Cars pass along a road in thick smog in

Cars pass along a road in thick smog in Beijing on February 21, 2011. Beijing went “beyond” measurable pollution levels, the US embassy said, as a Chinese official warned people to stay indoors and avoid outdoor activities. Air pollution in Beijing has been consistently listed as among the worst in the world by international organisations such as the United Nations. AFP PHOTO/GOH CHAI HIN (Photo credit should read GOH CHAI HIN/AFP/Getty Images)