[quote=Monochromatic Rain]
if anyone wonders why the US has the most cities, it's simply because I live in the US and am more familiar with it.
[/quote]

Actually with two on east coast and one in the Midwest it makes sense that the US could have 5, the other countries(like Russia) have their populations more heavily focused in one area.

Also, could you just say that all the pollutants caused poisonous gas clouds to form which engulfed the earth(because there is a delay between when pollutants are released and when they reach the "upper" atmosphere.