While rumors swarmed online about the cause of the unusual cloud, Chinese police arrested two internet users who said the pollution had been triggered by a chlorine leak at a chemical plant or an explosion at a steel refinery, notes The Economic Observer.

Meanwhile, government authorities told the Xinhua news agency straw burning was the cause and denied there had been any industrial accidents.

Air pollution is increasingly becoming a major problem in China, and the government is often accused of downplaying the severity of the problem in metropolitan areas.