Beijing was cloaked in smog for the third consecutive day yesterday as the city's weather bureau issued a yellow alert for haze. Some emergency measures promised by authorities earlier have yet to be implemented. The city's air quality index, calculated on five types of pollutants, has stood above 250 since noon on Friday, with much of Saturday and Sunday morning over 300, meaning the air was "severely polluted", the worst on a six-level scale. Monitoring at the US embassy showed the air has been "very unhealthy" or "hazardous" for the past three days, with the amount of PM2.5 - tiny particles thought to be most damaging to health - higher than 200 micrograms per cubic metre. The upper daily limit set by the World Health Organisation is 20. A satellite image from the National Meteorological Centre showed other northern provinces, including Hebei , Henan and Shanxi , also shrouded in heavy smog. Video: Dangerous levels of smog shrouds Beijing and Huainan Weather officials in the city said yesterday that the smog would linger until today, when light rain was expected to clear the sky ahead of the week-long National Day holiday starting tomorrow. The earlier-than-expected return of lingering smog prompted many residents to air their frustrations on social media sites, who noted that such pollution usually started in mid-November when the city's central heating was turned on. "My nose and sinuses feel just terrible," wrote a microblogger at Sina Weibo. "Beijing's air gets worse every year. October is supposed to be the golden season." The smog comes just weeks after Beijing and nearby provinces announced plans to curb air pollution, including a promised emergency response mechanism on the most heavily polluted days. But Beijing authorities appeared to have been caught off-guard. When issuing its yellow alert for haze yesterday morning, the meteorological bureau instructed children, the elderly and those with chronic respiratory diseases to stay indoors, and encouraged others to wear masks when outside. It also suggested that kindergartens and schools cancel outdoor sports activities. An environmental ministry official, Wang Jiang, told China National Radio that emergency measures should be in place when smog was forecast to last for three days. "Officials need to inform the public 48 hours in advance, so that on the third day of smog, factories will cease production and half of all vehicles will be taken off the road," he said.