The Rang Dong warehouse fire last month was "a chemical incident that led to an environmental incident," he said. "Pollution following the fire is mainly limited to inside the warehouse."

Authorities have decontaminated polluted areas, Ha said, adding that materials left behind in the warehouse must be dealt with properly.

Professor Jozef Pacyna of the AGH University of Science and Technology in Poland said a map detailing the areas affected by the mercury released from the fire is needed, as it might take several months or years before the element could become toxic.

About 15.2-27.2 kilograms of mercury had been released into the environment following the fire, the Vietnam Environment Administration had said last Sunday.

27.2 kilograms of mercury by itself would not immediately have an impact on human health, Pacyna told VnExpress. The mercury released in the fire was inorganic, and it is its organic form that is toxic, he said, adding that the element could become toxic after it got absorbed in the soil and water and contaminate food sources like seafood.

A map to illustrate mercury concentration would help to visualize the number of people possibly affected by the element, said Pacyna.

Hair samples would need to be tested to determine how much one is affected by mercury, he noted, adding that food consumption is the most likely way mercury is absorbed into the human body.

Pacyna drew special attention to an environment ministry report saying that mud samples in an area about 1 km away from the Rang Dong warehouse was found to contain mercury concentration six times higher than normal. Such a high concentration showed that the element has been accumulating in the soil for a long time, and not immediately after the fire, he said.

The report's finding is a serious problem that has never been investigated before, and required utmost attention and due response from Vietnamese authorities, Pacyna said.

The five-hour blaze that started at 6 p.m. on August 28 destroyed a third of the inventory at the 6,000-square-meter warehouse belonging to the Rang Dong Light Source and Vacuum Flask JSC.

Investigators have so far said that inadequate storage procedures were the cause of the fire. The exact cause is yet to be determined.

People living nearby have been moving out of their homes due to health concerns.

Three hundred and twenty people living in Ha Dinh and Thanh Xuan Trung wards have been tested for mercury poisoning as of Friday, but "none showed signs of abnormal, acute poisoning," Tran Nhi Ha, deputy director of the city Department of Health, said.

Authorities deployed both army and civilian personnel to decontaminate areas surrounding the Rang Dong warehouse Thursday.

Rang Dong, a leading manufacturer of lighting products and equipment, has estimated the loss from the fire at VN150 billion ($6.4 million).