bone fractures (Jowsey et al., 1972; Riggs et al., 1982) However, this has now been shown to be untrue (National Research Council, 1993) FIGURE 15.3 Dental caries and dental fluorosis in relation to fluoride in public water supplies (Source: Reprinted by permission of the publisher from Principles of Dental Public Health by J M Dunning, Cambridge, MA: Harvard University Press, copyright © 1962 by the President and Fellows of Harvard College.) WATER FLUORIDATION 15.9 Various studies in fluoridated communities over the last 30 years have shown a dramatic increase in the number of teenagers who are completely caries free Teenagers without lifetime exposure realize benefits from fluoridation, and benefits increase for those with lifetime exposure Conservative estimates indicate that 20 percent of the teenagers in a fluoridated community will be caries free (CDC, 1978) This is about six times as many as are caries free in a fluoride-deficient community Fluoridation can substantially reduce costs associated with restorative dentistry According to the CDC, for every dollar spent (1992) on water fluoridation, a potential $80 in dental bills may be saved (CDC, 1992) In 1992, the CDC estimated the cost of fluoridation to be about the cost of a candy bar per person per year OPERATIONAL AND DESIGN CONSIDERATIONS Optimal Fluoride Levels The optimal fluoride level in water is the level that produces the greatest protection against caries with the least risk of fluorosis Initially, this figure was obtained after examining the teeth of thousands of children living in various places with differing fluoride levels It was not based on any direct or accurate knowledge of how much water children drank at various times and at different places Early in such investigations, variation in the optimal figure was observed depending on the local air temperature, which had a direct bearing on the amount of water children consumed at different ages Studies in California and Arizona, where temperatures are considerably above the average of other parts of the United States, showed a definitely lower optimal fluoride level This was demonstrated by observing the prevalence of dental fluorosis in places with various natural fluoride levels in their public water supplies and by estimating the actual quantities of water ingested by children of various age groups and weights The optimal fluoride level for a water system is usually established by the appropriate state regulatory agency Optimal fluoride concentrations and control ranges recommended by the USPHS and CDC may be used as guidelines if state limits have not been established These levels, shown in Table 15.3 (Reeves, 1986), are based on the annual average of the maximum daily air temperature in the particular school or community Many water supplies contain fluorides from natural sources For these systems, the question of the practicability of supplementing natural fluoride with enough fluoride to bring the concentration up to the optimal level must be addressed Usually, addition of the small amount of fluoride needed to reach the optimal level can be shown to be economically justified based on the resulting benefits to the community Fluoride Chemicals and Chemistry Fluorine, a gaseous halogen, is the thirteenth most abundant element found in the earth’s crust It is a pale yellow noxious gas that is highly reactive It is the most electronegative of all elements and cannot be oxidized to a positive state Fluorine is not found in a free state in nature, as it is a gas It is always found in combination with chemical radicals or other elements as fluoride compounds When dissolved in water, these compounds dissociate into ions All fluoride chemicals commonly used in water fluoridation dissociate to a great degree 15.10 CHAPTER FIFTEEN TABLE 15.3 Optimal Fluoride Levels Recommended by the U.S Public Health Service, Centers for Disease Control Recommended control range Annual average of maximum daily air temperatures*, °F Community, mg/L School†, mg/L 0.1 below 0.5 above 20% low 20% high 50.0–53.7 53.8–58.3 58.4–63.8 63.9–70.6 70.7–79.2 79.3–90.5 1.2 1.1 1.0 0.9 0.8 0.7 5.4 5.0 4.5 4.1 3.6 3.2 1.1 1.0 0.9 0.8 0.7 0.6 1.7 1.6 1.5 1.4 1.3 1.2 4.3 4.0 3.6 3.3 2.9 2.6 6.5 6.0 5.4 4.9 4.3 3.8 Recommended fluoride concentration Community systems, mg/L School systems, mg/L * Based on temperature data obtained for a minimum of years † Based on 4.5 times the optimum fluoride level for communities Source: Reeves (1986) Fluoride can be found in a solid form in fluoride-containing minerals such as fluorspar, cryolite, and apatite Fluorspar is a mineral containing from 30 to 98 percent calcium fluoride (CaF2) Cryolite (Na3AlF6) is a compound of aluminum, sodium, and fluoride Apatite is a mixture of calcium compounds that includes calcium phosphates, calcium fluorides, and calcium carbonates Trace amounts of sulfates are usually present as impurities Apatite contains from to percent fluoride and is the main source of fluoride used in water fluoridation at the present time Apatite is also the raw material for phosphate fertilizers Cryolite is not a major source of fluoride in the United States Fluoride is widely distributed in the lithosphere and hydrosphere Because of the dissolving power of water and the movement of water in the hydrologic cycle, fluoride is found naturally in all waters High concentrations of fluoride are not common in surface water, but may occur in groundwaters, hot springs, and geothermal fluids Fluoride forms compounds with every element except helium, neon, and argon Polyvalent cations such as aluminum, iron, silicon, and magnesium form stable complexes with the fluoride ion The extent to which complex formation takes place depends on several factors, including the complex stability constant, pH, concentrations of fluoride, and complexing species (Eichenberger, 1982) Sodium fluoride, sodium fluorosilicate, and fluorosilicic acid are the three most commonly used fluoride chemicals in the United States Standards for these chemicals are published by AWWA for use by the water industry (AWWA, 1994a–c) All chemicals used for fluoridation should be comparable in quality to the requirements of these standards From time to time, shortages of fluoride chemicals have occurred Generally, most of these are not shortages at the manufacturer’s plant, but temporary shortages at the local distributor level Local shortages are usually eliminated quickly In the past, shortages at the manufacturing level, especially of fluorosilicic acid and sodium silicofluoride, have occurred Fluorosilicic acid and sodium fluorosilicate (and most sodium fluoride) are byproducts of the manufacture of phosphoric acid, the main ingredient of phosphate fertilizer Sales of fertilizer will have a direct effect on the volume of fluoride chemicals produced In the past, slow sales of fertilizer have resulted in a temporary shortage of these two chemicals Shortages have been relatively mild because the number of fluo- 15.11 WATER FLUORIDATION ridated communities was smaller and lower volumes of sodium fluorosilicate and fluorosilicic acid were needed than at the present time Shortages occurred in 1955 to 1956, in the summer of 1969, in the spring and summer of 1974, in the summer of 1982, and in the early part of 1986, with the last one being the most severe (Reeves, 1985) During production of fluoride chemicals, trace amounts of impurities may be introduced into the chemical, especially arsenic, lead, and/or zinc Normally, impurities are at levels far below those that would necessitate the establishment of maximum impurity limits Sodium Fluoride Sodium fluoride (NaF) is a white, odorless material available either as a powder or as crystals of various sizes It has a molecular weight of 42.00, a specific gravity of 2.79, and a practically constant solubility of 4.0 g/100 mL (4 percent) in water at temperatures generally encountered in water treatment practice When added to water, sodium fluoride dissociates into sodium and fluoride ions: NaF ⇔ Na+ + F− (15.1) The pH of a sodium fluoride solution varies with the type and amount of impurities present Solutions prepared from common grades of sodium fluoride have a pH near neutrality (approximately 7.6) Sodium fluoride is available in purities ranging from 97 to over 98 percent, with impurities consisting of water, free acid or alkali, sodium silicofluoride, sulfites, and iron, plus traces of other substances Approximately 8.6 kg (19 lb) of sodium fluoride will add mg/L of fluoride to 1.0 mil gal (3.8 ML) of water Sodium fluoride is used in the manufacture of vitrified enamel and glasses, as a steel degassing agent, in electroplating, in welding fluxes, in heat-treating salt compounds, in sterilizing equipment in breweries and distilleries, in paste and mucilage, as a wood preservative, and in the manufacture of coated paper In the past it was also used as a rodenticide It is no longer used as such and is not included on the Environmental Protection Agency’s list of registered rodenticides Sodium Fluorosilicate Sodium fluorosilicate (Na2SiF6) is a white, odorless crystalline material with a molecular weight of 188.06 and a specific gravity of 2.679 Its solubility varies from 0.44 g/100 mL of water at 0°C to 2.45 g/100 mL at 100°C When sodium fluorosilicate is dissolved in water, virtually 100 percent dissociation occurs rapidly: Na2SiF6 ⇔ 2Na+ + SiF6= (15.2) = Fluorosilicate ions (SiF ) may react in two ways Most common is hydrolysis of SiF6= releasing fluoride ions and silica (SiO2): SiF6= + 2H2O ⇔ 4H+ + F− + SiO2 ↓ (15.3) Silica, the main ingredient in glass, is very insoluble in water Alternatively, SiF6= dissociates very slowly, releasing fluoride ions and the gas silicon tetrafluoride (SiF4): SiF6= ⇔ 2F− + SiF4 ↑ (15.4) Silicon tetrafluoride reacts quickly with water to form silicic acid or silica: SiF4 ↑ + 3H2O ⇔ 4HF + H2SiO3 (15.5a) SiF4 ↑ + 2H2O ⇔ 4HF + SiO2 ↓ (15.5b) HF ⇔ H + F + − (15.5c) Solutions of sodium fluorosilicate are acidic, with saturated solutions usually exhibiting a pH of between and (approximately 3.6) Sodium fluorosilicate is 15.12 CHAPTER FIFTEEN available in purities of 98 percent or higher Principal impurities are water, chlorides, and silica Approximately 6.3 kg (14 lb) of sodium fluorosilicate will add mg/L of fluoride to 1.0 mil gal (3.8 ML) of water Sodium fluorosilicate has some other industrial uses including laundry scouring (neutralizing industrial caustic soaps), the manufacture of opal glass, and mothproofing woolens It has been used in the past as a rodenticide, but like sodium fluoride, it is no longer used in this way The U.S Environmental Protection Agency does not list it as a registered rodenticide As in the case of sodium fluoride, the principal hazard associated with handling sodium fluorosilicate is dust Fluorosilicic Acid Fluorosilicic acid, also known as hexafluorosilicic, silicofluoric, or hydrofluorosilicic acid (H2SiF6), has a molecular weight of 144.08 and is available commercially as a 20 to 35 percent aqueous solution It is a straw-colored, transparent, fuming, corrosive liquid having a pungent odor and an irritating action on the skin Solutions of 20 to 35 percent fluorosilicic acid have a low pH (1.2), and at a concentration of mg/L of fluoride in poorly buffered potable waters, a slight depression of pH can occur If the alkalinity of the drinking water is less than mg/L as CaCO3 and the pH of the water is 6.8–7.0, then adding mg/L of fluoride could lower the pH of the drinking water to 6.2–6.4 Fluorosilicic acid dissociates in solution virtually 100 percent Its chemistry is very similar to that of Na2SiF6: H2SiF6 ⇔ 2HF + SiF4 ↑ (15.6a) SiF4 ↑ + 2H2O ⇔ 4HF + SiO2 ↓ (15.6b) SiF4 ↑ + 3H2O ⇔ 4HF + H2SiO3 (15.6c) HF ⇔ H + F + − (15.6d) Fluorosilicic acid should be handled with great care because of its low pH and the fact that it will cause a “delayed burn” on skin tissue Fluorosilicic acid (23 percent) will freeze at approximately 4°F (−15.5°C) Approximately 20.8 kg (46 lb) of 23 percent acid are required to add mg/L of fluoride to 1.0 mil gal (3.8 ML) of water Hydrofluoric acid and silicon tetrafluoride are common impurities in fluorosilicic acid that result from production processes Hydrofluoric acid is an extremely corrosive material Its presence in fluorosilicic acid, whether due to intentional addition (i.e., “fortified” acid) or normal production processes, demands careful handling Unlike chlorine fumes, fluorosilicic acid fumes are lighter than air and will rise instead of settling to the floor (Silicon tetrafluoride [SiF6] is a gas that is heavier than air but is not toxic.) Fluorosilicic acid seems to have a special affinity for electrical switches, contacts, and control panels, as well as concrete Other Fluoride Chemicals Ammonium silicofluoride, magnesium silicofluoride, potassium fluoride, hydrofluoric acid, and calcium fluoride (fluorspar) are being used or have been used for water fluoridation Each material has properties that make it desirable in a specific application, but each also has undesirable characteristics None of these chemicals has widespread application in the United States Calcium fluoride, however, is sometimes used in South America Ammonium silicofluoride has the peculiar advantage of supplying all or part of the ammonium ion necessary for the production of chloramine, when this form of disinfectant is preferred to chlorine in a particular situation It has the disadvantage of hindering disinfection if there are short contact times Also, it is more expensive than sodium fluorosilicate WATER FLUORIDATION 15.13 Magnesium silicofluoride and potassium fluoride have the advantage of extremely high solubility, of particular importance in such applications as school fluoridation, where infrequent refills of solution containers are desired In addition, potassium fluoride is quite compatible with potassium hypochlorite, so a mixture of the two solutions (in the same container) can be used for simultaneous fluoridation and chlorination They cannot be fed in dry form Also, they are both (especially potassium fluoride) more expensive than sodium silicofluoride Magnesium silicofluoride is widely used in Europe as a concrete curing compound and thus is mass produced, but it is still more expensive than sodium silicofluoride Potassium fluoride is one of the main ingredients in the manufacture of nerve gas Calcium fluoride (fluorspar) is the least expensive of all the compounds used in water fluoridation, but it is also the most insoluble It has been successfully fed by first dissolving it in alum solution and then utilizing the resultant solution to supply both the alum needed for coagulation and the fluoride ion Some attempts have been made to feed fluorspar directly in the form of ultrafine powder, on the premise that the powder would eventually dissolve or at least remain in suspension until consumed These attempts have not been very successful APPLICATION Fluoride Feed Systems Three methods of feeding fluoride are common in community water supply systems They are: Dry chemical feeder with a dry fluoride compound Chemical solution feeder with a liquid fluoride compound or with a prepared solution of a dry chemical Fluoride saturator The first two methods are also commonly used to feed other water treatment chemicals The saturator is a unique method for feeding fluoride Selection of the best fluoridation system for a situation must be based on several factors, including population served or water usage rate, chemical availability, cost, and operating personnel available (AWWA, 1988; Reeves, 1986) Although many options will be possible, some general limitations are imposed by the size and type of facility In general, very large systems will use the first two methods, whereas smaller systems will use either an acid feeder or the saturator Manuals describing considerations and alternatives involved in selecting the optimal fluoridation system are available (AWWA, 1988; Reeves, 1986) Factors important in the selection, installation, and operation of a fluoride feed system are the type of equipment used, the fluoride injection point, safety, and waste disposal Types of Equipment Fluoride chemicals are added to water as liquids, but they may be measured in either liquid or solid form Solid chemicals must be dissolved into solution before feeding This is usually accomplished by using a dry chemical feeder that delivers a predetermined quantity of chemical in a given time interval.Two types of dry feeders exist Each has a different method of controlling the rate of delivery A volumetric dry feeder delivers a measured volume of dry chemical per unit of time A gravimetric dry feeder delivers a measured weight of chemical per unit of time Many water treatment plants that treat surface water utilize dry feeders to add other treatment chemicals and so use dry feeders for fluorides to maintain consis- 15.14 CHAPTER FIFTEEN tency with other equipment Dry feeders are used almost exclusively to feed sodium fluorosilicate because of the high cost of sodium fluoride The saturator feed system is unique to fluoridation and is based on the principle that a saturated fluoride solution (4%) will result if water is allowed to trickle through a bed containing a large amount of sodium fluoride A small pump is used to feed the saturated solution into the water being treated.Although saturated solutions of sodium fluoride can be manually prepared, automatic feed systems are preferred Selection of Fluoridation Systems While there is no specific type of fluoridation system that is solely applicable to a specific situation, there are some general limitations imposed by the size and type of water facility For example, a large metropolitan water plant would hardly be likely to consider a fluoridation installation involving the manual preparation of sodium fluoride solution, nor would a small facility consisting of one unattended well consider the use of a gravimetric dry feeder installation Prior to the actual design of a fluoridation system, a decision must be made on the type of chemical to be used This will largely determine the type of fluoridated water system that will be designed To determine the type of fluoride chemical to use and thus the type of fluoridation system to be designed, the following items must be considered: Chemical availability Water usage Type of existing facilities a Compatibility with proposed system b Space available c Number of treatment sites required (fluoride injection points) Characteristics of the water a Natural fluoride and optimal fluoride levels b Type of flow (variable or steady state) c Pressure (discharge) Estimated overall cost a Capital (initial) cost b Operation and maintenance costs c Chemical costs Operator preference and skill State rules, regulations, and preference The selection of the chemical is a judgment made after considering some or all of the above items There is no exact or perfect solution, and different people will make somewhat different judgments While in many cases the facts clearly favor one chemical, sometimes they will not; therefore, well-informed, knowledgeable persons could come to different conclusions The following example illustrates how fluoride chemical selection might be made and the chemical equipment selected EXAMPLE 15.1 The Town of Pelion, Iowa (population 525) has decided to fluoridate its water supply system The water system also serves a large rural school (population 2,000) Pelion’s water system consists of two city wells that are not attended on a full-time basis The average daily production rate is 0.2 MGD The optimal fluoride level for this community’s water system is 0.8 mg/L All three fluoride chemicals are readily available from a nearby chemical supplier Well No has a maximum pumping rate (capacity) of 290 gpm (417,600 gpd) and a discharge pressure of 65 psi.Well No has a capacity of 250 gpm (360,000 gpd) and 15.15 WATER FLUORIDATION a discharge pressure of 60 psi The natural fluoride level in the water from both wells is 0.1 mg/L The wells are located approximately mile apart Both wellhouses are large and contain equipment for feeding chlorine, polyphosphate, and soda ash Also, the wellhouses contain electricity and the necessary piping 0.7 mg/L × 0.2 MGD × 8.34 lb/gal Sodium fluoride feed rate = ᎏᎏᎏᎏ 0.45 × 0.98 Sodium fluoride feed rate = 2.65 lbs/day NaF needed/yr = 2.65 lbs/day × 365 days/yr = 967.3 lb/yr Amount of fluorosilicic acid solution (H2SiF6, 23% solution) needed: Fluorosilicic acid solution feed rate (lb/day) dosage (mg/L) × capacity (MGD) × 8.34 lb/gal = ᎏᎏᎏᎏᎏᎏ AFI × solution strength AFI = atomic weight of fluoride in H2SiF6 divided by the molecular weight of H2SiF6 = grams of fluoride per gram of H2SiF6 = (6 × 19)/144.1 = 0.79 Solution strength = 23 grams of H2SiF6 per 100 grams of solution = 0.23 grams H2SiF6 per gram solution 0.7 mg/L × 0.2 MGD × 8.34 lb/gal Solution feed rate = ᎏᎏᎏᎏ = 6.4 lb/day 0.79 × 0.23 Solution (23%) needed/yr = 6.4 lb/day × 365 days/yr = 2336 lb/year Comparing the cost of the two chemicals: Chemical Costs Comparison Chemical (item) NaF Acid Cost, ¢/lb Chemical used, lb/yr Chemical cost, $/yr Difference, $ 90 30 967 2336 870 703 −167 As the town has relatively small unattended wells, the use of sodium fluorosilicate for dry feeders should be ruled out immediately Thus, the choice is between using sodium fluoride and a saturator and using fluorosilicic acid in carboys and a metering pump A saturator will require slightly more space, but that is not a problem here Both the acid system and the saturator system are compatible with the water system There will be two fluoride injection points, one at each well, because of the location of the wells There is a steady flow and adequate pressure at each well While the chemicals are readily available, there will be a difference in cost of both chemicals and equipment These costs can be estimated, compared, and evaluated: Data: Average daily production rate = 0.2 MGD Fluoride dosage (mg/L) = optimal fluoride level (mg/L) − natural fluoride level (mg/L) Fluoride dosage = 0.8 mg/L − 0.1 mg/L = 0.7 mg/L 15.16 CHAPTER FIFTEEN Amount of sodium fluoride (NaF with saturator) needed: Sodium fluoride feed rate (lbs/day) Fluoride dosage (mg/L) × capacity (MGD) × 8.34 lb/gal = ᎏᎏᎏᎏᎏᎏᎏ AFI × chemical purity of the sodium fluoride AFI = fluoride content of sodium fluoride = gram atomic weight of fluoride (19) divided by the molecular weight of sodium fluoride (42) = 19/42 = 0.45 There is approximately a $167 difference in yearly chemical costs between using fluorosilicic acid and sodium fluoride.Also, there is a difference of approximately $2,200 in capital costs As the equipment for the acid installation and the yearly chemical costs are cheaper, a judgment can be made that the acid system is preferred (The fact that the Pelion water system also serves a large rural school is not a factor that will influence the selection of the fluoride chemical It is generally best to base the decision on which chemical to use on cost.) Thus, in this problem, the decision is to use hydrofluorosilicic acid As the project develops and specific kinds of equipment are selected, rough designs are made, and additional information is gathered, the estimated costs may become very inaccurate If this happens, another cost comparison should be made to ensure that it is still more economical to use the acid The type of feeder chosen for a particular fluoride installation is determined by cost (primarily), availability, service reputation of the manufacturer or sales representative, and, again, personal preference Once it has been decided which fluoride chemical to use, the choice of the fluoride feeder will be limited If fluorosilicic acid is to be used, then a metering pump will be required If a saturator (with sodium fluoride) is to be used, a metering pump is necessary If sodium fluoride (as a dry chemical) or sodium fluorosilicate is to be used, then a dry feeder is required Only the specific model of each general type of feeder will need to be determined after the chemical has been selected Fluoride Injection Point Ideally, the fluoride injection point should be at a location through which all water to be treated passes In a treatment plant, this could be a channel where other water treatment chemicals are added, a main coming from the filters, or the clear well If a combination of facilities exists, such as a treatment plant for surface water plus supplemental wells, a point where all water from all sources passes must be selected If no common point exists, a separate fluoride feeding installation is needed for each facility Another consideration in selecting the fluoride injection point is the possibility of fluoride losses through reaction with and adsorption on other treatment chemicals Whenever possible, fluoride should be added after filtration to avoid substantial losses that can occur, particularly with heavy alum dosages or when magnesium is present and the lime-soda ash softening process is being used A fluoride loss of up to 30 percent can result if the alum dosage rate is 100 mg/L (Bellack, 1984) If aluminum or iron salt coagulants are used and a fluoride compound is added before the metal hydroxide precipitate is removed, soluble aluminum and iron complexes can be formed, especially when the coagulation pH is less than about 6.5 In some situations, addition of fluoride before filtration may be necessary, such as in cases where the clear well is inaccessible When other chemicals are being fed, the question of chemical compatibility must be considered The fluoride injection point should be as far away as possible from the injection points for chemicals that contain calcium, in order to minimize loss of WATER FLUORIDATION 15.17 fluoride by local precipitation For example, if lime is being added to the main leading from the filters for pH control, fluoride can be added to the same main but at another point, or it can be added at the clear well If lime is added to the clear well, fluoride should be added to the opposite side If injection point separation is not possible, an in-line mixer must be used to prevent local precipitation of calcium fluoride and to ensure that the added fluoride dissolves In a single-well system, the well pump discharge can be used as the fluoride injection point If more than one well pump is used, the line leading to the distribution system can be used as the injection point In a surface water treatment plant or softening plant, the ideal location of the fluoride injection point is in the line from the filters to the clear well This location provides for maximum mixing Sometimes the clear well is located directly below the filters, and discharging any chemicals directly to the clear well is difficult In this situation the fluoride injection point must be at another location, such as in the main line to the distribution system or before the filters All fluoride injection points should have an antisiphon device included Safety Considerations Manuals describing operational hazards and safety practices for fluoride chemical feed systems are available (AWWA, 1983, 1988; Reeves, 1986) Treatment plant operators must use proper handling techniques to avoid overexposure to fluoride chemicals Dusts are a particular problem when sodium fluoride and sodium fluorosilicate are used The use of personal protective equipment (PPE) should be required when any fluoride chemical is handled or when maintenance on fluoridation equipment is performed Recommended Emergency Procedures for Fluoride Overfeeds When a community fluoridates its drinking water, a potential exists for a fluoride overfeed Most overfeeds not pose an immediate health risk; however, some fluoride levels can be high enough to cause immediate health problems All overfeeds should be corrected immediately because some have the potential to cause serious long-term health effects Specific actions should be taken when equipment malfunctions or when an adverse event occurs in a community public water supply system that causes a fluoride chemical overfeed.The CDC publishes recommended actions for handling fluoride overfeed events in community water systems (CDC, 1995) BIBLIOGRAPHY American Dental Association Council on Access, Prevention and Interprofessional Relations, JADA, vol 126, Chicago, IL, p 19-S, June 1995 Ast, D B., D J Smith, B Wachs, H C Hodges, H E Hilleboe, E R Schesinger, H C Chase, K T Cantwell, and D E Overton “Newburgh-Kingston Caries-Fluorine Study: Final Report.” J Am Dental Assoc., 52: 290, 1956 AWWA B701, Standard for Sodium Fluoride AWWA, Denver, CO, 1994a AWWA B702, Standard for Sodium Fluorosilicate AWWA, Denver, CO, 1994b AWWA B703, Standard for Fluorosilicic Acid AWWA, Denver, CO, 1994c AWWA, Safety Practice for Water Utilities, AWWA Manual M3 AWWA, Denver, CO, 1983 AWWA, Water Fluoridation Principles and Practices (3rd ed.), AWWA Manual M4 AWWA, Denver, CO, 1988 Bellack, E Fluoridation Engineering Manual U.S Environmental Protection Agency, Washington, DC, 1972; reprinted September 1984 15.18 CHAPTER FIFTEEN Brown, H K., and M Poplove “Brantford-Sarnia-Statford, Fluoridation Caries Study, Final Survey, 1963.” J Can Dental Assoc., 31 (8): 505, 1965 Burt, B A., S A Eklund, and D W Lewis Dentistry, Dental Practice and the Community (4th ed.) Saunders, Philadelphia, 1992 CDC FL-98 Caries-Free Teenagers Increase with Fluoridation United States Department of Health and Human Services, Public Health Service, Centers For Disease Control,Atlanta, 1978 CDC NIOSH Recommendations for Occupational Safety and Health Standards MMWR 1985; 34 (Supplement): 175, 1985a CDC “Dental Caries and Community Water Fluoridation Trends—U.S.” Morbidity and Mortality Weekly Report, 34 (6): 77, 1985b CDC “Public Health Focus: Fluoridation of Community Water Systems.” Morbidity and Mortality Weekly Report, 41:372–375, 381, 1992 CDC Fluoridation Census 1991 U.S Department of Health and Human Services, Public Health Service, CDC, Atlanta, 1993 CDC Engineering and Administrative Recommendations for Water Fluoridation, 1995 MMWR, vol 44, No RR-13, 1995 Churchill, H V “The Occurrence of Fluorides on Some Waters of the United States.” Jour AWWA, 23 (9): 1399, 1931 Corbin, S B “Fluoridation Then and Now.” Am J Public Health, 79, 1989: 561–563 Dean, H T “Chronic Endemic Dental Fluorosis (Mottled Enamel).” JAMA, 107: 1269, 1936 Dean, H T., F A Arnold, J Phillip, and J W Knutson Studies on Mass Control of Dental Caries through Fluoridation of Public Water Supply Public Health Reports 65, Grand RapidsMuskegon, MI, 1950 Dunning, J M Principles of Dental Public Health (4th ed.) Harvard University Press, Cambridge, MA, p 399, 1986 Eichenberger, B A., and K Y Chen “Origin and Nature of Selected Inorganic Constituents in Natural Waters,” In Water Analysis, vol 1: Inorganic Species, Part 1, (R A Minear and L H Keith, eds.) Academic Press, New York, 1982 Gish, C “Relative Efficiency of Methods of Caries Prevention in Dental Public Health,” In Proc Workshop on Preventive Methods in Dental Public Health, University of Michigan, Ann Arbor, MI, June 1978 Hammer, C T The Status of Fluoridation in the State of Israel, unpublished report, February– May 1996 Hill, I N., J R Blayney, and W Wolf “Evanston Fluoridation Study—Twelve Years Later.” Dental Prog., 1: 95, 1961 Hodges, H C., and F A Smith In Fluorine Chemistry, vol (J H Simons, ed.) Academic Press, New York, 1965 Jowsey, J., L B Riggs, P J Kelly, and D L Hoffman “Effect of Combined Therapy with Sodium Fluoride, Vitamin D, and Calcium in Osteoporosis.” Am J Med., 53: 43, 1972 Keyes, P H.“Present and Future Measures for Dental Caries Control.” J.Am Dental Assoc., 79: 1395, 1969 Leverett, D “Prevalence of Dental Fluorosis in Fluoridated and Nonfluoridated Communities—A Preliminary Investigation.” J Pub Health Dentist., 46: 4, 1986 Maier, F J Manual of Water Fluoridation Practice McGraw-Hill, New York, 1963 McClure, F J “Ingestion of Fluoride and Dental Caries—Quantitative Relations based on Food and Water Requirements of Children 1–12 Years Old.” Am J Diseases Children, 66: 362, 1943 McClure, F S Water Fluoridation, The Search and the Victory National Institutes of Health, Bethesda, MD, 1970 McNeil, D R The Fight for Fluoridation Oxford University Press, New York, 1957 Mellberg, J R., and D E Mallon “Acceleration of Remineralization, in vitro, by Sodium Monofluorophosphate and Sodium Fluoride.” J Dental Res., 63(9): 1130, 1984 WATER FLUORIDATION 15.19 Michigan Department of Public Health Michigan Department of Public Health Policy Statement on Fluoridation of Community Water Supplies and Synopsis of Fundamentals of Relation of Fluorides and Fluoridation to Public Health, 1979 National Institute of Dental Research, National Caries Program The Prevalence of Dental Caries in United States Children, 1979–1980 The National Dental Caries Prevalence Survey, National Institutes of Health, Bethesda, MD, December 1981 National Institute for Occupational Safety and Health Pocket Guide to Chemical Hazards U.S Department of Human Services, Public Health Service, CDC; DHHS(NIOSH) publication no 94-116, 1994 “National Primary and Secondary Drinking Water Regulations; Fluoride; Final Rule.” Federal Register, 51 (April 2): 11396, 1986 National Research Council Health Effects of Ingested Fluoride National Academy of Sciences, National Academy Press, Washington, DC, 1993 Newburn, E “Effectiveness of Water Fluoridation.” J Public Health Dentistry, 49 (5): special issue, 1989 Occupational Safety and Health Administration “Respiratory Protective Devices: Final Rules and Notice.” Federal Register 60:30336–30402, 1995 Public Health Service Review of Fluoride: Benefits and Risks—Report of the Ad Hoc Subcommittee on Fluoride of the Committee to Coordinate Environmental Health and Related Programs U.S Department of Health and Human Services, Public Health Service, Washington, DC, 1991 Reeves,T G.“The Availability of Fluoride Chemical Supplies.” JADA, 110 (April): 513–515, 1985 Reeves, T G Water Fluoridation: A Manual for Engineers and Technicians U.S Department of Health and Human Services, Public Health Service, CDC, Atlanta, 1986 Reeves, T G Water Fluoridation: A Manual for Water Plant Operators U.S Department of Health and Human Services, Public Health Service, CDC, Atlanta, 1994 Riggs, L B., E Seeman, S F Hodgson, D R Taves, and W M O’Fallon “Effects of the Fluoride/Calcium Regimen of Vertebral Fracture Occurrence in Postmenopausal Osteoporosis.” N Engl J Med., 306 (8): 446, 1982 Ripa, L W “A Half-Century of Community Water Fluoridation in the United States: Review and Commentary.” J Public Health Dent 53: 17–44, 1993 Silverstone, L M “The Significance of Remineralization in Caries Prevention.” J Can Dental Assoc., 50 (2): 157, 1984 Smith, M C., E M Lantz, and H V Smith “The Cause of Mottled Enamel, a Defect of Human Teeth.” University of Arizona Agricultural Experiment Station Bulletin, No 32, 1931 United States Environmental Protection Agency (USEPA), Letter of Support, 1986 United States Public Health Service (USPHS), Policy Statement, 1992 United States Public Health Service (USPHS), Surgeon General Statement, 1995 Whitford, G M The Metabolism and Toxicity of Fluoride (2nd ed.) Karger, New York, 1996 World Health Organization Fluorides and Oral Health (WHO Technical Report Series: 846) World Health Organization, Geneva, 1994 World Health Organization Fluorine and Fluorides Environmental Health Criteria 36 (WHO Technical Report Series: 846) World Health Organization, Geneva, 1984 ... SiF4 ↑ (15. 4) Silicon tetrafluoride reacts quickly with water to form silicic acid or silica: SiF4 ↑ + 3H2O ⇔ 4HF + H2SiO3 (15. 5a) SiF4 ↑ + 2H2O ⇔ 4HF + SiO2 ↓ (15. 5b) HF ⇔ H + F + − (15. 5c) Solutions... similar to that of Na2SiF6: H2SiF6 ⇔ 2HF + SiF4 ↑ (15. 6a) SiF4 ↑ + 2H2O ⇔ 4HF + SiO2 ↓ (15. 6b) SiF4 ↑ + 3H2O ⇔ 4HF + H2SiO3 (15. 6c) HF ⇔ H + F + − (15. 6d) Fluorosilicic acid should be handled with... and effectiveness of alternatives to municipal water fluoridation are shown in Table 15. 2 (Gish, 1978) TABLE 15. 1 Alternative Means of Fluoride Supply Topical methods Systemic methods Fluoride