Fifteen people were killed when ammonia leaked in a seafood refrigeration plant in Shanghai's Baoshan District yesterday.
The accident also left 25 people requiring hospital treatment for the effects of the caustic gas, including five in a critical condition, local authorities said.
Doctors said most victims suffered internal burns after inhaling the highly corrosive fumes, while some of those who died had severe facial burns.
All the dead and injured were in the plant at the time of the leak, and authorities said there was no danger to the wider population.
The accident happened at about 10:50am at Shanghai Weng's Cold Storage Industrial Co Ltd on Fengxiang Road, the municipal government said in a statement.
The injured were rushed to Baoshan's Dachang Hospital. Last night, five were still in critical condition, the local government said.
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Thirteen people died after being taken to the hospital, and another two died after they were transferred to Shanghai Liqun Hospital.
Three people in a critical condition have been transferred to Tongji Hospital and Changhai Hospital, according to a report by eastday.com.
Doctors at Dachang Hospital said most of the victims suffered respiratory tract burns after breathing in the ammonia.
Some victims had burns to their skin, and a number of those who died had severe facial burns, making identification difficult, said doctors.
The report said those who suffered light injuries were given oxygen and transfusions at Dachang Hospital and some were able to leave afterward.
More than 200 firefighters in 25 fire vehicles and 100 police were dispatched to the scene, where firefighters sprayed water to dilute ammonia in the air.
A chief firefighter told the Xinhua news agency that there were no naked flames after the leak, so the team carried out rescue work immediately and closed down valves that are suspected of having been the source of the leak.
"It was so pungent that I could hardly open my eyes," said a 23-year-old worker, surnamed Jiang.
Jiang and other workers were sleeping when someone knocked at the door and told them there had been an ammonia leakage. They then ran out immediately, said Jiang.
Nurses at Dachang Hospital said a pungent odor filled the air when the patients arrived.
"I was awakened by a harsh smell," Li Qiufen, a hospitalized worker, told Xinhua. "I could not breathe. I stumbled out of the room and found the smell coming from the workshop. There was a clamor to escape and chaos."
"There was no place to hide. I rolled on the ground, choking," said another injured worker surnamed Wang.
Severe burns
Wang Xiaofei, spokesman for Tongji Hospital, told Shanghai Daily that two patients were sent to the hospital at noon yesterday. Both had suffered severe burns but were in a stable condition following treatment.
The majority of workers at the plant are women from neighboring provinces. Many have worked for Shanghai Weng's for just a couple of weeks, the Xinhua news agency said.
Liquid ammonia is widely used as a refrigerant because it can absorb a large amount of heat. However, the colorless liquid must be stored under high pressure or at a very low temperature because it boils at minus 33.5 Celsius degrees.
The cause of the accident is still under investigation, the city government statement said.
A report on Xinmin Evening News website said the accident was caused by a pipeline leak in a workshop, while a report by China Central Television said the leak occurred in an ice glazing workshop in a food processing unit inside the plant.
Shanghai Weng's Cold Storage specializes in import and export, logistics, purchasing, processing, cold storage, sales and international trade of seafood products, according to its website.
It has a cold storage covering an area of more than 30,000 square meters and a 3,000-sq-m temperature-regulated storage and a processing workshop for frozen foods.
During the rescue, workers in the plant were evacuated and traffic police cordoned off Fengxiang Road.
Environmental emergency monitoring vehicles were also sent to monitor if there were air pollution. Baoshan government said no environmental pollution was detected and the plant was later reopened to staff.
Han Zheng, Party secretary of Shanghai, instructed the work safety administration to find out the cause of the accident and publish their findings as soon as possible.
As liquid ammonia is volatile and high corrosive, it has a high incidence of causing chemical accidents.
In June, 121 people were killed and 76 injured in an explosion caused by a liquid ammonia leak in a poultry company in Changchun City of northeast China's Jilin Province.
And last October, 479 people were poisoned in an ammonia leakage attributed to worn machine parts in Honghu City of central China's Hubei Province.