The Dark box aging

[fancyman]

I was wondering if anyone is finding that their older boxes are aging visibly.

Most notably, my The Dark box is developing yellow splotching under the plastic that wasn't obviously there a year ago. It's been stored in the same place as other boxes of the same era which don't show yellowing (but no The Dark boxes). Is anyone else seeing this?
http://imgur.com/a/9OE1T

In a semi-related note, does anyone know what the old plastic wrapping was made from? I know that PVC slowly leeches chlorine that can damage paper (Source, see section V. "Comic Bags" : http://www.hoboes.com/pub/Comics/About% ... %20of%202/)