Nguyen Nhat Cam, director of the Hanoi Center for Disease Control, said all eight tap water samples collected Monday met safety standards for all 107 parametric values including styrene, a carcinogenic that had been found in excessive quantities in supplies to southwestern Hanoi.

"All values are within allowed limits. However the samples were collected three days ago, so we still recommend that people do not use the water supplied by the Da River water plant from Wednesday night for drinking or cooking," he said, adding that the city had instructed that the water supply resumes so people could clean their water tanks.

The center is still working with the National Institute of Occupational and Environmental Health to collect water samples daily, and another announcement would be issued once the water is safe for consumption, Cam said.

Regarding the concentration of styrene 1.3-3.6 times the allowed limit in samples collected Thursday and Friday last week, Cam admitted that while Vietnam has had no specific study on the topic, such high concentration has the possibility of affecting people's health.

Styrene is a colorless oily liquid that does not dissolve in water, evaporates easily and has an unpleasant odor in high concentrations. The chemical compound could be found in vehicles' exhaust smoke, rubber, plastic, foam food containers and cigarette smoke.

According to U.S. and European studies, when inhaled, styrene at concentrations of more than 1,000 times higher than levels normally found in the environment is harmful. When consumed orally, half the amount of styrene entering the human body gets broken down and leaves the body within eight hours.

Hanoi authorities have asked the Vinaconex Water Supply Company (Viwasupco), the company responsible for supplying the contaminated water, to supply water for free until all affected residential areas finish cleaning their water tanks and pipes; and until the water is tested again and found safe for consumption.

The city is also working with authorities in neighboring Hoa Binh Province, where Viwasupco's water source was contaminated, to decontaminate the source. The test results of Viwasupco's water quality would be updated and publicized daily, officials have said.

On Tuesday last week, a 2.5 ton tanker was seen dumping used oil into a creek in Hoa Binh Province that feeds into the Da River, which is sourced by Viwasupco to process and supply clean water to 10 districts in the southwestern part of Hanoi.

The oil contamination led to the company’s tap water stinking, and authorities said Tuesday the malodor was caused by high concentrations of styrene in the water.

On Thursday, Viwasupco restored its water supply after a one-day suspension for cleaning its pipes and reservoirs.

Police in Hoa Binh Province have opened a criminal probe and are looking for the truck and the driver that illegally dumped used oil into the environment, as well as the company or organization behind the incident.

They have also said Viwasupco must take responsibility for not suspending the water supply to residents immediately despite knowing about contamination at source.

Viwasupco currently supplies 300,000 cubic meters of water per day. Around one million people, or one eighth of the capital city's residents, depend on its supply.