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February 10, 2003
Volume 81, Number 06
CENEAR 81 06 p. 29
ISSN 0009-2347


For those seeking a dazzling white smile, polishers and peroxide may do the trick


As far back as the Middle Ages, people have been willing to go to great lengths for a bright-white smile. In those days, the neighborhood barber-surgeon--the guy you went to for anything from minor surgery to a haircut--would also file down your not-so-pearly whites and soak them in a concentrated solution of nitric acid. Patients did get the white teeth they were looking for. But the concentrated acid destroyed their tooth enamel, causing massive tooth decay later in life.

SMILE Most whitening products rely on hydrogen peroxide to remove stains from teeth.

Today, people seeking dazzling white choppers, à la Julia Roberts, can turn to less extreme methods. A vast array of pastes, gels, and strips promises to give you a movie-star smile, and they work at home or in the dentist's chair.

The outermost part of your teeth consists of a translucent layer of highly ordered hydroxyapatite, a mineral made up of calcium phosphate. This protective layer, known as enamel, is the hardest tissue in the body. Underneath it lies dentin, an off-white matrix of amorphous calcium phosphate and collagen that surrounds the blood vessels and nerves that nourish your teeth. Light passing through the enamel is reflected by the dentin, giving your teeth their pearly white color.

The root cause of most tooth discoloration lies just at the surface of your teeth's enamel. Drinking red wine, coffee, and tea can discolor your teeth. Colored molecules found in these beverages, including tannins and other polyphenols, adsorb to the enamel's surface. Dark pigments in cigarettes, blueberries, and other foods can also be deposited on your tooth enamel. Much of this superficial staining can be combated by regular brushing. But over time, these compounds can diffuse into the enamel, where they cannot be removed by brushing alone.

Aging is another big culprit. As we get older, our dentin gradually takes on a yellowish hue. "We don't yet know what chromophore or chromophores cause this tooth yellowing," says Paul A. Sagel, principal engineer of Procter & Gamble's oral care product delivery. His group is using ion mass spectrometry on teeth of varying ages to try to answer this question.

In addition, the antibiotic tetracycline can gray children's teeth if taken during their early years, when tooth enamel has yet to completely harden. Tetracycline is incorporated into children's enamel and dentin. Over time, it photooxidizes, giving teeth a grayish-blue tint that's difficult to remove.

Whether the culprit is diet, age, or drugs, those who have visited their dentist or drugstore seeking whiter teeth know that there are a dizzying number of products available. But many of them work the same way, by removing surface stains. Instead of actually altering your teeth's natural color, most whitening toothpastes just remove the surface stains that mask your teeth's intrinsic hue.

All toothpastes rely on abrasives to scrub stains from the tooth surface. The first "toothpowder," created in England in the late-18th century, contained rather harsh abrasives, including brick dust and ground-up cuttlefish. Today, toothpastes contain milder polishing agents such as silica, aluminum oxide, calcium phosphates, or calcium carbonate. Some toothpaste manufacturers also offer products that rely on proteolytic enzymes and chelators to lift stains on the teeth.

To combat stains below the surface, you'll probably need to turn to whitening gels containing hydrogen peroxide. The tooth-whitening power of peroxide was first recognized in the early 1970s. Dentists noticed that an oral antiseptic containing peroxide not only helped heal lesions in their patients' mouths but also gave them whiter teeth. Today, peroxide-based gels for tooth whitening at home and at the dentist are a booming business.

Hydrogen peroxide works its magic by breaking down into water and oxygen via radical intermediates. It's thought that these radical intermediates react with polyphenols and other pigments that stain teeth, at least in part by destroying the double-bond network that lends such compounds their color.

H2O2 can diffuse through the enamel and into the dentin in about 15 minutes, Sagel says. So, with proper use, H2O2 bleaching can combat stains both at and beneath the teeth's surface.

But close inspection of the labels of many of the whitening products on the market shows that most contain carbamide peroxide--a 1-to-1 compound of urea and H2O2 that contains 33% H2O2 by weight--instead of H2O2 itself. The reason is historical, Sagel says. Because the earliest peroxide antiseptics had to be packed into oral wounds, solid carbamide peroxide was used instead of liquid hydrogen peroxide. Carbamide peroxide remains popular, despite being more expensive and harder to formulate than H2O2.

Most peroxide-containing whiteners come in a gel that also contains glycerin and carbopol as thickeners and, in some cases, flavoring agents. Stannate and pyrophosphate salts are often added to scavenge metals and prevent peroxide decomposition while the gel is still on the store shelf.

Gels containing 10 or 20% carbamide peroxide can be brushed directly onto teeth, delivered in a mouth-guard-like tray, or embedded in an adhesive plastic strip that is stuck on the teeth. Several weeks of use is often necessary to see whitening with these at-home products.

In professional procedures for teeth whitening, dentists use up to 35% H2O2. Because whitening gels can cause tooth sensitivity and gum irritation, extra precautions are taken to apply the gel only to the teeth. Lasers and other light sources are sometimes used in combination with the gel; it's thought that these light sources simply accelerate the rate of peroxide decomposition, thereby speeding up the bleaching process. But whether lights add real benefit remains controversial.

If none of these options works for you, there's still one left: tooth veneers. Popular with movie stars and models, thin veneers made of porcelain are bonded to the existing teeth. Porcelain veneers won't last as long as your real teeth. But unlike your own teeth, these pearly whites don't stain--leaving you to dazzle maintenance-free.


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Copyright © 2003 American Chemical Society

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