Discussed by locals as opaque with yellowish and greenish tinges, the fog descended all of a sudden in the morning, forcing men and women to wear face masks, witnesses informed AFP.
Xinhua news agency quoted the environmental safety department of Hubei province stating in a statement: "Children, the elderly and people with heart or respiratory diseases are cautioned to remain indoors."
Xinhua reported straw burning has been the reason and waived there was any kind of industrial accidents in or near Wuhan, after Internet speculation indicated that there was a blast at the at a chemical complex northeast of the city.
"I checked out of the window of my office and I couldn't believe my eyes," stated resident Li Yunzhong.
"initially I believed it was likely to rain. In 31 years in Wuhan I have not known something similar to it. We're very anxious due to the fact we don't understand what it is."
France's consulate-general in the central city cautioned citizens to stay at home, shut their windows and reduce the usage of air-conditioning.
"The origin of the thick cloud which has covered the city of Wuhan since this early morning is at present unknown," it reported on its website.
