The air pollution that has sullied the city over much of the last 10 days will persist until at least Sunday, the Shanghai Environmental Monitoring Center reported Thursday.

The city's Air Quality Index (AQI) peaked at 246 Thursday, breaching the threshold for severe pollution and forcing authorities to advise residents to avoid spending time outdoors, according to the center.

The AQI has shown good air quality on only two of the last 11 days. The air has been severely polluted on three of the days and lightly or moderately polluted during the other six.

Like during past days of severe pollution, the lack of strong air currents allowed pollutants to accumulate in the city, said Zhao Qianbiao, a monitor with the center. The recent spate of morning fog also contributed to the pollution as the water vapor in the air prevented pollutants from dispersing.

The pollutant most responsible for the increase in the AQI Thursday was PM 2.5, which stands for particulate matter less than 2.5 microns in diameter, Zhao told the Global Times.

PM 2.5 is especially dangerous because the size of the particles allows them to lodge deeply in the lungs, where they can cause or aggravate respiratory illnesses.

The city's PM 2.5 reading rose as high as 280 micrograms per cubic meter Thursday morning. Authorities warn residents to take precautions whenever the reading exceeds 75 micrograms per cubic meter.

PM 2.5 levels fell in the afternoon after the wind picked up as a cold front blew in, but the front was too weak to disperse the pollutants, Zhao said.

The Shanghai Meteorological Bureau issued a yellow haze alert at 1:42 pm Thursday. It warned drivers to be extra cautious on the roads due to low visibility. Authorities also advised residents with heart and respiratory illnesses to spend less time outdoors.