Except that they aren't, because they're made of Polyvinyl Chloride (PVC), which is unstable and will off-gas acidic chlorine vapour. Ever get a 'plastic-y' smell when you open one of these cases? That's the off-gassing in action.
There is a simple test to determine if your Zip-Loc's contain PVC--it's called the Beilstein test, used to determine if a compound contains chlorine, and particularly effective with plastics. Copper wire burns clean by itself, but green in the presence of chlorine, so get yourself a sample baggie, a piece of copper wire, and a small propane torch. Heat the copper wire in the flame of the torch until it burns cleanly. This will burn off any residues that might be on the wire. Make sure that you hold the wire with pliers or an insulator to avoid injury. Touch the hot wire to the baggie. Some of the plastic will melt and be stuck to the wire. Put the wire back into the flame. If the flame burns yellow or clear, no PVC present. If the flame burns bright green, then some PVC is present.
As always, try to avoid breathing any fumes off burning plastic.
For what it's worth, I have my long-term storage figures individually wrapped in acid-free tissue and put in an acid-free cardboard box--and I have post-graduate credentials in material culture preservation, and more than 20 years' experience in storing and preserving museum and archaeological collections.
