Residents of Jianbi township of the city's Jingkou district reported around 10 am that they smelled irritating gas and had a hard time breathing.
Some residents were immediately taken by ambulance to downtown hospitals.
Zhu Wenzhong, director of the district's environmental protection bureau, said the gas leak was the fault of workers at the Zhenjiang Sopo Chemical Industry New Development Co, which makes chemical products and raw materials.
"The leak lasted about five minutes," Zhu said. "The company turned off the device that leaked the sulphur dioxide immediately after noticing the problem. The pollution was halted, and residents could not smell any gas by the afternoon."
The city's Jiangsu University showed that the amount of sulphur dioxide in the air did not exceed the country's legal limits. Of the seven air-monitoring sites the environmental bureau has around the company, five did not detect any sulphur dioxide, and two detected only small amounts of it.
But some residents disagreed, complaining on the Internet that they couldn't breathe and the density of the sulphur dioxide wasn't low.
"Some people even said on the Internet that they smelled chlorine, which was totally wrong," Zhu said.
"Many people who are now in the hospital were not sent by the ambulance. They just went to hospitals for a health check on their own, to make sure that there was nothing wrong."
Zhu said the company will not be allowed to resume operations until the problem that created the leak has been solved.
"Sopo will be punished for the leak after the environmental bureau's evidence collection and investigation are done," Zhu said.
Company Chairman Song Qinhua apologized to the public Thursday afternoon.
"We'll investigate the leak and do what we can to keep this from happening again," Song said.