A strange yellow haze has shrouded cities in central China. Residents from Henan, Anhui and Hubei provinces are all wary of the haze, which some experts describe as harmful.

Last week a dark yellow haze enveloped Wuhan city. Days later, the weather forecasting agency issued a yellow alert, and medical face masks quickly sold out.

Authorities say the haze is caused by farmers burning straw. Slogans now hang along the country roads in Henan province warning farmers against the practice. Farmers say they don't know what to do now, because historically they have burned straw to clear their fields.

[Farmer from Henan Province]: "All the straw is still in the field. Nobody here dares to burn it. Nobody."

Zhuang Guoshun, Director of the Atmospheric Chemistry Research Center at the Department of Environmental Engineering at Fudan University disagrees with the authorities. He has a different answer as to what's causing the haze.

[Zhuang Guoshun, Director of Atmospheric Chemistry Research Center at Fudan]: "As to the cause of the haze I have said in many meetings that there are mainly three reasons: first are emissions from motor vehicles and from factories; second are emissions from burning biomass; and the last one is from sand dust. Normally in big cities, the main reason is from emissions of motor vehicles."

Experts warn that the yellow haze may be harmful. It may contain particles that can enter the throat and stick in the respiratory tract and lungs. Patients with respiratory diseases are advised to stay indoors.