Vehicles move slowly on the Chang'an Avenue shrouded in the heavy fog in Beijing, capital of China, Jan. 19, 2012. Beijing's air in some districts was rated as "hazardous" on Thursday, as north China was enveloped in fog for the fourth day. A wave of cold air will drive away the haze and refresh the air quality, but local meteorological authorities forecasted that this will arrive no earlier than Saturday. (Xinhua/Liu Changlong)