Are Scoville numbers constant, or do they increase/decrease over time and with heating/cooling?

Scoville numbers (SHU – Scoville Heat Units) are a measure of spiciness in Chilis and peppers.

Are these numbers constants? Once measured, does the SHU change when the spice has been heated in a recipe, or cooled in the refrigerator? Does a SHU remain constant over time? (for instance, a pepper sitting in a bowl for a month)