Shanghai will continue Tuesday to endure the severe air pollution that descended on the city Monday, which marked the first time that the Air Quality Index (AQI) exceeded its highest threshold.

The AQI hit 317 at noon Monday, breaking the threshold of 300 for "very heavy" air pollution. At that level, even healthy people can develop "intense symptoms or certain illnesses," according to the Shanghai Environmental Monitoring Center, which calculates the AQI.

The Shanghai Environmental Protection Bureau ordered industrial enterprises to reduce emissions after issuing a heavy air pollution warning at 7 pm Sunday, according to local media reports and a statement from the monitoring center.

The primary pollutant affecting the AQI was PM 2.5, which stands for particulate matter that is 2.5 micrometers or smaller in diameter. PM 2.5 is especially dangerous because the small size of the particulates allows them to lodge deeply in the lungs, where they can aggravate respiratory illnesses.

The hourly PM 2.5 level hovered above 280 micrograms per cubic meter from around 8 pm Sunday to 10 am Monday. It began to fall Monday afternoon, but remained at high level.

The air pollution has persisted because stable atmospheric conditions have allowed pollutants to accumulate, said Huo Juntao, a monitor with the Shanghai Environmental Monitoring Center.

According to pollution regulations that took effect in April, major industrial enterprises, such as power and steel plants, must reduce emissions when a heavy pollution warning is in effect. Construction sites must cease activities that create dust, such as digging and transporting construction materials.

However, these measures will not improve the city's air quality in the short term because the stable atmosphere prevents the existing heavy pollutants from dispersing, said Zhuang Guoshun, the director of the Center for Atmospheric Chemistry Study at Fudan University.

The Shanghai Environmental Protection Bureau has urged residents to take public transportation instead of driving their own vehicles. It has also advised children and the elderly to stay indoors and others to cut down on their outdoor activities. It has required people who work outdoors to take extra precautions.

The Shanghai Municipal Education Commission required schools Monday morning to stop all outdoor activities.