Air pollution will continue to linger in the city Wednesday following two days of moderate pollution, the local environmental authority said.

Shanghai's Air Quality Index (AQI) hit 170 Monday and continued to rise Tuesday, reaching 190 citywide, according to the Shanghai Environmental Monitoring Center. The air was more polluted in the districts of Qingpu, Putuo, Jing'an and Xuhui, where the AQI surpassed 200, which marks the threshold for heavy pollution.

The air quality in the city is expected to improve Thursday, according to the center.

The pollutant most responsible for the rise in the AQI has been PM 2.5, or particulate matter less than 2.5 microns in diameter, according to a staff member with the monitoring center surnamed Zhao. PM 2.5 is especially dangerous because the size of the particulates allows them to lodge deeply in the lungs, where they can cause or aggravate respiratory illnesses.

The citywide PM 2.5 reading was 160 micrograms per cubic meter Tuesday morning, according to the monitoring center. The level began to drop in the afternoon as the wind picked up.

The main sources of PM 2.5 are automobile exhaust, chemical plants and road dust. Sand and smog blown in from neighboring regions also caused an increase in PM 2.5 levels, Zhao said.

Low wind speeds and high humidity helped the pollution hang around the city over the last few days, Zhao told the Global Times.

The local environmental authority reminded residents that children, senior citizens and people suffering from heart and respiratory illnesses should spend less time outdoors while air pollution levels are high.