Combo photo taken on January 23, 2013 (up) and January 22, 2013 (bottom) show the tower of the Zhengyang Gate in Beijing, capital of China. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Wang Yueling)
A woman and a baby boy wearing masks are seen in fog-enveloped Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Luo Xiaoguang)
Citizens walk past the fog-enveloped National Stadium, or the "Bird's Nest", in Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Wang Zhen)
Citizens wearing masks walk in fog in Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Luo Xiaoguang)
Tourists watch the national flag-raising ceremony at the fog-enveloped Tian'anmen Square in Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Wang Yueling)
Buildings are seen in fog-enveloped Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Wan Xiang)
Women wearing masks do morning exercises in fog-enveloped Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/He Junchang)
Vehicles run on the fog-shrouded Guomao Bridge in Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/Luo Xiaoguang)
Citizens and vehicles are seen on the fog-enveloped Yinhe Street in Beijing, capital of China, January 23, 2013. The air quality hit the level of serious pollution in Beijing on Wednesday, as smog blanketed the city. (Xinhua/He Junchang)