BEIJING, Dec. 7 -- Shanghai woke up to another choking day yesterday with the air quality index and PM2.5 density at their worst since official records began about a year ago.

But conditions should start to get better from today, forecasters said.

The city’s PM2.5 density surged past 600 micrograms per cubic meter yesterday afternoon, more than eight times the nation’s limit of 75, making the tiny particles the main pollutant.

Average PM2.5 density peaked at 602.5 micrograms per cubic meter at 1pm, while the highest — 627.6 micrograms — was recorded in Hongkou District. PM2.5 refers to airborne particles smaller than 2.5 micrometers in diameter. They are the main cause of urban smog and can cause heart and lung problems.

The air quality index surged past 400 yesterday morning into the severely polluted range and stayed there throughout the day.

The average climbed to 482 at 6pm, with the highest recording of 500 in Xuhui District.

Density of the larger PM10 particles peaked at 671 micrograms per cubic meter at 1pm, more than four times the nation’s limit of 150 micrograms.

The Shanghai Environmental Monitoring Center said a lack of wind and rain made it difficult for the pollutants to disperse and pollutants from the north added to problems which have plagued the city since Monday.

The center forecast severe pollution this morning, with the AQI reaching 350 to 400, and 290 to 350 in the afternoon. But it said the density of pollutants would gradually decline with the arrival of a southeast wind.

Authorities issued a severe pollution alert at 1pm, upgraded from a warning, and launched emergency measures to cut down on emissions.

About a third of government vehicles were taken off the road.

Other measures included the suspension of large-scale public activities and sports events. A Christmas market on Qinhuangdao Road scheduled to open today is likely to be shut down, the organizers said yesterday. Fireworks were banned and work on construction sites and factories suspended. Vehicles carrying construction waste were banned on the streets, and the burning of straw in the open air prohibited.

The Shanghai Electric Power Co suspended operations at a third coal-fired unit yesterday and opened two natural gas units instead.

City authorities called on residents to take public transport and leave their cars at home.

Children, the elderly, and people with heart and lung diseases were told to stay indoors, while others were advised to cut down on outdoor activities and not leave windows open for too long.

The Shanghai Education Commission ordered schools to cancel outdoor activities and said that if students were late or absent their absence would not be recorded. The directive had been in place earlier in the week. Some parents expressed concern that schools and kindergartens had not been ordered to suspend classes, as was the case in Nanjing, capital of neighboring Jiangsu Province, over the past two days.

“It was the second day of severe pollution, so why were classes not canceled since the air quality was that poor?” said Jennifer Chen, whose son is a grade one student at a primary school in Xuhui District.

Fu Yi, a chief service officer at the Shanghai Meteorological Bureau, said the haze should improve today because of the wind.

Moderate haze would turn to light but fog is still expected throughout the morning.

Wind, and the arrival of a cold front from the north, should see pollutants begin to disperse from Monday, forecasters predicted.