Before you think, Toronto! or New York city! try this article on for size.
Norilsk, Russia â€” This city above the Arctic Circle contains the world's largest metal smelting complex and, therefore, some of the world's worst smog. "There is so much pollution going into the air from this place that there is no living piece of grass or shrub within 30 kilometers of the city," Fuller says. "Contamination [with heavy metals] has been found as much as 60 kilometers away."
It disturbs me how (reportedly) easy some of these problems are to solve, or lessen. Maija points out that China have the resources to rebuild most of their factories and could run them pollution free, but they just don't care. I can't wait till I'm a wealthy philanthropist...
The Blacksmith Institute (who did the research) have a Google-map fed view of the "dirty 30".