Dozens of labourers at a copper smelter in Yunnan are being treated for suspected poisoning after they passed out. The company's deputy manager Yang Yuhe told Xinhua a high concentration of arsenic in raw materials supplied to the plant may have made the workers ill. Poor ventilation and their failure to wear protective masks might have contributed to the accident, Xinhua reported. Provincial health and safety inspectors are investigating the incident at the Dianzhong smelting plant owned by the Dayao copper mine. Fifty-five workers have been taken to hospital. All were in stable condition, Xinhua said. The first of the 55 collapsed at work on Sunday morning, the agency reported. At first, his colleagues thought he had heatstroke. As more workers fell ill, they became alarmed. Yesterday, neither the provincial government nor industry representatives in the southwestern province would confirm the workers had been poisoned.