The Ghiringhelli Specialty Foods site, based in North America, reportedly had a gas leak in a kitchen fed by an external carbon dioxide tank.
Workers at the plant began having difficulty breathing and some started vomiting on Friday. One worker remained in a serious condition on Friday while the remaining people exposed have recovered, said an Associated Press report.
Firefighters at the scene saw workers outside suffering from nausea, respiratory problems, fainting and having difficulty seeing, reported local media.
The plant manufactures private label fresh and frozen natural foods, including salads, pizzas and grilled meats.