About coin holders what manufacturers and generations cause toning and such damage to coin ?
As previously discussed here, early holders of the major tpgs were generally not sealed as well as subsequent generations. Electrostatic fusing of the plastic seam is pretty good now. I don't know what testing has been done to prove a level of hermetically sealed proof. Others probably know more. Peace Roy
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Ok. Did holders contain questionable chemicals or plastic mixes that effect coins ?
The effects of PVC, a known coin killer, was known before slabbing commenced o/a 1986 so it was not used in the production of holders. Slabs are not hermetically sealed and coins can be effected by air; particularly temperature and humidity. Anything else that is in the air could also effect them (use your imagination on this one) over longer periods of time. In short, toning is caused by the environments interaction with the surface of the coin. How much effort went into ensuring the holder or label don't negatively effect the coin MIGHT be found through research. I have not seen it discussed here. Your "some say" are opinions. Use Google to find information or become/hire a chemist. Here's one article I found:
USAF veteran 1984-2005