Shanghai is to include air pollution in weather alerts from around the end of the month and the city government will suspend school classes in the event of high air pollution warnings, according to Zhang Quan, director of the city's Environmental Protection Bureau.

The air pollution alerts will be in four levels — blue, yellow, orange and red like the city's current weather alerts — with guidance to residents on each level, Zhang said yeserday.

"The city government will issue air pollution alerts at around 8pm on the previous day and adjust them at 6am the next day to let schools know if they should suspend classes," Zhang said.

The alerts will be published on television, radio, various websites and microblogs.

Schools should suspend classes but remain open for any students who fail to be informed not to go to school.

The bureau is working with the educational authority over what advice to give to students of different ages, he said.

"Currently, it is quite difficult for local schools to decide whether to suspend classes," said Guo Xiong, headmaster of Shanghai Yan'an High School.

He said the school sometimes resumed outdoor activities when the index declined, but found the air quality remained poor because the index was actually the average number over the previous 21 hours.

The city government didn't issue any closure notice to schools over the past week when Shanghai suffered its most serious pollution on record.

Even when the air quality index surged over 500 last Friday, five times the healthy level, no Shanghai school was asked to suspend classes.

Yesterday's announcement was the first time the local authority had said that classes would be suspended on highly polluted days although the central government has said large-scale outdoor activities should be stopped and middle and primary schools as well as kindergartens suspend classes on seriously polluted days.

Serious air pollution has affected Shanghai from November 30, driving the air quality index to record highs. PM2.5 particles, tiny airborne particles that can pose dangerous health risks, remained the main pollutants in the city.

High pressure cyclone

"There are external and internal causes for the long-term and serious pollution days in Shanghai," Zhang said.

A cold front brought polluted air from northern China's industrial cities while a high pressure cyclone south of Shanghai prevented dirty air from being blown further south to the sea.

Internal causes included vehicle and factory emissions, and dust from construction sites.

Vehicle and factory emissions accounted for 50 percent of the city's pollution, followed by dust from construction sites (10.5 percent), power stations (7.3 percent) and straw burning (10 percent).

The other 20 percent was from other provinces, he said.

The city government is to launch a joint campaign with Zhejiang and Jiangsu provinces to curb PM2.5 levels by 20 percent within three years beginning from the end of the month, Zhang said.

"Only regional and even nation-wide corporation can root out the city air pollution problem in the end," he said.