Stockpiles can get quite old (unfortunately I don't have any concrete examples to hand). The US DoD actually runs a shelf life program for testing all sorts of

Stockpiles can get quite old (unfortunately I don't have any concrete examples to hand). The US DoD actually runs a shelf life program for testing all sorts of things to find out if they're still good. To your question, there are many different kinds of masks. For cheap disposables, the foam or adhesive could deteriorate and it would no longer seal to your face properly. Some of the more expensive ones have cartridges; I have no idea what parts might deteriorate and under what conditions, but it wouldn't surprise me at all if they didn't work after sitting on a shelf for 25 years or so. And I have no idea what various filter membranes are made of; chemical stability could conceivably pose a challenge in the long term, particularly regarding what they come into contact with inside the package.