Beijing's once-welcome qing shuang, or cool and crisp, autumns seem to be something of the past as a pall of heavy smog has once again descended over the city. A Central Forecasting Bureau forecaster, Yang Guiming, said a lack of wind had resulted in warm air from southern China settling over Beijing. He said these conditions were typical in the capital in the run up to 'deep autumn'. A policy researcher with the Sinosphere environmental consulting firm in Beijing, Ma Jun, said early autumn was once a period of clear skies, warm days and cool nights. But in the past decade, smoggy days have spread to all four seasons thanks to rising pollution levels. On August 2, for example, one of Beijing's hottest days this year, thick fog blanketed the city. The same weather covered Beijing for a week in October last year, he said. Coal smoke and increasing emissions from motor vehicles and dusty construction sites have added to this week's fog, threatening the health of anyone doing heavy exercise outdoors, Mr Ma said. The pollution index was about 230 on Thursday, with 100 the ceiling for healthy air quality. City workers were also seen wetting down streets to keep dust under control. Beijing residents said they noticed the pasty skies and heavy air this week. 'I realised it's fog, and the moisture is heavier than usual,' said Beijing-based newspaper editor Ye Jun, who added he was coughing more than usual. However, Mr Yang forecast some relief. He said the smog should start clearing by today, when a cold front moves in from the north.