BEIJING - Beijing's air was rated as "hazardous" on Thursday, as north China was enveloped in fog for the fourth day.

The rating comes even though the city continues to gauge air quality through the PM 10 measure ahead of its much-anticipated use of the more sensitive PM 2.5. The Beijing Meteorological Bureau said Thursday that the air in two districts -- Tongzhou and Fangshan -- has been rated "hazardous," meaning particular matter over 500 micrometers in size was detected in each cubic meter of air.

The bureau vowed earlier this month that it would apply the tighter PM 2.5 standard, which measures finer matter considered more hazardous to people's health, ahead of the upcoming Spring Festival which will start on Sunday.

The Beijing Environmental Protection Monitoring Center said on its official microblog Thursday that the PM 10 data soared after Wednesday's evening rush hour. The average reading in the city Thursday was over 300 micrometers per cubic meter of air, which was rated as "medium pollution."

A wave of cold air will drive away the haze and refresh the air quality, but the bureau forecasted that this will arrive no earlier than Saturday, adding clear skies are not expected before Tuesday.