** FILE ** A cyclist wears a face mask while cycling through polluted air in Lanzhou, in China’s western Gansu Province, in this Dec. 5, 2006 file photo, as visibility fell to only 300 meters in the morning. According to a study, overseen by the National Academy of Sciences, the result of a 2-1/2 year collaboration between U.S. and Chinese academies of engineering and sciences, China is the world’s biggest emitter of sulfur dioxide; both countries lead the world in their emissions of industrial carbon dioxide, a heat-trapping gas blamed for warming the atmosphere like a greenhouse. (AP Photo/EyePress, File) ** CHINA OUT **

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