Rice is a staple food for billions of people around the planet, but a new study suggests climate change may increase arsenic levels contained within the grain.

Rice is a staple food for more than half of the global population. It is consumed on a daily basis by more people than either wheat or maize, also known as corn.

So it is with some concern that scientists have unveiled a recent finding: that as carbon emissions rise and the Earth continues to warm, so too will arsenic levels in rice.

The presence of arsenic in rice has long been known as a problem. Almost all rice contains arsenic. The harmful, naturally occurring chemical can accumulate in the soil of paddy fields, leaching into the grains of rice grown there. But the amounts found in rice grains can vary considerably from well below the recommended limits set by regulatory bodies to several times higher.

Yet, consuming even low amounts of inorganic arsenic through food or drinking water can lead to cancers and a range of other health problems, such as cardiovascular disease and diabetes.

Researchers around the world have been working on ways to reduce the levels of arsenic in rice – and in the meantime, there are ways of cooking it that can extract some of this harmful element from grains (see the box further down the story for more).

But a new study of inorganic arsenic accumulation has found it may become a greater problem due to climate change. The researchers grew 28 different strains of paddy rice at four different locations in China in experimental conditions over a 10-year period.

They found that arsenic levels in the rice increased as carbon dioxide (CO2) levels in the atmosphere and temperatures rise. Then epidemiologists modelled how, at current rice consumption levels, these arsenic levels could affect people's health. They estimated that the corresponding increases in arsenic levels in rice could contribute approximately 19.3 million more cancer cases in China alone.

"Inorganic arsenic has been shown in more studies than I can throw a stick at to be a carcinogen, to have adverse effects with respect to pulmonary health, with respect to cardiovascular health – it's a long laundry list," says Lewis Ziska, associate professor of environmental health science at Columbia University in New York, who co-authored the study. "And two metrics of climate change – the increase in CO2 and the increase in temperatures – are resulting in greater amounts."

It's worth noting that the researchers' worst-case scenario is beyond the high emissions "business as usual" scenario used by the Intergovernmental Panel on Climate Change, the United Nations body on climate change. The most dire predictions assume temperatures rise by 2C, and CO2 levels increase by an additional 200 parts per million between 2025 and 2050. But it does provide a snapshot of what may happen to rice crops in the future if carbon emissions are not reduced.

While the researchers focused on locations in China for their experiments, they say such impacts are likely to be seen in rice grown in regions including Europe and the US too, as inorganic arsenic is common in rice grown around the world.

"We're not the first ones to look at CO2, we're not the first ones to do temperature – but we're the first ones to put them together in the field. And that's what stunned us," says Ziska.

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Of course, the study has limitations, aside from the metrics chosen for the 2050 scenario. For one, it assumed that people will continue to consume the same amount of rice per person in 2050 that they were eating in 2021, even though, as countries get wealthier, their rice consumption tends to drop. On the other hand, it also assumed that people would continue to eat far more white rice than brown rice, like they do now. Because of how it is processed, white rice contains less inorganic arsenic than brown rice – so a shift in the other direction could make numbers even worse.

Still, this is "one of the most comprehensive studies" ever done on the topic, says Andrew Meharg, professor at the school of biological sciences at Queen's University Belfast, and a long-time researcher of rice and arsenic, who was not........

© BBC