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Message:: [QUOTE="RickieB, post: 805650, member: 2486"]Does this answer the question if they need to breath or not..well no, it address what happens where they breathe... From my understanding, some TPG have Humidity Controlled rooms in which slabbing is performed..now I can not verify this with 100% accuracy, but what I have heard from some reputable sources say this is the case. My feeling is that unless any holder is non-gas permeable, it breathes. Therefore it must be porous and when examined under high power microscope or even an electron scanning scope pores looks like gaping holes when magnified.I work with some small pore size filtration devices around 1 to 2 microns that effectively keep out potential contaminants, viruses even smaller than that pore size and can flow through easily. I am not sure if Mylar-D which PCGS uses is gas permeable or not..perhaps you should ask them the question. They have to have Public Record of MSDS Sheets on file. A good way to do this would be to ask for a sealed empty sample of the slab..expose it to warm temps and then place it in a refrig..if it condensates inside, well it's permeable material or has a leak. Regards, RickieB[/QUOTE]