Beijing and several other cities across northern China ordered 1,200 factories to cut production or shut on Saturday in response to a blanket of dangerous smog moving over the area.

Half the vehicles in the capital were ordered off roads, schools shut and construction halted after China's environmental body on Friday issued a five-day smog red alert and urged residents to stay indoors.

Levels of microscopic PM2.5 - small, inhalable particles used to gauge air quality - rose above 300 by early evening in Beijing, according to the monitor Beijing Air. 

Levels of PM2.5 in the 300-500 range are considered hazardous. 

The red alert was the first issued this year in the capital of 22 million people. More than 40 cities have issued warnings, with 22 on red alert.

Of the 1,200 factories affected, 500 were ordered to limit output and 700 were forced to shut down. A major oil refinery run by state oil giant Sinopec and steel mills were among those factories to reduce output.

Northern China experiences air quality problems in the winter as coal-powered plants increase output. Choking air pollution highlights the trade-off between rapid economic growth in the world's second-largest economy and health and environment concerns.

cw/tj (AP, Reuters)