Shanghai's air quality improved slightly Sunday after a cloud of pollution sullied the city over Friday and Saturday.

The city's Air Quality Index (AQI) had fallen to 160 as of 8 pm Sunday, after peaking above 200 Saturday, indicating severe pollution, according to the Shanghai Environmental Monitoring Center.

A high pressure system that hung over the city over the weekend allowed pollutants to accumulate in the air, said a monitor surnamed Zhang with the center.

At 160, the AQI still showed moderate pollution in the city, despite a plunge in PM 2.5, which was the primary pollutant responsible for the increase in the AQI.

PM 2.5 stands for particulate matter less than 2.5 microns in diameter. 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 conditions.

The city's PM 2.5 reading rose well above 160 micrograms per cubic meter Sunday morning.

Authorities advise residents to take precautions whenever the reading exceeds 75 micrograms per cubic meter.

The AQI readings began to decline when PM 2.5 levels fell after the southern wind picked up Sunday, Zhang told the Global Times.