For reasons that are not quite clear, if there's one item that seems to have become visual shorthand for post-apocalyptic settings, it's the gas mask (part or not of a Hazmat Suit). Generally this is due to the air being poisoned in some way (which leads to Fridge Logic if you think about it too much; in real life, poisoned air never lasts that long, and if it does, it might involve things where a gas mask wouldn't be sufficient), although there are occasional cases where the masks are present for no obvious reason. They are wandering round after an apocalypse with the only protection being their own oxygen supply.