SCIENCE & TECHNOLOGY WHAT'S
THAT
STUFF?
FLUORIDE
A
VOCAL MINORITY ASIDE, MOST
people approve of the addition of fluoride to their toothpaste and drinking water to help prevent tooth decay. For better or worse, fluoride is ubiquitous, yet many people don't know what that stuff is. According to a history of fluoridation prepared by the National Institute of Dental & Craniofacial Research, the understanding of fluoride's role in tooth decay prevention got its start in 1901, when a young dental school graduate named Frederick S. McKay decided to open a practice in Colorado Springs, Colo. W h e n he arrived, McKay was astounded to find scores of townspeople with brown stains on their teeth. At the same time, he later learned, the mottled teeth were surprisingly resistant to decay. McKay's quest to determine the cause of the staining ended three decades later with the discovery that the town's drinking water had high levels of naturally occurring fluoride. Subsequent research determined that water fluoride levels below 1 ppm prevent decay without the attendant staining. In a trial launched in 1945, Grand Rapids, Mich., became the first city to fluoridate its water; 11 years later, the benefits of the practice were clear and widespread fluoridation began. FLUORIDE PREVENTS decay in two main ways: It reduces the ability of bacteria in plaque to generate tooth-weakening acids, and it helps remineralize those tooth areas where acid attack has already begun. The agent doing the work is the fluoride ion; the chemical compound it's part of does not matter, as long as the ion is available in solution. Perhaps because of this flexibility, separate classes of compounds have evolved as fluoride sources for drinking water and for toothpaste. For toothpaste, the Food & Drug Administration has blessed three decaypreventing compounds: stannous fluoride, sodium fluoride, and sodium monofluorophosphate (MFP). All are derived from hydrofluoric acid, which in turn is made by reacting sulfuric acid with fluorspar, a calcium fluoride-rich ore. Ozark Fluorine Specialties in Tulsa, Okla., makes all three compounds, and, in 42
C&EN
/ APRIL
1 6,
2001
fact, it claims to be the only U.S. company producing U.S. Pharmacopeiagrade fluorides at all. Business Manager Philip Rakita says the company sells "millions of pounds of dentifrice fluoride a year" to toothpaste makers worldwide. The trick to making fluoride toothpastes is marrying the fluoride with a compatible abrasive. For example, the simplest and most obvious fluoride choice, sodium fluoride, isn't very soluble in the presence of calcium phosphates, the abrasive used in most early toothpastes. Procter & Gamble was the first company to find a fluoride-abrasive combination that worked, pairing stannous fluoride with calcium pyrophosphate. It launched this formula in 1955 as Crest and proceeded to dominate the fluoride toothpaste market for the next 10 years. P&G was on top, but the next dental fluoride breakthrough—MFP—was already in the works. Ozark had discovered the compound in 1949 and by the early 1960s was selling it to toothpaste makers overseas. M F P really took off in 1967 when Colgate-Palmolive first launched Colgate with MFP, providing new competition for Crest and cementing fluoridated toothpaste as the marketplace standard. Sodium fluoride didn't debut until 1982, when P & G came out with Advanced Formula Crest, which uses a hydrated silica abrasive that doesn't hinder sodium fluoride dissociation. The later rise of gel toothpastes, which owe their translucence to silica, also created sodium fluoride demand. Today, Rakita says, sodium fluoride is probably the leading dentifrice fluoride. All three are still in use, though, their choice determined by compatibility with other ingredients in the wide variety of toothpastes made today MFP, for example, is in some versions of Colgate and in Chesebrough-Ponds's Close-Up and Aim. A recent survey of drugstore shelves turned up stannous fluoride only once, in a Colgate toothpaste with 5% potassium nitrate that treats sensitive teeth.
Water fluoridation, while serving the same decay-preventive purpose as toothpaste fluoridation, uses a different set of chemicals derived from a different source. The chief water fluoridation chemical is hydrofluosilicic acid (HFS), a phosphate fertilizer coproduct. The fertilizer manufacturing process starts with the digestion of phosphate rock—fluorapatite or [Ca3(P04)2l3#CaF2—in sulfuric acid. The hydrogen fluoride generated reacts with silica impurities in the rock to form silicon tetrafluoride, which further combines with hydrogen fluoride to produce HFS. THE SODIUM SALT of HFS, sodium silicofluoride (SSF), is also used in water fluoridation, as is a water treatment grade of sodium fluoride made directly from hydrofluoric acid. According to Mark Looney, inorganic fluorides business vice president at Solvay Fluorides, a major marketer of HFS and SSF, most U.S. phosphate fertilizer makers collect HFS at their plants. These firms tend to be in phosphate rock-rich Central Florida and North Carolina. Because it's a coproduct, HFS is subject to the vagaries of the world's need for fertilizer, much of which is supplied by the U.S. The export market is slow right now, Looney notes, but the HFS market is still in balance. In contrast, a severe fertilizer industry downturn in the mid-1980s led to shortages of HFS and the temporary cessation of fluoridation in some municipalities; however, Looney doesn't see that happening again. One trend Looney does see is a slow shift from granular SSF to easier-tohandle liquid HFS. Water-treatment-grade sodium fluoride, made in North America only by Solvay, is popular with small and medium-sized water systems because dispensing systems are efficient and easy to maintain, he adds. Although the water fluoridation business is pretty mature, Looney notes that opportunity still knocks: Los Angeles began fluoridating its water supply just two years ago, and eight of the country's largest 50 cities still go without. In contrast, virtually every toothpaste is fluoridated, so the only market spark Ozark's Rakita can hope for is a request—unlikely he admits— for a new toothpaste fluoride. —MIKE
Separate classes of compounds have evolved as fluoride sources for drinking water and for toothpaste.
MCCOY
HTTP://PUBS. ACS.ORG/CEN
