The leak occurred when a truck carrying 30 tonnes of liquid ammonia was unloading at the factory located in China's Inner Mongolia Autonomous Region.
According to investigations, workers failing to operate the equipment properly and old pipes on the truck caused the leak.
Twenty-one people were hospitalised while 181 others were kept under observation, Xinhua news agency reported.
However, no casualty was reported due to the leak.
Meanwhile, authorities were testing the city's water supply for potential ammonia contamination.