YICHUAN, Sept. 8 -- Thirty-three people have been poinsoned by ammonia gas leaked from a local chemical plant in this capital of northwest China's Ningxia Hui Autonomous Region, local authorities said on Monday.

The accident occured on Saturday afternoon in the Jiemeifengyou Chemical Plant in Ningdong Energy and Chemical production base in Yinchuan, according to sources with the management committee of the production base.

Most of the victims showed minor poisoning symptoms. Four people who were seriously poisoned have been out of danger after overnight emergency medical treatment, they said.

Witnesses saw ammonia gas and highly concentrated ammonia liquids spurted out from a flare piping in the southeast corner of the plant at 15: 45 on Sunday.

The company kept spraying water in the polluted areas to dilute the gas in the air. Currently, the air quality in the periphery of the plant has met the safety standards.

Once inhaled by human beings, ammonia will damage the oxygen carring capacity of blood cells and cause soar throat, tear-shedding, coughs, chest distress, difficult breathing and even the life-threatening cardiac arrest.

Causes of the accident are being investigated.