Police in Thanh Hoa Province are investigating a suspected water poisoning case in which hundreds of workers at local companies were sent to hospital after drinking water from the same source.

 Worker admitted to Hop Luc General Hospital on May 15

The workers affected were all from the Hong Fu Footwear Company and the Hong My Company. According to the workers, they drank more water than usual during their breaks on May 15 because of the heat, and within five to ten minutes they started experiencing headaches, nausea and itchiness in the throat. Hundreds of workers were admitted to Hop Luc General Hospital.

The labour union in Thanh Hoa said the companies employ a combined 16,000 workers. In March of 2011,  hundreds of workers at Hong Fu Company were also hospitalised as a result of food poisoning after eating at company's cafeteria.

The vice head of the provincial Department of Health, Vu Quang Vinh, said Hong Fu Company promptly reported the incident to the local Department of Health, and about half of the 735 workers recovered the same day. There were no fatalities.

The Department of Health has collected samples and sealed the water sources. However, initial test results show no signs of heavy metal poisoning. They also conducted tests involving a lab rat who has shown no signs of poisoning.

Colonel Bui Hong Ky, deputy head of police force in Thanh Hoa, said that the situation was complicated by the large number of victims' relatives and concerned citizens who gathered around the companies. Police presence had to be increased to ensure control of the situation.

The police are continuing their investigation into the case to rule out deliberate poisoning. All workers have been allowed a three-day leave with pay.