BEIJING (BLOOMBERG) - Acrid-smelling smog rolled back into Beijing, shrouding the city of 20 million people in a gray haze four days after northern China reported the worst pollution in a year.

Beijing authorities issued an orange alert - the second highest warning level - on Saturday before smog levels rose so that factories could be prepared to shut down. The alert requires schools to cancel outdoor activities.

Air quality was "very unhealthy" as of 10 am on Monday (Dec 7), according to a US Embassy monitor, and clear skies were not expected again until after the smog peaks on Tuesday or Wednesday.

Monday's bad air, coupled with five days of hazardous pollution on Nov 27-Dec. 1, raised fresh concern about the government's ability to tackle air quality despite repeated statements from leaders that cleaning up the environment in the country is a top priority.

Last week, the concentration of fine particulates that pose the greatest risk to human health rose to 666, more than 25 times World Health Organization-recommended levels.

The latest round of bad air was the result of "factory discharges and unfavourable weather conditions," the state-run China Daily reported, citing National Meteorological Centre Senior Engineer Xue Jianjun.

China will strengthen inspections of polluting factories, Environmental Protection Minister Chen Jining said, according to China Daily.

Last week, Beijing's traffic authority said it would consider a congestion fee to ease traffic and smog in the city. The idea was met with impatience from China Daily's editorial page, which said such a fee should have been put in place long ago.

"The severity of the city's traffic and pollution problems leaves no time for policymakers to postpone such moves," the newspaper said.