Beijing is battling its fourth bout of smog this month, as air pollution levels rose yesterday morning and reinforced fears that foul air could become the norm in the capital during winter months if drastic measures are not taken to address the problem. Residents awoke to find more acrid air enveloping their city, and municipal weather authorities responded by issuing a "yellow" smog warning as visibility dropped below 3,000 metres. Pollution readings by the local environmental watchdog, as well as the US embassy, showed the air quality at a "hazardous" level for most of the day. The US embassy's air quality readings, published on its Twitter feed hourly, showed the level of health-threatening PM2.5 - particles with a diameter of less than 2.5 micrometres - above the "hazardous" level of 300 micrograms per cubic metre for most of the day and into the night. The air quality was bad enough for the Beijing Municipal Environmental Monitoring Centre to warn the public to go outside as little as possible for the next two or three days, as the air would not improve. The centre is under the city's Environmental Protection Bureau, which said the poor air quality index yesterday was largely due to excessive levels of PM2.5. Zhou Rong, a Greenpeace clean-air campaigner in Beijing, said authorities must do more in terms of addressing high emissions. Air quality is increasingly in the hands of Mother Nature during the winter, Zhou noted, as pollution increases significantly on days with little to no wind. "Authorities should consider regulations to allow workers to work from home on such bad air days, to minimise the impact on people's health," she said. Yesterday's resurgence of smog again made pollution a hot topic in cyberspace. One internet user questioned whether the air quality inside was better than it was outside. "Why not go about life as usual if there is not much I can do?" he said.