Bleaching or teeth whitening is a process that uses Carbamide Peroxide gel to lighten teeth. An impression is taken of your teeth and a custom tray is filled with the gel and is worn during the day or at night. The process takes about one or two weeks. There are some over the counter bleaching products that have the same active ingredient but in a lower concentration. These products lighten teeth but at a slightly slower rate. Bleaching can sometimes cause teeth to become sensitive to cold or even air. This condition is usually temporary and after a couple of days of not wearing the trays the sensitivity goes away.