How Does Teeth Whitening Work?

Whitening involves applying bleach solutions to the teeth.
There are highly colored organic molecules lodged between the crystals of tooth enamel (the outermost tooth covering) or in the dentin (the tooth material under the enamel) which give the teeth a stained appearance
The bleach attacks these molecules and the process continues until these organic particles are rendered colorless.
For surface stains, the solution is left on the teeth usually for 30 minutes to one hour, depending on the method used. For internal stains on non-living teeth that have had root canal work, the bleach might be sealed inside the tooth.
Bleaching works best on yellowish stains and even some brown stains, but may not work at all on gray discoloration. It's important to keep in mind that bonding material and fillings cannot be whitened with bleach

What Causes Teeth Staining or Discoloration?

There are two types of tooth discoloration:

Extrinsic (external or surface) staining

Caused by substances that come in contact with the tooth.
Big culprits are tobacco, either smoked or chewed
Food containing tannins such as red wine, coffee and tea.

Intrinsic (internal) tooth discoloration

Happens with aging as enamel loses its youthful translucency, becoming less porous; and making the underlying dentin more visible as it thickens and becomes more yellow over time.
Exposure to excessive fluoride or the antibiotic tetracycline during tooth-formation, inherited developmental disorders, and jaundice in childhood.
For erupted teeth, the main causes of intrinsic discoloration are tooth decay, restorations, pulp death (root canal problems) and trauma to developing teeth.
It's possible to have both extrinsic and intrinsic discoloration at the same time.

Is Whitening Safe for My Teeth and What Are the Risks of Teeth Whitening?

Whitening is safe because the chemicals used to attack the organic molecules do not materially affect the mineral structure of the tooth itself. There are many studies supporting the overall safety of whitening by bleaching.
The main risk is tooth sensitivity and that varies with a given product's concentration and the amount of time it is left on the teeth. If sensitivity does occur, it usually lasts no more than one to four days. An anti-sensitivity toothpaste will be prescribed prior to treatment to lessen sensitivity.
Gums can also become irritated on contact with bleaching solutions or by an ill-fitting mouth tray. It's important to wipe off excess gel from your gums during whitening and to inform your dentist of any problems.

How Long Do the Results Last?

No matter which whitening method you choose, you will probably find that the results fade over time. Whitening usually lasts from six months to two years, though longer-lasting results have been reported.

How Can I Maintain My White Smile?

You can make the brightness last longer by avoiding the foods and habits mentioned above that cause staining. Some individuals may need a touch-up whitening treatment in the dentist's office or at home once or twice each year.