A man wearing mask is at Tian'anmen square in Beijing on Wednesday. [Photo/CFP]

The Beijing Meteorological Bureau on Wednesday morning issued a code yellow alert to warn residents of smog in several districts via its weibo account, according to people.com.cn.

It is the first smog to hit the capital since the APEC Economic Leaders' Week ended on November 12.

The alert warned that Daxing, Tongzhou and Fangshan districts would be shrouded by smog from Wednesday to Thursday.

Another forecast by the bureau on Wednesday morning said air pollution across the capital would be worse in the evening, as high pressure had been preventing pollutants from dispersing.

During the APEC meetings, Beijing had been free of smog as emission-heavy production drew to a halt and half of cars across the capital were banned to lessen pollution.

Peng Yingdeng, an environmental protection expert, believes improving the air quality in Beijing is a long-term project. He said: "It's impossible to expect Beijing's air will change fundamentally through some short-term temporary regulations and measures."

According to Peng, the blue sky during the APEC shows that temporary measures can work to decrease the air pollution, which could be referenced as the method for long-term air pollution prevention.

The bureau issued a code blue alert of air pollution in the city at 3pm, according to an update on chinanews.com.

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