I know that Jeff Laudeman stored dice in them for years and never had a problem that I know of. If it is polyester there should not be a problem as long as not exsposed to heat. Polyethelene foam "ethafoam" is supposed to be "archive safe" so polyethelene fiber fill would seem to be the best choice if you could get it?? I hav been looking for a sorce of ethafoam to make dice inserts from.