[drcy]

The major cause of damage to cards is physical handling-- so you if you're taking cards in and out of various holder systems regularly, the physical handling is the biggest potential source.

A serious study of holder, materials etc would be excellent for the hobby, but I think keeping a card in a holder/penny sleeve in dry/dark conditions is good.

As a side note, I got a storage rooms tour of the Chicago History Museum by the head historian, and they have different environmental conditions for different things. Photos are stored in different conditions than say medals. The highlight of the tour was the bed Lincoln died on. It had what they believed were his blood stains, but they weren't willing to do a DNA test yet until it could be done by taking only a very small piece of the cloth (the current required amount of cloth isn't to their liking). Though he said with scientific advancement he assumed it would be done in the near future.