The victims had received free treatment in three hospitals and clinics in Xifeng County, including 14 who reported serious headaches and cramps.

Environment officials in Guiyang, the provincial capital, said the air quality had returned to normal by Wednesday. The sulfur dioxide concentration was 180 to 200 mg per cubic meter, below the national safety benchmark.

The 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 the air on Monday morning due to a malfunctioning treatment device.

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 and slow air movement at the time, said Huang Qiubin, the county magistrate.

As a result, 450 people, including five teachers and 135 children from two primary schools and a middle school, reported respiratory problems and were sent to a local hospital, Huang said.

Local authorities declined to give the exact number of the people still in hospital, saying many were quickly discharged and they are carrying out further investigations into the incident.