The research behind this method found that as the teeth get older, there is some sort of phenomenon inside the tooth structure that prevents the oxygen from getting in or doing its job. So, younger teeth are more permeable to oxygen (they are whiter when you are young) and older teeth are less (they become darker) because the oxygenation is more and more inhibited. This whitening method we have available in our office starts with conditioning the teeth first, so the tooth structure is going to become microscopically more permeable to the oxygen, and this oxidation process will break down the darker long-chain stain molecules into shorter molecules that are white. We'd start with at- home bleaching trays that are fabricated in a very special way, so you will not loose the bleaching material in your mouth and get all the uncomfortable taste .This will condition the teeth MUCH more than typical bleaching, so not only are the teeth much whiter after the at-home portion, but the oxygen from the high concentration IN OFFICE session that comes next, can REALLY dive into the tooth structure and WHITENS like you've never seen before!
So, the question : Why can I just do bleaching in ONE session over at the mall or in any office? You just have the answer above. The tooth structure is not conditioned to receive the high concentration of oxygen, there is only dehydration during the bleaching, that gives the sensation of whitening for two days and the original darkness is coming back. And the sensitivity can be very disturbing sometimes.
The conclusion is: if you decide to do whitening, do some research and became informed for what really works and is worth the money!