This slideshow from Popular Science looks at some of the most heavily polluted cities in the world.
"The World Bank estimates that 16 of the world's 20 most polluted cities are found in China's industrial areas."
"In fact, according to a study by Italian environmental group Legambiente, Milan has more smog than any other city in Europe and the continent's second-highest level of ozone. Most of the problem comes from the city's love of driving, but that's changing quickly: Congestion pricing in downtown Milan implemented in January has dropped traffic by 26 percent and, residents hope, will lead to drops in smog as well."
"Pittsburgh recently wrested the title of America's most polluted city from Los Angeles-at least when it comes to short-term particle pollution like soot, aerosols, heavy metals and exhaust. But the city of 335,000, which has transitioned from an industrial town to one of the country's most livable cities, may not be to blame for its bad air. According to some research, much of Pittsburgh's pollution is blown in from factories and power plants in Ohio."