BBC NEWS ONLINE: 'Alarmingly high' levels of arsenic in Pakistan's ground water

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For David Polya, professor of environmental chemistry at the University of Manchester, there is a "considerable amount of uncertainty in the new figures".

"Even if the population at risk was only half that estimated, it would mean that the estimates of the number of people around the world impacted by such high arsenic hazard groundwaters would need to be substantially upwardly revised," he said.

"This reflects a trend over the the last few decades, where increasing numbers of people have been recognised to be exposed to high arsenic concentrations in their drinking water.

"As further detailed studies such as this are conducted in other areas, no doubt the number of people known to be exposed to this poison through drinking water will further increase."