Heavy fog with a visibility of around 1 km persisted in Beijing for a third day Wednesday, keeping the city's air pollution at the most hazardous levels measured this year.

Heavy fog persisted in Beijing for three days. [CFP]

The municipal environment bureau's readings indicated the heaviest air pollution on Wednesday was monitored in Daxing District in southern Beijing, where the air quality index (AQI) reading hit 362.

The AQI ranges from 0 to 500: the higher the number, the more severe the pollution. Readings over 300 are considered hazardous.

The average AQI reading for the city Wednesday was 207, down from 270 on Tuesday and 333 on Monday.

The highest AQI reading over the last three days -- at 394 -- was recorded in the eastern Chaoyang and northern Haidian districts on Tuesday.

The Beijing Meteorological Bureau said a weak cold front would hit the city on late Wednesday to dispel haze. Winds, however, were unlikely to completely clear the air pollution on Thursday.