Tobbe Arnesson, on 17 December 2010 - 10:01 AM, said:
Why would it be worse for the bricks if they had to endure humid air before they got cold? Naturally the paper items would not like humid air, but the bricks?
Moisture can condense in scratches on the surface of bricks. Unlike most liquids, water expands under normal freezing conditions. This puts stress on the plastic when it is cold and in a very fragile state. This alone usually isn't enough to cause damage unless the environment undergoes a lot of hot and cold temperature cycles.
One of the factors is the speed of the temperature change. Fast changes in temperature creates more stress on any material because the core will expand/contract slower than the surface. That's why when you drop ice cubes in a glass of warm water, the ice cubes will sometimes crack due to a rapid temperature change. It is best to warm up pieces that have been out in the cold slowly.
New, unopened sets are in little danger because they haven't been handled very much (no scratches) and are still in their sealed polybags (factory humidity). The air in the polybags also act as insulation, slowing down temperature changes with the bricks. The polybags play a part in making sure the pieces don't get damaged on the long trip from the factory to the retail stores. TLG can't afford to use temperature and humidity controlled shipping services.