Tooth dehydration (desiccation) is a common side effect with most teeth bleaching treatments, but most frequently with UV light-activated processes. Teeth and dental tissues have water content that the heat of bleaching evaporates, causing teeth to appear whiter. When rehydration happens (usually within a few days), the dehydrated teeth relapse to a darker color. Some whitening methods, such as Deep Bleaching, actively work to minimize dehydration.
