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DougHere's a bit of info I pulled off a comic supply site regarding the chemicals in the bags:
"In recent years polypropylene bags have been sold under the guise of being archivally sound. This is far from the truth. Only uncoated and untreated material is suitable for archival protection. Currently, the only way to seal polypropylene is to add a substance called PVDC (Polyvinyl Dichloride which is a relative of PVC) to allow the material to be heat sealed. Therefore, once you add the harmful additive, the sleeve now becomes non archival and should not be used for long term storage."
Apparently this PVDC/PVC coating in the bags is the stuff that's rubbing off on the holders and causing the cloudy spots.