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1.5: Fluoridation

  • Page ID
    5727
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    Learning Objectives

    • Describe the regulation concerning fluoride
    • List the chemicals that provide fluoride ions in water supplies
    • Calculate fluoride dosage for water supply

    Water fluoridation is the controlled addition of fluoride to a public water supply to reduce tooth decay. Fluoridated water contains fluoride at a level that is effective for preventing caries or cavities; this process can occur naturally or by adding fluoride. Typically a fluoridated compound is added to drinking water. However, defluoridation is needed when the naturally occurring fluoride level exceeds recommended limits. In 2011 the World Health Organization (WHO) suggested a level of fluoride from 0.5 to 1.5 mg/L (milligrams per liter), depending on climate, local environment, and other sources of fluoride. Bottled water typically has unknown fluoride levels.

    Dental remains a major public health concern in most industrial countries, affecting 60–90% of schoolchildren and the vast majority of adults. Water fluoridation reduces cavities in children. Efficacy in adults is less clear. A review of the prevalence of dental decay estimates a reduction in cavities when water fluoridation was used by children who had no access to other sources of fluoride. Most European countries have experienced substantial declines in tooth decay without its use, however, milk and salt fluoridation are widespread.

    Although fluoridation can cause dental fluorosis at levels above 2 mg/L per day, which can alter the appearance of developing teeth. Also, no clear evidence exists of other adverse effects of fluoridation. Fluoride's effects depend on the total daily intake of fluoride from all sources. Drinking water is typically the largest source of fluoride, and other methods of fluoride therapy include fluoridation of toothpaste, salt, and milk. The views on the most efficient method for community prevention of tooth decay are mixed. The World Health Organization reports that water fluoridation, when feasible and culturally acceptable, has substantial advantages, especially for subgroups at high risk of dental decay. European Union (EU) finds no benefit to water fluoridation compared with topical use.

    Public water fluoridation was first practiced in the U.S. As of 2012, 25 countries have artificial water fluoridation to varying degrees, 11 of them have more than 50% of their population drinking fluoridated water, and 28 countries have water that is naturally fluoridated—though in many of them the fluoride is above the recommended safe level. As of 2012, about 435 million people worldwide received water fluoridated at the recommended. About 214 million of them living in the United States.

    The goal of water fluoridation is to prevent tooth decay by adjusting the concentration of fluoride in public water supplies. Tooth decay is one of the most prevalent chronic diseases worldwide. Although it is rarely life-threatening, tooth decay can cause pain and impair eating, speaking, facial appearance, and acceptance into society, and it greatly affects the quality of life of children.

    The goal of water fluoridation is to prevent a chronic disease whose burdens particularly fall on children and the poor. Fluoridation does not affect the appearance, taste, or smell of drinking water. It is normally accomplished by adding one of three compounds to the water: sodium fluoride, fluorosilicic acid, or sodium fluorosilicate.

    Sodium fluoride (NaF) was the first compound used, and it is the reference standard. It is a white, odorless powder or crystal; the crystalline form is preferred if manual handling is used, as it minimizes dust. It is more expensive than the other compounds, but is easily handled and is usually used by smaller utility companies. It is toxic in gram quantities by ingestion or inhalation.

    Fluorosilicic acid (H2SiF6) is the most commonly used additive for water fluoridation in the United States. It is an inexpensive liquid by-product of phosphate fertilizer manufacture. It comes in varying strengths, typically 23–25%; because it contains so much water, shipping can be expensive. It is also known as hexafluorosilicic, hexafluosilicic, hydrofluosilicic, and silicofluoric acid.

    Sodium fluorosilicate (Na2SiF6) is the sodium salt of fluorosilicic acid. It is a powder or very fine crystal that is easier to ship than fluorosilicic acid. It is also known as sodium silicofluoride.

    These compounds were chosen because of their solubility, safety, availability, and low cost. A 1992 census found that, for U.S. public water supply systems reporting the type of compound used, 63% of the population received water fluoridated with fluorosilicic acid, 28% with sodium fluorosilicate, and 9% with sodium fluoride.

    Fluoride plump
    Figure \(\PageIndex{1}\): Feed pump application of fluoride compound into drinking water – Image by the Library of Congress is in the public domain

    The Centers for Disease Control and Prevention (CDC) developed recommendations for water fluoridation that specify requirements for personnel, reporting, training, inspection, monitoring, surveillance, and actions in case of overfeed, along with technical requirements for each of the compounds used. The maximum amount of fluoride from all sources (including non-dietary sources) is 0.05 mg/kg body weight per day for children and adults, including pregnant and lactating women.

    In 2011, the U.S. Department of Health and Human Services (HHS) and the U.S. Environmental Protection Agency (EPA) lowered the recommended level of fluoride to 0.7 mg/L. In 2015, the U.S. Food and Drug Administration (FDA), based on the recommendation of the U.S. Public Health Service (PHS) for fluoridation of community water systems, recommended that bottled water manufacturers limit fluoride in bottled water to no more than 0.7 milligrams per liter (mg/L). Previous recommendations were based on evaluations from 1962, when the U.S. specified the optimal level of fluoride to range from 0.7 to 1.2 mg/L. These standards are not appropriate for all parts of the world, where fluoride levels might be excessive and fluoride should be removed from water. In 2011, the World Health Organization stated that 1.5 mg/L should be an absolute upper bound and that 0.5 mg/L may be an appropriate lower limit.

    Fluoride naturally occurring in water can be above, at, or below recommended levels. Rivers and lakes generally contain fluoride levels less than 0.5 mg/L, but groundwater, particularly in volcanic or mountainous areas, can contain as much as 50 mg/L.

    In most drinking waters, over 95% of total fluoride is the F-, with the magnesium, fluoride complex (MgF+) being the next most common. Because fluoride levels in water are usually controlled by the solubility of fluorite (CaF2), high natural fluoride levels are associated with calcium-deficient, alkaline, and soft water. Defluoridation is needed when the naturally occurring fluoride level exceeds recommended limits. It can be accomplished by percolating water through granular beds of activated alumina, bone meal, bone char, or tricalcium phosphate; by coagulation with alum or with precipitation with lime.

    Fluoride's adverse effects depend on total fluoride dosage from all sources. At the commonly recommended dosage, the only clear adverse effect is dental fluorosis, which can alter the appearance of children's teeth during tooth development. In April 2015, recommended fluoride levels in the United States were changed to 0.7 ppm from 0.7–1.2 ppm to reduce the risk of dental fluorosis. Fluoride is the only chemical contaminate that has a primary contaminate level, 4 mg/L, and a secondary contaminate level, 2 mg/L.

    Fluorosis can be prevented by monitoring all sources of fluoride, with fluoridated water directly or indirectly responsible for an estimated 40% of risk and other sources, notably toothpaste, responsible for the remaining 60%. Fluoridation has little effect on risk of bone fracture (broken bones. No clear association between fluoridation and cancer or deaths due to cancer, for cancer in general and also specifically for bone cancer (osteosarcoma).

    In rare cases, improper implementation of water fluoridation can result in over fluoridation that causes outbreaks of acute fluoride poisoning, with symptoms that include nausea, vomiting, and diarrhea. Three such outbreaks were reported in the U.S. between 1991 and 1998, caused by fluoride concentrations as high as 220 mg/L; in the 1992 Alaska outbreak, 262 people became ill and one person died. In 2010, approximately 60 gallons of fluoride were released into the water supply in Asheboro, North Carolina in 90 minutes which was an amount that was intended to be released in a 24-hour period.

    Like other common water additives such as chlorine, hydrofluosilicic acid and sodium silicofluoride decrease pH and cause a small increase of corrosivity; however, this problem is easily addressed by increasing the pH. Although it has been hypothesized that hydrofluosilicic acid and sodium silicofluoride might increase human lead uptake from water, a 2006 statistical analysis did not support concerns that these chemicals cause higher blood lead concentrations in children. Trace levels of arsenic and lead may be present in fluoride compounds added to water, but no credible evidence exists that their presence is of concern.

    The effect of water fluoridation on the natural environment has been investigated, and no adverse effects have been established. Issues studied have included fluoride concentrations in groundwater and downstream rivers; lawns, gardens, and plants; consumption of plants grown in fluoridated water; air emissions; and equipment noise.

    Fluoride exerts its major effect by interfering with the demineralization mechanism of tooth decay. Tooth decay is a disorder that features an increase within dental plaque of bacteria such as Sreptococcus mutans and Lactobacillus. These organisms produce organic acids when carbohydrates, especially sugar, are eaten. When enough acid is produced to lower the pH below 5.5, the acid dissolves tooth enamel in a process known as demineralization. After the sugar is gone, the mineral loss can be recovered from ions dissolved in the saliva. Cavities result when the rate of demineralization exceeds the rate of remineralization.

    Sources of Fluoride

    Toothpaste enhanced with fluoride

    Fluoride supplements

    Food processed with fluoridated water

    Mouthwash enhanced with fluoride

    Fluoridated water supplies

    All fluoridation methods, including water fluoridation, create low levels of fluoride ions in saliva and plaque fluid that exerts a surface effect on teeth. A person living in an area with fluoridated water may experience rises of fluoride concentration in saliva to about 0.04 mg/L several times during a day. Technically, this fluoride does not prevent cavities it controls the rate at which they develop. When fluoride ions are present in plaque fluid along with dissolved hydroxyapatite, and the pH is higher than 4.5, a fluorapatite-like remineralized veneer is formed over the remaining surface of the enamel; this veneer is much more acid-resistant than the original hydroxyapatite and is formed more quickly than ordinary remineralized enamel would be.

    The MCLG for fluoride is 4.0 mg/L or 4.0 ppm. EPA has set this level of protection based on the best available science to prevent potential health problems. EPA has set an enforceable regulation for fluoride, called a maximum contaminant level (MCL), at 4.0 mg/L or 4.0 ppm. MCLs are set as close to the health goals as possible, considering cost, benefits, and the ability of public water systems to detect and remove contaminants using suitable treatment technologies. In this case, the MCL equals the MCLG, because analytical methods or treatment technology do not pose any limitation.

    EPA has also set a secondary standard (SMCL) for fluoride at 2.0 mg/L or 2.0 ppm. Secondary standards are non-enforceable guidelines regulating contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. EPA recommends secondary standards to water systems but does not require systems to comply. However, states may choose to adopt them as enforceable standards. Tooth discoloration and/or pitting is caused by excess fluoride exposures during the formative period prior to the eruption of the teeth in children. The secondary standard of 2.0 mg/L is intended as a guideline for an upper bound level in areas that have high levels of naturally occurring fluoride. The level of the SMCL was set based upon a balancing of the beneficial effects of protection from tooth decay and the undesirable effects of excessive exposures leading to discoloration.

    States may set more stringent drinking water MCLGs and MCLs for fluoride than EPA. The drinking water standards are currently under review. The Safe Drinking Water Act requires EPA to periodically review the national primary drinking water regulation for each contaminant and revise the regulation, if appropriate. In 2003 and as part of the first Six Year Review, EPA reviewed the drinking water standard for fluoride and found that new health and exposure data were available on orally ingested fluoride. EPA requested that the National Research Council (NRC) of the National Academies of Science (NAS) conduct a review of this data and in 2006, the NRC published their evaluation in a report entitled, Fluoride in Drinking Water: A Scientific Review of EPA's Standards. The NRC recommended that EPA update its fluoride risk assessment to include new data on health risks and better estimates of total exposure.

    In order to calculate the fluoride dosage, or any chemical dosage, one has to know the pounds of chemical and volume of water in millions of gallons per day.

    • Dosage, mg/L = (Chemical, lb)/(Water, MGD)(8.34 lb/day)

    In order to determine the amount of feed solution in gallons or gallons per day to treated water, one must know the amount of water to be treated in gallons or gallons per day, the feed dose in mg/L, and the feed solution in mg/L.

    • Feed Solution, gal = (Feed, gal)(Feed dose, mg/L)/(Feed Solution, mg/L)

    In blending waters or mixing chemicals of different strengths, the volumes or flows of the chemicals and their strengths must be known.

    • Mixture Strength, % = (Vol I, gal)(Strength I, %) + (Vol II, gal)(Strength II , %)/(Vol I, gal + Vol II, gal) or...
    • = (Flow I, gpd)(Strength I, %) + (Flow II, gpd)(Strength II, %)/(Flow I, gpd) + (Flow II, gpd)

    When using chemicals for fluoridation, one needs to know the percentage fluoride ion purity. This information allows for the conversion of pounds of chemical dosage to pounds of fluoride ion available.

    • Fluoride Ion Purity, % = (Molecular Weight of Fluoride)(100%)/Molecular Weight of Chemical

    Review Questions

    1. Describe the regulation concerning fluoride.
    2. List the chemicals that provide fluoride ions in water supplies.
    3. Calculate fluoride dosage for water supply if 2.4 million gallons are treated each day and 14.2 pounds of fluoride is added during a 24-hour period.

    Test Questions

    1. EPA has set an enforceable regulation for fluoride, called a maximum contaminant level (MCL), at ______.
      1. 4.0 mg/L
      2. 2.0 mg/L
      3. 15 ppb
      4. 80 ppb
    2. The goal of water fluoridation is to ______ by adjusting the concentration of fluoride in public water supplies.
      1. Enhance skeletal fluorosis
      2. Improve kidney function
      3. Prevent tooth decay
      4. Disinfect water supplies
    3. Like other common water additives such as chlorine, hydrofluosilicic acid and sodium silicofluoride ______ pH and cause a small increase of corrosivity.
      1. Decrease
      2. Increase
      3. Has no effect on
      4. Helps to buffer
    4. In rare cases, improper implementation of water fluoridation can result in over fluoridation that causes outbreaks of ________, with symptoms that include nausea, vomiting, and diarrhea.
      1. Increased lead in drinking water
      2. Acute fluoride poisoning
      3. Dental fluorosis occurring
      4. Over fluoridation has no side effects
    5. If a water system is treating a total daily flow of 2.6 million gallons of water that has a natural fluoride level of 0.2 mg/L, how many pounds of fluoride should be added to the water each day at the water treatment plant?
      1. 12 pounds of fluoride
      2. 11.2 pounds of fluoride
      3. 10.8 pounds of fluoride
      4. 10 pounds of fluoride

    1.5: Fluoridation is shared under a CC BY license and was authored, remixed, and/or curated by John Rowe.

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