Beijing saw some of its worst air pollution in the past six months yesterday, with its skyline engulfed in a blanket of smog. This followed a brief respite during the Olympic Games, when prolonged traffic bans were put in place to clear the air. The air pollution index, which measures air quality from noon to noon, reached 169, considered 'slightly polluted' by national standards. Statistics from the Beijing municipal environmental protection bureau showed 27 out of 28 monitoring stations across the capital recorded figures designated as 'slightly polluted' or 'polluted'. It was the third highest pollution level recorded since the Beijing Olympics, and came just days after the authorities declared Beijing had reached a self-imposed target number of clear-sky days for this year, thanks to the capital's all-out effort to cut pollution ahead of the Games. The capital cleared its smog-plagued air just in time for the Olympics with a series of temporary measures, such as pulling half the city's 3.3 million cars off the road, halting construction and closing factories. But with success comes higher public expectations for good air quality, prompting questions over the government's ability to stamp out pollution, analysts said. Choking smog had returned to the city of 17 million soon after strict anti-pollution regulations were lifted at the end of the Paralympics in September. Under pressure, city authorities adopted a watered-down version of the Olympic traffic rules, banning private cars from roads on one weekday every week. But the new restrictions have not worked. The air pollution index reached 176 on October 18 and hit a six-month high of 186 on November 12. Zhu Tong , an environmental expert at Peking University, said the existing restrictions did little to reduce emissions of airborne particles and sulfur dioxide from tens of thousands of coal-fired boilers, the main source of pollution in winter. 'Beijing still has a long way to go to convert coal-fuelled boilers and use clean energy,' Professor Zhu said. He said stagnant weather conditions, meaning little wind or rainfall, had also worked against anti-pollution efforts. Environmentalists and local residents have long cast doubts over the lasting environmental impact of the Olympics and the authorities' promise that clear skies would remain after the Games. Last week, Beijing said it had reached its target number of 256 'blue-sky days' this year, compared with 100 clear-sky days in 1998. The central government has spent more than 150 billion yuan (HK$169 billion) in the past decade to clean up pollution in the capital.