"Sometimes, the air near the ground is cold while above it is hot, like a lid on a big pan. The lower air cannot diffuse," he said. This was the cause for serious smog in Beijing in January 2013.

Sources of the smog should be thoroughly studied, Yang said, adding nine government bodies, including the Ministry of Science and Technology and the Chinese Academy of Sciences, are working on the matter.

Lu Hao, chairman of the NPC Environment and Resources Protection Committee, said farmers' burning straw should be regulated.

"However, if the straw is not burned, farmers will have a problem as there is no place to store it," Lu said, adding the government should find a solution to the issue, such as using straw to generate electricity, and that farmers should not shoulder the cost of disposing straw.

Noting that many industries have shifted to less-developed western China, lawmakers warned of air pollution in the hinterland.

China has vowed to strengthen air pollution control. Beijing will adopt a special air pollution control plan for the upcoming APEC meetings, featuring temporary closure of factories in the event of a pollution warning.