Twenty-one of the 246 people who fell ill after an ammonia gas leak Wednesday in north China's Inner Mongolia Autonomous Region remained in hospital Thursday, a local work safety official said.

A poisoned man receives treatment in a local hospital in Chifeng City of north China's Inner Mongolia Autonomous Region, August 5, 2009. [Xinhua]

Among others made ill, 69 remained under hospital observation and 156 had been discharged from hospital by 3 p.m. Thursday, said Zhao Xiangyang, work safety chief of Hongshan District, Chifeng City.

The leak occurred at 8:40 a.m Wednesday. when a truck carrying 30 tonnes of liquid ammonia was unloading at the factory of Chifeng Pharmaceutical Group, but was sealed in about 50 minutes, the Hongshan district government said.

A pipe suddenly burst during unloading. There were no fatalities.

The 246 victims were factory workers, responders to the emergency and residents living nearby.

An initial investigation suggests three factors -- old pipes, failure of emergency valves for sealing in the ammonia tanker and overloading of the truck -- were responsible for the leak, said Zhao.

The Environmental Protection Bureau of Chifeng City said the leak did not pollute the environment.

(Xinhua News Agency August 6, 2009)