A man wearing a mask takes a picture in front of Tiananmen Square in Beijing on Wednesday. Photo: CFP

Smog shrouded Beijing on Wednesday after blue skies lasted more than a week in the city after the APEC Summit, during which clear skies were dubbed "APEC blue."

Beijing air control authorities have issued a hazardous air alert and urged residents to stay indoors as smog enveloped the city on Wednesday.

The city's severe air pollution emergency response office issued a blue alert, the lowest of a four-level warning system, at 3 pm, for Wednesday and Thursday, blaming regional pollution and unfavorable weather conditions for the build-up of pollutants. Beijing's air quality index hit 348 at 4 pm with the index of PM2.5, the smaller but more harmful particles, also reaching the same level, said the website of the Beijing Environmental Protection Monitoring Center.

 Prior to the smog, Beijing experienced an unusual two weeks of clear skies, with the help of cold fronts and a spate of measures like traffic control and shutting down factories and suspending construction activity and the "APEC blue" was achieved. The monitoring center's director Zhang Dawei said that after the APEC meetings, temporary measures to curb smog were stopped and the pollutants rapidly accumulated.

Zhang said that because of the large amount of pollutants being discharged in Beijing, the air quality will suffer within two or three days. "All we can do is wait for a new cold front to clear the air," he said. 

The air is forecast to gradually clear up on Thursday night when a cold front arrives.

In response, the Beijing government has ordered factories to take further measures to cut emissions and advised residents to stay indoors and wear masks while outdoors.

Children, the elderly and those who suffer respiratory and cardio-cerebrovascular diseases are urged to reduce outdoor activity.

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