Smog in China's capital Beijing has climbed back into the unhealthy range as the second pollution red alert of December goes into effect.

A wave of smog settled over the city of 22.5 million on Saturday and is forecast to last into Tuesday.

Levels of PM2.5, the smallest and deadliest airborne particles, rose as high as 303 in some parts of the city and are predicted to top 500 in coming days. That is more than 20 times the level that is considered safe by the World Health Organization.

Schools have been ordered closed and half the city's cars forced off the road on any given day. Barbecue grills and other outdoor smoke sources are banned and factory production restricted.

The smog is largely blamed on coal burning power plants.