A blanket of humid, still air resulting in smog that is expected to shroud Beijing for at least three days triggered the capital's first ever pollution "red alert" on Tuesday as many residents ignored warnings to limit their time outdoors. Although smog has always been a public health concern in Beijing, the government's response system has come under extra scrutiny in the past week because it came under heavy criticism for not issuing a red alert during an episode of heavy smog which exceeded hazardous levels. A red alert means that 30 percent of vehicles will be taken off the roads, heavy vehicles will be banned, most schools will be advised to cancel classes, businesses are recommended to implement flexible working hours and all "large-scale, outdoor activities" should be stopped.