Each of your teeth is made up of an inner dentine layer and a hard outer enamel layer which protects your teeth. When you put items in your mouth – food, cigarettes smoke, coffee, etc – another layer gradually forms on top of the enamel layer. The foreign material accumulates to form a pellicle film over the enamel layer.
A dentist or hygienist can clean away this film, through scraping, air flow and chemical treatments. Even brushing your teeth can remove some of it – brushing with an abrasive toothpaste cleans the tooth in the same way scrubbing with an abrasive pad cleans a dish. “Whitening toothpastes” are designed to work even harder on this layer.
The problem is, as this pellicle layer sits on your teeth for years and years, the foreign material gets into the enamel. The enamel is made up of hydroxyapatite crystals, which form microscopic hexagonal “rods”. Simply put enamel is porous, which means staining agents can work their way down into the tooth, where you can’t simply scour them away. The deeper stains are basically harmless, but many people find them unattractive.
This is where true tooth whiteners come into play. Basically, the whiteners use bleaching chemicals to get down into the tooth enamel and set off a chemical reaction (specifically an oxidation reaction) that breaks apart the staining compounds.
Most tooth whitening treatments use one of two chemical agents: carbamide peroxide or hydrogen peroxide. When used in the mouth carbamide peroxide breaks down to give hydrogen peroxide and urea, with hydrogen peroxide being the active whitening ingredient.