Parts of China have been covered under a thick blanket of smog for the last four days.The air quality in China can sometimes be so bad, that the government have to give people warnings about the level of pollution in the air.When the levels are high they can close schools, stop planes taking off, close factories and limit the number of cars on the road.In north and central China, cities like Beijing, are currently under a red alert air pollution warning – the highest level.Millions of people, especially children and elderly people have been warned to stay indoors when the levels are high.