Over the past decade, more than 20 percent of deaths recorded in a study that monitored nearly 12,000 people in the Araihazar district of the capital Dhaka appear to have been caused by arsenic-tainted well water.

By some estimates, between 35 and 77 million people in Bangladesh have been chronically exposed to arsenic-contaminated water as a result of a catastrophically misguided campaign in the 1970s.

Millions of tube wells were drilled in the aim of providing villagers with clean, germ-free water. Many wells were inadvertently dug into shallow layers of soil that were heavily laced with naturally occurring arsenic.

The UN's World Health Organisation (WHO) has called Bangladesh's arsenic crisis "the largest mass poisoning of a population in history."

Several previous investigations highlighted the health risks of contaminated groundwater, but they failed to explain how much of the tainted water a person may have drunk and what level of contamination was enough to cause sickness.

In the new study released Saturday, physicians checked the volunteers' overall health and took blood and urine samples every two years. They also took samples of local well water to monitor for arsenic levels.

After six years, 407 deaths had occurred from all causes, 21 percent of which could be attributed to arsenic concentrations above the UN's recommended threshold. Of deaths linked to chronic disease, 24 percent were associated with exposure to the poison at such levels.

The death rate rose in line with the exposure.

Bangladesh authorities reacted cautiously to the study.

"To be frank, I have my doubts about these findings -- I would like to examine their methodology more closely," Bangladeshi Health Minister A.F.M. Ruhal Haque told AFP on Sunday.

"Arsenic is a problem in Bangladesh, there is no question about that, but the risk that contaminated groundwater poses to the majority of the population has been blown out of proportion by this study," he said.

"Tube wells are a problem, but the government has been testing all wells, and when we find a contaminated one, we clearly mark them and stop villagers using them."

The government is distributing a new peat filter which can purify arsenic contaminated water in the worst-affected areas of the country, and it also expanding its surface water cleaning and distribution network, he added.

The study found that when compared to those exposed to the lowest arsenic levels (less than 10 microgrammes of arsenic per litre of water), people with levels of 10-50 microgrammes had a 34 percent higher risk of death.

Those with the highest level of exposure (between 150 and 864 microgrammes) had a 64-percent higher risk of death.

But even exposure at relatively lower levels carried a risk, a finding that is important for other countries -- there are more than 70 of them, including the United States, India and Mexico -- that face a serious arsenic problem.

In the study, 23 percent of the volunteers were exposed to water with up to 10 microgrammes of arsenic per litre, which is the UN recommended maximum.

Twenty-one percent were exposed to concentrations of between 10 and 50 microgrammes, with 50 microgrammes the current Bangladesh standard.

Thirty-one percent were exposed to between 50 and 150 microgrammes; and 25 percent to between 150 microgrammes and the maximum tested level, of 864 microgrammes.

Chronic exposure to arsenic is linked with cancers of the liver, kidney, bladders and skin, as well as heart disease.

Given the long-term effect of arsenic, taking a temporary break from exposure was no solution, for the risk of death remained the same, the researchers found.

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