The Ideal Solution

While mouth trays are one key differentiator between tray whitening kits, the other key difference between them is in the intensity of the peroxide solution these kits deliver. Because whitening takes place over time with tray whitening, the peroxide is usually in a lower concentration than with in-office procedures, and this isn't a bad thing. Lower concentrations of peroxide minimize side effects and permit a gradual whitening of teeth - so you or your dentist can stop when you've reached the proper shade.

Hydrogen peroxide and carbamide peroxide are the two kinds of peroxide used for bleaching teeth. Hydrogen peroxide is a very strong whitening agent that needs to be fresh to deliver its full power - and it requires dental supervision to be correctly applied. Carbamide peroxide is a more user-friendly, shelf-stable version of hydrogen peroxide–meaning that it isn’t quite as strong and can be kept fresh for a longer time than hydrogen peroxide - an important consideration, especially for do-it-yourselfers purchasing pre-packaged kits.

Still, carbamide peroxide is a strong whitening agent and caution should be used when making the decision to whiten teeth without dental supervision. Unreasonable expectations can lead to the mis-use or overuse of whitening trays and the results may be harmful to teeth and gums.

From : http://www.teethwhiteningreviews.com/artman/publish/all-about-teeth-whitening-trays.php