The warning came just a day after a company insisted its water supply was safe for human consumption.

Tests have found that the water, contaminated by used oil, contained high levels of styrene, a substance that is probably carcinogenic. 

People can use the water for other purposes like washing clothes and bathing, but should not drink or use it for cooking, authorities said Tuesday.

Nguyen Van Ton, director of the Vinaconex Water Supply Joint Stock Company (Viwasupco), had said Monday that the water it supplies to Hanoi "meets standards" and "contains no toxin" despite oil contamination warnings.

He said they detected the contamination last Wednesday and immediately hired people to clean it up and informed local authorities, including the police of northern Hoa Binh Province, about the situation. A test performed by the company revealed that the contaminated water’s quality was within safety standards set by the Ministry of Health, he added.

The company hired about 50 people to take the oil out of the canals that divert water into its factory, but one of them reported that the water was heavily polluted and that the company had cheated the workers by not telling them what the actual job was, only saying they had to remove grass and bush from a stream.

Ton said he could not say for sure that the oil contamination had not reached the water used to supply residents. The company provides 300,000 cubic meters per 24 hours for the entire southwestern part of Hanoi, including Thanh Xuan, Hoang Mai, Cau Giay, Ha Dong and several downtown districts.

On reports saying hundreds of thousands of families in Hanoi said their tap water smelled unpleasant and was unusable, Ton said the smell was probably due to the chlorine in the water, since the company had increased its levels to deal with the oil contamination.

Some others reported smelling burnt oil in their tap water, which Ton said was a "subjective perception," adding that the source of the smell would be announced by authorities in the future.

Residents of an apartment complex in Hanoi's Hoang Mai District collect water
from water tankers as their tap water turned smelly and became unusabl ,
October 14, 2019. Photo by VnExpress/Ngoc Thanh.

Higher styrene levels

Meeting with Party chief and President Nguyen Phu Trong Tuesday morning, Hanoi Chairman Nguyen Duc Chung affirmed that the company's water source was contaminated.

Chung said initial tests have found that the levels of styrene in the water were 1.3-3.6 times higher than normal.

Styrene, an organic compound used in the production of latex and synthetic rubber, among other things, is believed to have carcinogenic properties. The World Health Organization has reported cases of leukaemia and lymphoma among workers exposed during the styrene manufacturing process. It can cause irritation of the skin, eyes and respiratory tract and central nervous system. 

Since last Thursday, the daily life of thousands of people in the capital city has been badly disrupted by a burned and pungent smell coming from their tap water. Many have stopped using tap water and are buying bottled water for consumption.

The reason for the smell was revealed to be a significant amount used oil dumped into an upstream section of the Da River, the biggest branch of the Red River, which flows from Yunnan in Southwest China through northern Vietnam to the Gulf of Tonkin in Vietnam.

A truck with a loading capacity of 2.5 tons was seen dumping the used oil into a mountain creek in Phu Minh Commune, Ky Son District of Hoa Binh Province, a northwestern neighbor of Hanoi last Tuesday, the Vietnam Environment Administration under the Ministry of Natural Resources and Environment said Monday.

Viwasupco, which processes water from the Da River and supplies to Hanoi, was aware of the problem even before citizens of Hanoi began to detect the strange smell, but went ahead with the supply anyway, the administration said.

Hanoi authorities have demanded that Hoa Binh and local police find the truck and the driver that illegally dumped used oil into the environment, as well as the company or organization behind the incident. It also said Viwasupco must take responsibility because it was aware that the water was polluted but processed and supplied to residents anyway.