Twenty-eight workers at a bioethanol refinery in central Hungary were injured Friday after a chlorine leak, the company that owns the facility said in a statement. The workers became ill after the accident at the Irish-owned Pannonia Bio plant at Dunafoldvar, 80 kilometres (50 miles) south of Budapest. Emergency services told local media that 13 staff were seriously injured, suffering from shortage of breath. A further 15 reported minor symptoms of poisoning such as irritated throats and eyes. Some of the injured were transported to hospitals in nearby cities by four ambulance helicopters. Pannonia Bio said in the statement sent to AFP that the accident occurred when a large quantity of chlorine gas was released into the air after nitric acid was added to a storage tank "for as yet unclear reasons". Production at the plant has continued uninterrupted, the company said. Emergency services added that dangerous levels of the gas have not been measured outside the immediate vicinity of the accident area. As a precaution, the town's mayor advised schools to keep children indoors and residents to close windows. The refinery, in operation since 2012, is "the largest single site bioethanol plant in Europe," according to Pannonia Bio's website. The company processes corn to produce corn oil, animal feed, and bioethanol -- an alternative to fossil fuels. pmu/jsk/adp