That would be an excellent way of mixing the two ideas, actually!  That way we could have another contrast of all these well-to-do individuals with a coordinated government, upper class, and technology, and a largely barren world filled with empty cities and roaming survivors as opposed to the overpopulation idea.<br/><br/>I had actually played an interesting game called Metro 2033 a few weeks ago where some unnamed disaster struck, leading most people underground because the air above ground became difficult to breathe.  I'm not sure if the composition of the air was altered or if there were toxic gasses.  Those venturing above ground into the emptied cities for supplies had to wear gas masks with air supplies.  Meanwhile, the gated community could have some sort of field in place to regulate the atmosphere.<br/><br/>Maybe that's a bit<em>too</em>harsh though?  At this point I'm just throwing out different variations on this same idea, since that seems to be where we're narrowing things down to.