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John SIf the slabs are not 100% air tight I would think that the slabbed cards would be in greater danger in a damp environemnt than penny-sleeve top-loaded cards. Once moisture would enter a slab it would be almost like a terrarium...very difficult to escape. Thankfully I have yet to see a scan of mold/mildew growing on the inside of a SGC or PSA holder.
Barry, I was very disappointed upon entering my bank as they had obviously not prepared adequately for extra humid weather. I might have to consider a move...my third in three years.