In this photo by Andy Wong taken with a slow shutter speed, heavy traffic clogs Chang'an Avenue under a Beijing skyline shrouded with haze pollution. The air quality index climbed above 200, which is considered severely polluted and requires people to wear protective gear outdoors. Beijing's overall air quality has improved as compared to the past several years, but it remains a serious challenge to local authorities. In late August, authorities closed hundreds of polluting factories and ordered half of the city's cars off the roads to ensure a blue sky for a military parade on Sept. 3. Air pollution has worsened noticeably since the temporary measures ended.