Beijing's municipal government is considering the establishment of six air corridors to ease air pollution, which has returned just days after the weeklong Asia-Pacific Economic Cooperation meetings that gave rise to the term "APEC blue", according to the Beijing Municipal Institute of City Planning & Design.

Beijing's first blue alert on pollution since the APEC meetings was issued on Wednesday. Blue is the lowest of a four-level warning system. A new spell of smog is expected to begin on Saturday, covering major areas of northern China.

As the tight reins on production and vehicle exhaust were relaxed after the APEC meetings, pollutants accumulated again, with heavy pollution on windless days, said Zhang Dawei, director of the Beijing Municipal Environmental Monitoring Center.