Two factories were ordered to halt production near the school where about 200 pupils fell sick after an unidentified gas leaked in southwest China Thursday morning, said a local government spokesman.

The two factories, an electrolytic zinc plant and a lead and zinc washery, are located in the upstream area of a river which flows through Xingjing County of Sichuan Province. The river is not a water source for local residents.

There are no chemical plants near the school.

Initial investigation by local health department said the students might have been poisoned by gas discharged from the lead and zinc washery.

Students at Xinmiao Township Primary School in Xingjing County, Ya'an City, said a smell of bad eggs and something burning assailed them at about 9:00 AM before classes began. Many soon suffered headaches, vomiting and chest pains, said the spokesman from the city government.

The smell was similar to a mixture of carbon monoxide and sulfureted hydrogen, two exhaust gas which might have been discharged from the washery, according to the investigators.

About 200 students were sent to the county's two hospitals for checkups. Twenty-one were diagnosed with poison symptoms and another 110 also remained at the hospitals for further observation. The others have been discharged.

"I smelled something weird at around 9:00 AM and then I felt stomach cramps and got a headache and wanted to vomit," said Li Xiong, a 12-year-old student who was having an infusion at Xingjing People's Hospital, which had admitted 90 patients by 6:00 PM.

"I was told my son was sick and I rushed to the school. Later a minibus brought us to the hospital," said Li's mother, sitting at her son's bedside.

"The children are now in a stable condition, but they will stay in hospital for further treatment and observation," said Shi Jun, head of the hospital.

On April 16, a sulfur dioxide discharge from a chemical plant at Xifeng County in southwest China's Guizhou Province sent 450 people to hospital, including five teachers and 135 children from two primary schools and a middle school.

Investigators later found that a nearby plant, which produces triple superphosphate (TSP), a chemical fertilizer, was responsible for the accident because one of its major sulfur dioxide treatment devices malfunctioned.

(Xinhua News Agency June 22, 2007)