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Tooth bleaching, also known as tooth whitening, is a common procedure in general dentistry but most especially in the field of cosmetic dentistry. Many people consider white teeth to be an attractive feature of a smile. A child's deciduous teeth are generally whiter than the adult teeth that follow. As a person ages the adult teeth often increase in value--that is to say, they become darker. This darkening is due to changes in the mineral structure of the tooth, as the enamel becomes less porous. Teeth can also become stained by bacterial pigments, foodstuffs and tobacco.
As white teeth are subconsciously associated with youth, they have become desirable. This has been made more apparent with the spread of American culture worldwide, where an especially white smile is coined a "Hollywood smile". The procedure to bleach teeth uses oxidising agents such as hydrogen peroxide or carbamide peroxide to lighten the shade of the tooth. The oxidising agent penetrates the porosities in the rod-like crystal structure of enamel and oxidises interprismatic stain deposits; over a period of time, the dentine layer, lying underneath the enamel, is also bleached.
There are two main methods of bleaching. The first involves applying a high concentration of oxidising agent for a short period of time, which is the so-called office bleach. This produces quick results but risks chemical burns to the soft tissues. Therefore, most in-office bleaching procedures use a light-cured protective layer that is carefully painted on the gums and papilla (the tips of the gums between the teeth). The bleaching agent is either carbamide peroxide, which breaks down in the mouth to form hydrogen peroxide, or hydrogen peroxide itself. The bleaching gel typically contains up to 35% hydrogen peroxide equivalent.
The alternative method involves using a thin mouthguard or strip to hold a low concentration of oxidising agent next to the teeth for as long as several hours a day for a period of 5 to 14 days. This is known as take-home or over-the-counter bleaching. This is a slower process but has fewer risks to the soft tissues. The bleaching agent is typically less than 10% hydrogen peroxide equivalent.