A new blanket of smog covering Beijing and the northern provinces of Hebei and Henan could reach hazardous levels on Friday, the central meteorological authority said on Tuesday.

In some affected areas, visibility could be reduced to 2 kilometers, the Central Meteorological Observatory said.

Air quality in the region has deteriorated since Monday night. The smog, which won't peak until Friday, also could spread to parts of Shandong and Shanxi provinces, the meteorological authority said.

Forecaster Lyu Mengyao said it would be the longest smoggy spell of the year. The last round of severe smog in the northern region, from Jan 1 to 3, resulted in an index-busting PM2.5 reading of 500 micrograms per cubic meter. PM2.5 is particulate matter with a diameter less than 2.5 microns that poses risks to human health.

The observatory said weather patterns — a strong temperature inversion and lowered atmospheric mixed layer — have prevented the air pollution from dispersing. However, winds are expected to arrive on Tuesday and drive the smog away.