About 140 children and their teachers are in hospital after sulfur dioxide discharge by a plant in southwest China's Guizhou Province, a local official said on Tuesday.

A chemical plant, which produces triple superphosphate (TSP), a chemical fertilizer, at the Xiaozhaiba Township of Xifeng County, discharged a huge amount of sulfur dioxide into sky on Monday morning, said Huang Yonghui, executive deputy head of the county.

The sulfur dioxide, a colorless and extremely irritating gas or liquid which can affect the lungs and lead to breathing difficulties, could not disperse quickly because of the heavy fog at the time, said Huang.

As a result, five teachers and about 135 students from two primary schools and a middle school reported respiratory problems and were sent to a local hospital, Huang said.

An unspecified number of children and teachers were still receiving treatment at the hospital, including six children in "a serious, but not critical condition", he said.

He declined to give the exact number of the people still in hospital, saying many were quickly discharged.

Local authorities are carrying out further investigations into the incident, he said.