Under smog alert, Beijing orders factories to shut or cut output

On Friday, China's environmental watchdog issued a five-day warning about choking smog spreading across the north and ordered factories to shut, recommended residents stay indoors and curbed traffic and construction work.

Red alerts are issued when the air quality index (AQI), a measure of pollutants in the air, is forecast to break 200 for more than four days in succession, surpass 300 for more than two days or overshoot 500 for at least 24 hours.

The Beijing Municipal Environmental Monitoring Centre showed an air quality reading of 297 by Saturday afternoon as haze started to envelop the capital.

Traffic on the city's roads was lower than usual as residents complied with limits on car use.

The Beijing municipal government said in a statement on Saturday Sinopec's 10 million tonne-per-year Yanshan refinery, a Shougang Group steel product plant and a Cofco factory that makes instant noodles and crackers were among 500 companies it had ordered to limit output.

The statement also listed 700 companies that had been ordered to suspend operations altogether.

The national environmental watchdog was sending more than a dozen inspection teams to check that factories and heavy manufacturing plants were complying with the crackdown, it said.

The air had been relatively clear earlier on Saturday, with a reading of around 120, much to the relief of Beijing residents.

"The country is taking measures, the city government is taking measures ... and now there's no real pollution, it's really good," said 56-year-old Beijing resident Hou Liqiang while playing tuoluo, a top-spinning game.

The world's second-largest economy has been battling environmental degradation left by decades of breakneck economic growth. The government's colour-graded warning system was adopted as part of its crackdown on smog.

More than 40 cities have issued warnings about smog, with 22 on red alert, including top steelmaking city Tangshan in Hebei province that surrounds Beijing, and Jinan in coal-rich Shandong province.