I am interested in writing a survival-themed interactive fiction story (a sort of "what would you do" scenario game). Obviously, the zombie apocalypse might come to mind as the setting, but this has been done to death (pun intended), and I would like to avoid those tropes.
My alternative scenario is this: a dense, persistent gas/smog descends on a city. Visibility is extremely limited, and the gas does not blow away with the wind; it sits in the streets and open buildings of the city, and remains for the most part at the same density/thickness (i.e. so visibility remains constant, within some degree).
The important part is that the gas is problematic enough that the government is unable to reclaim the city; so it becomes a sort of Pripyat, or otherwise a 'besieged' city, where no one can leave easily. Hence limited visibility is an important obstacle.
The other effect of the gas is that when it was first deployed, it was lethal. A week later or so it is no longer immediately lethal, but is still potent; it is corrosive, and does hideous damage to exposed skin; it does damage to mucous membranes, upon contact; i.e. it will kill you if you are not protected. However, people need to be able to survive with amateur protection, such as scarves wrapped around their face/mouth, since gas masks won't be everywhere (though a month or so of this sort of exposure will kill you off).
I'm not sure whether the gas will suddenly be released onto the city, implying residents will need to survive, and will not be able to easily leave the city (nor will help be easily reached), or whether it is a known issue and the city was evacuated months before, and instead it becomes a hub of illegal activity, S.T.A.L.K.E.R like.
Basically; what sort of realistic 'gas' substance would I want to use here? Most importantly: What sort of gas/smog would persist in an urban environment and not get blown away/diffused by the wind? Secondly: what sort of gas of this type would have the health effects as listed above (mustard gas would seem obvious, but that doesn't meet criterion 1).