[File photo]

Heavy air pollution engulfed nine cities in China between Oct. 17 and 19, including Beijing and Shijiazhuang in Heibei province, with a total heavily polluted area of 260,000 square kilometers, environmental authorities said.

Due to unfavorable meteorological conditions, heavy smog descended on the Beijing-Tianjin-Hebei region and surrounding areas on Oct. 17. Beijing's average density of PM2.5 reached 244 micrograms per cubic meter on Oct. 19, the Ministry of Environmental Protection (MEP) announced on Wednesday.

A yellow alert for air pollution, the second tier of a four-level warning system, was issued on Oct. 18 in Beijing, the city’s third alert in October. The alert was later cancelled due to improved air quality on Oct. 20.

The Beijing government's heavy air pollution emergency plan states that the government may implement an odd-even license plate scheme to limit cars on the road when the city's pollution alert level reaches its highest tier. Pollution in Beijing is mainly caused by vehicle exhaust and the burning of industrial coal, the Chinese Research Academy of Environmental Sciences reported.

The MEP has dispatched eight investigation squads to supervise the sources of pollution in the Beijing-Tianjin-Hebei region. Some factories in the region are still discharging pollutants beyond the legal limit, and emergency measures to curb air pollution have not yet been implemented at certain construction sites and factories, People’s Daily reported on Oct. 20.