Rice tainted with high levels of arsenic has been linked to genetic damage that heightens the risk of cancer, a study published on Monday said.
Naturally-occurring arsenic in drinking water is a long-known health hazard, especially in Bangladesh, where tens of millions of people depend on wells drilled in the 1970s.
Scientists have also fretted about rice grown in contaminated groundwater, but this is the first time they have found proof of a risk.
Researchers at Britain's University of Manchester and the Indian Institute of Chemical Biology in Kolkata carried out a probe with the help of 417 villagers in India's West Bengal.
They asked the volunteers to provide details about their lifestyle and amount of rice eaten daily, and to provide samples of cooked rice and their urine.
The volunteers came from three different areas but had a similar diet and socio-economic status. All had a low exposure to arsenic through their drinking water.
The researchers sifted through the urine samples to extract cells from the lining of the urinary tract, which were then analysed for genetic signatures called micronuclei.
Micronuclei are tiny pieces of DNA that are left over from when a cell replicates and fails to copy its genetic code properly.
Previous research has found that the more frequently that cells make a mistake in copying the code, the greater the risk that they will become cancerous. A higher count of micronuclei is thus a barometer of the risk.
What they found was that micronuclei frequency rose with increasing arsenic levels in rice. The trend was true both for men and women and for tobacco and non-tobacco users.
The villagers ate around 500 grammes (1.1 pounds) of rice daily on average. Frequency of micronuclei began above consumption of cooked rice with more than 200 microgrammes of arsenic per kilo.
The study, published in Nature Group's Scientific Reports journal, did not have the scope to monitor the volunteers for health problems.
But it says the findings should sound the alarm for people who consume large daily helpings of rice grown in high-arsenic water areas.
Around the world, this could be as many as several hundred million people, it says.
"Although concerns about arsenic in rice have been raised for some time now, to our knowledge this is the first time a link between consumption or arsenic-bearing rice and genetic damage has been demonstrated," said David Polya, a professor at Manchester's School of Earth, Atmospheric and Environmental Sciences, who led the British side of the team.
"It vindicates increasing concerns expressed by the European Food Safety Authority and others about the adequacy of regulation of arsenic in rice," Polya said in a press release.
Ashok Giri, who led the Indian side of the team, cautioned against "any panic from the consequences," explaining that any health risks arose from long-term chronic exposure to arsenic.
More than 3 billion people worldwide eat rice as a staple, according to estimates published last year.
The new study says arsenic levels of 200 microgrammes or more per kilo are found in significant percentages of rice grown in China, Bangladesh, Japan, Pakistan and China as well Europe and the United States.
"This study raises considerable concerns over the threat to human health," it warns.
Independent commentator Parvez Haris, a scientist at the De Montfort University in Leicester, said it was important for people to realise that they do not have to stop eating rice, given its value in nutrition.
"Our own study revealed that some varieties of aromatic rice from Bangladesh are very low in arsenic and can significantly reduce arsenic exposure in humans," he said in an email exchange with AFP.
"Unfortunately, in the absence of regulations requiring labelling of rice packets, showing how much arsenic is present, it would be very difficult for the consumers to select low-arsenic rice from supermarket shelves."
Regulatory bodies should change labelling so that consumers "can make an informed decision," he said.