FOUR workers were killed yesterday in an explosion at a factory in Baoshan District that manufactures and repairs heavy industrial equipment, the district government said.

　　The incident happened about 12:20pm at Shanghai Zhizhuo Machine Equipment Co on Shitai Road, while the men were repairing a machine known as a cathode roller, Shanghai Administration of Work Safety said.

　　The cause of the blast is unknown, though the men were said to be using welding equipment at the time of the incident.

　　The four were identified as Xue Jianyong, Zhou Jun, Wu Jiasheng and Zhang Hongbo.

　　Two other men — Chang Yuejin and Yang Sanping — were injured in the blast and are being treated at Baogang Hospital for non-life-threatening injuries, the district government said.

　　The work safety authority described the incident as a “production safety accident,” but did not elaborate.

　　One of the two men hurt said he was on his way to inspect the work being carried out on the machine when the explosion lifted him off his feet.

　　“I was walking toward it when I heard a loud bang and I fell down,” the unnamed man told Shanghai TV.

　　He said he thought the explosion might have caused by expanding gas left inside the machine the men were working on.

　　The 2-meter diameter cathode roller belonged to a company based in Kunshan, east China’s Jiangsu Province, the work safety administration said.