Dangerously heavy smog has forced China to shut down schools, ground hundreds of flights and halt road and rail transport. A red alert has been issued in 24 cities across northeast China since Friday. Authorities have urged residents to stay indoors and implemented emergency measures to protect public from toxic miasma. Construction sites have been closed, and there's a restriction on the number of vehicles allowed on the roads. It is reported to be the worst smog spell of 2016.