China still has a long way to go to make its cities greener and its skies clearer. According to the World Bank, 16 of the world's 20 cities with the worst air are here in China. The smog-filled cities are ringed with heavy industry, metal smelters, and coal-fired power plants.

These are all critical to keeping the fast-growing economy going, but they spew tons of carbon, metals, gases, and soot into the air. Coal is the number one source of air pollution in China. The country gets 80 percent of its electricity and 70 percent of its total energy from coal, much of it high-sulphur coal.

Cars are another leading contributor to air pollution, as car ownership, heavy traffic and low-grade gasoline continue to rise in Chinese cities.Coal and car exhaust are the main causes of health-threatening suspended particulates, followed by construction dust and sand storms.

China still has a long way to go to make its cities greener and its skies clearer.