Shanghai braced for a second day of severe pollution on Monday after the city's Air Quality Index hit record hazardous levels.

The city's environmental authority issued its first orange alert — the second-highest in its four-level warning system — after the index reached 317.

The concentration of PM2.5 — particulate matter smaller than 2.5 microns in diameter that can penetrate the lungs — stood at an average of 266 micrograms per cubic meter in the previous 24 hours, running from noon to noon.

It was the third day of severe pollution within a month, and the Shanghai Environmental Monitoring Center predicted the Air Quality Index would remain between 285 and 305 on Tuesday.

The index is an integrated assessment of six main air pollutants: sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, PM10 and PM2.5.

China's meteorological departments issue an orange warning for haze when visibility is less than 2,000 meters and pollution is sustained.

A photo taken by a Shanghai resident at The Bund on Monday morning that was widely spread on social media showed the Lujiazui financial district, across the Huangpu River, shrouded in haze, with only a vague outline of the skyscrapers visible.

Some commuters wore face masks, and some reported having sore throats and eyes.

"I haven't stopped coughing since yesterday, and my eyes are uncomfortable, especially when I'm in the street," said bank clerk Liu Wei, 28.

She said the air purifier she bought in October reflected the poor air quality. "Usually the machine will show the air in the room is clean after working for 10 minutes, but it didn't show the air was clean until it worked for more than an hour on Monday morning, and the light flickered continually to show the air was substandard afterward."

The severe pollution began on Saturday night and became continuously worse, the Shanghai Environmental Monitoring Center said.

The city's education commission published two notices on Monday urging kindergartens and schools to cancel all outdoor activities.

Experts said the pollution was caused by an industrial discharge in the Yangtze River Delta region that was not dispersed by cold air from the north.

"A new wave of cold air from the north will help diffuse the pollutants," said Qian Hua, director of the research institute of atmospheric environment under the Shanghai Academy of Environmental Sciences.

However, the city's location on the east coast means its air quality will be less desirable in winter, he said.

"Winds from the southeast are dominant in summer, and ocean wind is clean. So when the air on the mainland is hit by air from the ocean, the air quality will improve," he said. "In winter, the wind from the north is usually dirty."

Moreover, leaves fall from trees in the cold weather, and that weakens the plants' ability to capture dust, experts said. The pollution worsened on Monday morning because of traffic and power consumption in factories and other workplaces.

"The dry climate also decreases the chance of dust being absorbed by moisture," Qian said.

With the heavy pollution, face masks have become coveted merchandise. Several stores on Taobao, a major online shopping site, that sell face masks claiming to filter PM2.5 particles said business is booming.