By Richard Black
Environment correspondent, BBC News website
About 140 million people, mainly in developing countries, are being poisoned by arsenic in their drinking water, researchers believe.
About 50 million people are affected in Bangladesh
Speaking at the Royal Geographical Society (RGS) annual meeting in London, scientists said this will lead to higher rates of cancer in the future.
South and East Asia account for more than half of the known cases globally.
Eating large amounts of rice grown in affected areas could also be a health risk, scientists said.
"It's a global problem, present in 70 countries, probably more," said Peter Ravenscroft, a research associate in geography with Cambridge University.
"If you work on drinking water standards used in Europe and North America, then you see that about 140 million people around the world are above those levels and at risk."
Arsenic consumption leads to higher rates of some cancers, including tumours of the lung, bladder and skin, and other lung conditions. Some of these effects show up decades after the first exposure.
"In the long term, one in every 10 people with high concentrations of arsenic in their water will die from it," observed Allan Smith from the University of California at Berkeley.
"This is the highest known increase in mortality from any environmental exposure."
The international response, he said, is not what the scale of the problem merits.
"I don't know of one government agency which has given this the priority it deserves," he commented.
The first signs that arsenic-contaminated water might be a major health issue emerged in the 1980s, with the documentation of poisoned communities in Bangladesh and the Indian state of West Bengal.
In order to avoid drinking surface water, which can be contaminated with bacteria causing diarrhoea and other diseases, aid agencies had been promoting the digging of wells, not suspecting that well water would emerge with elevated levels of arsenic.
Rice plants absorb arsenic from the soil as they grow
The metal is present naturally in soil, and leaches into groundwater, with bacteria thought to play a role.
Since then, large-scale contamination has been found in other Asian countries such as China, Cambodia and Vietnam, in South America and Africa.
It is less of a problem in North America and Europe where most water is provided by utilities. However, some private wells in the UK may not be tested and could present a problem, Mr Ravenscroft said.
Once the threat has been identified, there are remedies, such as as digging deeper wells, purification, and identifying safe surface water supplies.
As a matter of priority, scientists at the RGS meeting said, governments should test all wells in order to assess the threat to communities.
"Africa, for example, is probably affected less than other continents, but so little is known that we would recommend widespread testing," said Peter Ravenscroft.
His Cambridge team has developed computer models aimed at predicting which regions might have the highest risks, taking into account factors such as geology and climate.
"We have assessments of the Ganges and Brahmaputra river basins, for example, and then we look for similar basins elsewhere.
Arsenic contamination can be a problem in parts of the US
"There are similar areas in Indonesia and the Philippines, and very little evidence of tests; yet where there has been some testing, in (the Indonesian province of) Aceh for example, signs of arsenic turned up."
Asian countries use water for agriculture as well as drinking, and this too can be a source of arsenic poisoning.
Rice is usually grown in paddy fields, often flooded with water from the same wells. Arsenic is drawn up into the grains which are used for food.
Andrew Meharg from Aberdeen University has shown that arsenic transfers from soil to rice about 10 times more efficiently than to other grain crops.
This is clearly a problem in countries such as Bangladesh where rice is the staple food, and Professor Meharg believes it could be an issue even in the UK among communities which eat rice frequently.
"The average (British) person eats about 10g to 16g of rice per day, but members of the UK Bangladeshi community for example might eat 300g per day," he said.
The UK's Food Standards Agency is currently assessing whether this level of consumption carries any risk.