There were also 26 people injured in the leak at around 11:00 local time (03:00 GMT), Xinhua reports.
Local media published pictures of fire-fighters at the scene.
China's industrial safety regulations are sometimes ignored by local authorities focused on boosting economic development, observers say.
Local media say that the incident occurred in the city's northern district of Baoshan at a refrigeration unit owned by a seafood company, AFP reports.
- BBC
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