Cars move on the Chang`an Avenue in Beijing, capital of China, Feb. 21, 2014. Beijing upgraded the haze alert from yellow to orange on Friday. The city has a four-tier alert system, using blue, yellow, orange and red in order to indicate the air pollution level of increasing severity. (Xinhua)

　　People wearing respirators walk on a street in Beijing, capital of China, Feb. 21, 2014. Beijing upgraded the haze alert from yellow to orange on Friday. The city has a four-tier alert system, using blue, yellow, orange and red in order to indicate the air pollution level of increasing severity. (Xinhua/Hu Linqing)

　　People visit the Hall of Prayer for Good Harvests at the Temple of Heaven Park in Beijing, capital of China, Feb. 22, 2014. Smog continues to blanket China`s north and central regions on Saturday as the National Meteorological Center (NMC) issued a yellow alert for air pollution. (Xinhua/Jiang Kehong)