The massive smog cloud choking the eastern provinces since midweek expanded northwards to Beijing yesterday. The air quality index in the capital was as high as 356 at 9pm, ending a week-long streak of blue-sky days. Anything worse than 300 is considered "severe", the highest rating on the six-level rating system. Shanghai residents saw air pollution levels ease slightly. The pollution-monitoring station atop the US embassy in Chaoyang district showed PM2.5 levels - potentially harmful particles less than 2.5 microns in size - of 473 micrograms per cubic metre at 9pm. The World Health Organisation's safe limit for PM2.5 is 25 mcg. The situation was even worse in the surrounding province of Hebei . The AQI in the provincial capital, Shijiazhuang , hit the top of the 500-point air quality index yesterday evening. The National Meteorological Centre maintained the "orange" smog alert it issued nationwide on Friday. Serious pollution was expected to continue in Anhui , Beijing, Hebei, Jiangsu , Tianjin and Zhejiang until this afternoon. In Nanjing , which has been the centre of the current smog episode, the air quality index continued to hover around 300. Schools in the Jiangsu provincial capital have been closed since Wednesday to protect children from the potentially hazardous air. Some 170 domestic flights into and out of Beijing were affected by the smog yesterday and 93 were cancelled, according to the Civil Aviation Resource Net of China. A total of 111 domestic flights were cancelled in Nanjing yesterday, leaving 8,000 passengers stranded, according to CCTV. In Shanghai - a major aviation hub with two international airports - some 483 domestic flights were affected and 144 were cancelled, even as the air quality improved slightly. Authorities lifted a "yellow" smog alert yesterday afternoon after the air quality index drifted below 300. On Friday, the municipal government had closed schools, halted the worst- polluting factories, frozen road works and pulled nearly a third of government vehicles out of service as PM 2.5 levels exceeded 600 mcg. Chai Fahe , vice-director of the Chinese Research Academy of Environmental Sciences, told CCTV that the pollution had to be tackled by all regions simultaneous since smog was not bound by borders. "Every region has to clean its own mess instead of waiting for others to go first," Chai said. Many tried to live with the smog, with a group of catwalk models donning surgical masks during an outdoor jewellery show in Nanjing on Friday.