CHENGDU -- On Friday afternoon, 12 pupils poisoned by an unidentified gas leak in southwest China were still in hospital, and 185 others had returned home, as experts continued to try to pin down the cause of the incident.

Of the 12 students, three are being treated for upper respiratory tract infection but the others should leave the hospital soon, said Shi Jun, head of Xingjing County People's Hospital in Ya'an City of Sichuan Province.

The 197 pupils from Xinmiao Township Primary School were taken to hospital after a smell of bad eggs and something burning assailed them at about 9:00 a.m. Thursday. Many soon suffered headaches, vomiting and chest pains.

The smell was similar to a mixture of carbon monoxide and sulfureted hydrogen, two exhaust gases which might have been discharged from two nearby factories.

The factories -- an electrolytic zinc plant and a lead and zinc washery -- are located only a few hundred meters from the school.

But experts found the factories had not discharged untreated pollutants into the air or water, and speculated that the students might have been poisoned by a mixture of gases.

They said that exhaust gases from the factories might have mixed with gases from sanitary sewage, forming an inversion layer in the atmosphere which helped the mixture to spread.

Most of the students are eight to 10 years old. Six teachers and a cook who were also at the school were not affected, according to Yue Minggui, a teacher of the school.