Tooth whitening is achieved by changing the color of the tooth enamel that has become yellowed or discolored over time. The most active ingredient most commonly used for whitening is hydrogen peroxide. When it spreads into the tooth, it acts as an oxidising agent that will break down and produce unstable free radicals that attach themselves to inorganic salts in tooth enamel that results in less heavy and much smaller pigmentation components of the tooth. These smaller pigments reflect less light to create the whitening effect. Whitening, or bleach treatments can last from 6 months to about 2 years.