More than a fifth of deaths in a district of Bangladesh were caused by the country's notorious problem of arsenic-tainted well water, and even relatively low exposure to the poison was not risk-free, The Lancet reported on Saturday.
By some estimates, between 35 and 77 million people 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. By tragic irony, many wells were dug into shallow layers that were heavily laced with naturally-occurring arsenic.
Several investigations have highlighted the health danger from this water, but they have been questioned on methodological grounds.
These probes provided an overall snapshot but not an individual one. In other words, 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.
Seeking to find out, doctors assessed 11,746 people aged 18 to 75 in Araihazar in the Dhaka administrative division.
The physicians checked the volunteers' overall health and took blood and urine samples every two years. They also took samples of the 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.
Compared to those exposed to the lowest arsenic levels (less than 10 microgrammes of arsenic per litre of water), those with levels of 10-50 microgrammes had a 34 percent higher risk of death, and those with the highest level (between 150 and 864 microgrammes) a 64-percent higher risk.
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 around the world, 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.
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.
Chronic exposure to arsenic is linked with cancers of the liver, kidney, bladders and skin, as well as heart disease.
The UN's World Health Organisation (WHO) has called Bangladesh's arsenic crisis "the largest mass poisoning of a population in history."