The capital's office for severe air pollution emergency response issued a yellow alert, the second lowest, at 4 p.m., with forecast of heavy or serious air pollution for Tuesday night and Wednesday.

Air quality index (AQI) in Beijing hit 236 at 6 p.m. with the index of PM 2.5 exceeding 200, according to the Beijing environmental protection monitoring center website.

Based on China's standard, an AQI between 201 and 300 is considered "heavy pollution", and an AQI of over 300 is defined as "serious".

The office advised enterprises and the public to cut emissions; citizens, especially the elderly and children, to stay indoors; and schools to avoid outdoor activities.

"Currently, the main emissions can be attributed to coal heating and raw coal use during the heating season," said Yu Jianhua, an official with the Beijing municipal environmental protection bureau.