Choking smog returned to blanket Beijing and surrounding areas yesterday, four days after a dangerous smog alert was lifted. Meteorologists said pollution levels were likely to remain high until after the National Day "golden week" ends tomorrow - and the traffic crush as people returned from holiday would not help the problem. The capital's air quality index registered above 300 for most of the day, according to the Beijing Municipal Environmental Monitoring Centre. That means the air was "severely polluted", the highest on the six-level scale. The National Meteorological Centre issued a yellow alert for haze for Beijing, Tianjin and parts of Shanxi and Hebei yesterday evening. The US embassy's air-quality monitoring station showed that Beijing's air had been "very unhealthy" or "hazardous" since Friday night, with the level of PM2.5 - tiny particles that pose the highest health risks - reaching 372 per cubic metre at 9am yesterday. The World Health Organisation recommends a daily limit of 20. Weather officials said a simultaneous rise in humidity and drop in wind had prevented pollutants from dissipating. Beijingers went online to vent their frustrations, with many complaining the heavy pollution had arrived even before the city's central heating was turned on. "It is very depressing to wake up to this kind of weather, especially on holidays," one internet user wrote on Sina Weibo. Another said: "October's supposed to be the best time of year, but even that's gone now. I wonder how bad it'll get this winter." Xinhua yesterday said the capital would replace four coal-burning heating plants with natural gas ones by the end of next year. Building the new plants, and some 40 other related projects, would cut annual sulphur dioxide emissions by 10,000 tonnes.