I would think heat and humidity are the most dangerous.

I am also in agreement with those who do not believe that age, alone... such as the older Vichy and Monaco plaques, has caused the warping, bleeding and discoloration; having seen those plaques in their original state of preservation in collections. Yes, the plasticizers and other additives do break down, though not the sole cause of some ugly pieces we see.

The are articles of advice from collectors in Monaco regarding safe storage posted here (archived).