In places that are hot and have lots of pollution/ozone (e.g. Los Angeles) foams and rubber and plastics can become brittle and crumble/fall apart much more readily than other places. Sunlight combined with heat and ozone is particularly brutal. Of the places you name, Phoenix is the one I know of that has some pollution/ozone issues but in my limitied time there (springtime, with temps in the 90's) the pollution didn't seem so awful. Tim.