GUIYANG, April 25 (Xinhua) -- More than 450 people hospitalized after a sulfur dioxide leak from a chemical plant in southwest China's Guizhou Province have now been discharged, local authorities said on Wednesday.

The chemical plant, which produces triple super phosphate (TSP), a chemical fertilizer, at Xiaozhaiba Township of Xifeng County, leaked a massive amount of sulfur dioxide into the air on April 16when a treatment device malfunctioned.

Sulfur dioxide is a colorless and extremely irritating gas or liquid which can affect the lungs and lead to breathing difficulties. The gas could not disperse quickly because of heavy fog and slow air movement, said county magistrate Huang Qiubin.

More than 450 people, including five teachers and 135 children from two primary schools and a middle school, reported respiratory problems and were sent to hospital.

The local environment authority said it was hard to say how much sulfur dioxide was leaked, but that air quality has now returned to normal. Huang said they will impose "a heavy fine" on the company, and government staffs are looking into compensation for those affected.