A whitening gel is placed in a tray that fits over your teeth. As the active ingredient in the gel, carbamide peroxide is broken down, oxygen enters the enamel and bleaches the colored substances. The structure of the tooth is not changed; only the tooth is made lighter and whiter. Fillings, Crowns, and Bonding will not lighten.

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, gels containing hydrogen peroxide can be used. 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.

Whitening gels work 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.

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.
Find out more about these teeth whitening products.