Beijing schools kept children indoors January 14, 2013, and hospitals saw a spike in respiratory cases following a weekend of excessive pollution in China's smoggy capital.

A teacher leads her students doing body exercise during class break in a classroom on a foggy day in Jinan, Shandong province, January 14, 2013. Heavy fog enveloped a large swathe of east and central China, with the city's residents being advised to stay

Cranes atop a residential building under construction in central Beijing, April 18, 2012.

Buildings and the Guomao Bridge are pictured amid heavy haze and smog in Beijing, October 29, 2011.

Visitors to Tiananmen Square shield themselves from the sun with umbrellas on a hot and hazy day, July 28, 2010.

Smoke billows from a chimney of a heating plant in Beijing, February 13, 2012.

Haze blankets the National Stadium, also known as the Bird's Nest, as seen from inside the Olympic Green area during the Olympics, August 7, 2008.

A coal-burning power station at night in Xiangfan, Hubei province, September 15, 2009.

Paramilitary policemen practice drills inside the Forbidden City amid haze in central Beijing, December 4, 2011.

Buildings in Beijing are pictured on a day with heavy haze and smog, October 28, 2011.