Several major cities in Central China's Henan province were recently covered by thick fog partly due to farmers burning straw.

Farmers burn straw in their fields after harvesting in Pingba county of Anshun, a city in Guizhou province, on Sunday, Oct 9, 2011. Burning straw is popular in rural areas. [China Daily]

The fog, which occurred about three days ago, covered large areas of the fertile and densely populated North China Plain.

On Sunday in the provincial capital of Zhengzhou, a heavy mist caused 52 flights to be cancelled and 42 to be delayed. A major highway running north-south through the province was blocked for two hours due to poor visibility, Henan Business Daily reported.

The fog also doubled the number of patients with respiratory problems at the No 3 hospital affiliated with Zhengzhou University, the paper said.

Illegal burning of straw should be severely punished, Zhang Dawei, deputy governor of the province, said at an emergency meeting on Sunday.

Zhang said such behavior severely pollutes the air and harms people's health.

Farmers in the province have been burning straw in recent years as an easy way to dispose of agricultural waste. The practice also occurs in other provinces such as Shandong, Anhui and Guizhou, especially in the autumn harvest season.

"Many farmers asked for leave from work in cities to return home for the harvest,' said Hong Fulin, a villager in Songji village, Xiping county in Zhumadian of Henan. "They don't have enough time and energy to transport the straw back home, and they can't use it for anything."

Hiring a grating machine would cost more than 450 yuan ($70) a hectare, which is too expensive for many farmers.

"During harvest time there were many people visiting villages in cars with loudspeakers telling us that it was illegal to burn straw," Hong said.

"But they just said violators will be fined 500 yuan and be detained for 15 days. They didn't explain to us why we can't do it."

Lu Bu, a researcher in agricultural resources and regional planning at the Chinese Academy of Agricultural Sciences, said farmers burn straw because recycling agriculture waste requires extra manpower and money.

"It is not cost-effective for individual farming households to recycle their waste," Lu said.

Lu suggested that local governments should offer subsidies to enterprises that produce organic fertilizers to encourage them to collect straw from farmers for batch treatment.

Burning straw also poses dangers to farmers.

A woman in her 60s in Shangshui county choked to death on Sunday after she inhaled too much smoke while burning straw in a field, the local newspaper of Henan Dahe Daily reported.

In Xinyang, the government encouraged the public to report any illegal burning of straw.