Climate change is quietly making rice more toxic

Rice has long been a symbol of nourishment and security. Across vast stretches of Asia, it serves not just as a meal but as a cultural constant – present in everyday dishes and sacred celebrations alike. Now, climate pressure threatens that deep-rooted connection.

But a new study led by researchers at Columbia University’s Mailman School of Public Health, in collaboration with teams from China and the United States, reveals a brewing crisis.

As the climate warms and carbon dioxide levels climb, a quiet transformation is taking place deep in paddy soils. This change could soon place millions at risk – not from scarcity, but from toxicity.

The findings, published in The Lancet Planetary Health, detail how a hotter, CO₂-rich atmosphere is likely to increase levels of inorganic arsenic in rice. This toxic compound is linked to a wide array of serious health problems, including cancers, cardiovascular disease, and developmental disorders.

With rice being a staple food for over half the world’s population, the implications are far-reaching and deeply concerning.

Climate factors and arsenic accumulation

Until recently, studies examining arsenic accumulation in rice focused on isolated climate factors. Researchers had explored how elevated temperatures or increased CO₂ levels might influence arsenic uptake, but never both at once in a natural setting.

This new study is the first to simulate real-world growing conditions over multiple years and varieties using Free-Air CO₂ Enrichment (FACE) platforms.

Over a ten-year span, researchers observed 28 genetically diverse rice cultivars across four field sites in the Yangtze River Delta, a region emblematic of Asian rice agriculture.

The FACE system introduced CO₂ levels 200 ppm above ambient conditions and used infrared heating to simulate a 2°C rise in temperature. The results were clear: temperature and CO₂ together had a synergistic effect, raising inorganic arsenic levels in rice grains more than either factor alone.

“Our results suggest that this increase in arsenic levels could significantly elevate the incidence of heart disease, diabetes, and other non-cancer health effects,” said Dr. Lewis Ziska, associate professor of Environmental Health Sciences at the Columbia Mailman School.

Climate change increases arsenic uptake

What makes these findings especially alarming is the mechanism behind them. The study uncovered that elevated temperature and CO₂ don’t just influence rice physiology – they reshape the microbial and chemical makeup of paddy soils.

Increased temperatures lowered redox potential in the soil, creating more anaerobic conditions. These conditions favor the release of arsenic into the water where rice roots grow.

Additionally, key soil microbial genes responsible for transforming arsenic into its more toxic forms became more abundant. One gene in particular, arsC, saw a sixfold increase in presence during the grain-filling stage when both temperature and CO₂ were elevated.

Dr. Ziska explained that the higher arsenic levels are likely due to climate-related changes in soil chemistry that favor arsenic that can more easily be absorbed into rice grain.

Health risks of arsenic exposure

The study didn’t stop at biochemistry. Using the latest population and consumption data, researchers modeled health risks for seven Asian countries: Bangladesh, China, India, Indonesia, Myanmar, the Philippines, and Vietnam.

These countries were selected because of their high rice consumption and the prevalence of irrigated paddy systems, where arsenic mobility is highest.

Their projections for the year 2050 are alarming. Under the combined CO₂+Temp scenario, mean inorganic arsenic exposure through rice was highest in Vietnam and Indonesia.

Lifetime cancer risks, particularly for lung and bladder cancer, saw a 44% rise compared to ambient conditions. China alone may face 19.3 million excess cancer cases related to arsenic exposure through rice.

“Ingesting rice in regions like southern China and Southeast and South Asia is already a significant source of dietary arsenic and cancer risk,” said Dr. Ziska.

Diseases linked to arsenic in rice

From a public health perspective, arsenic is not just a cancer risk. Long-term exposure to inorganic arsenic has been linked to diabetes, ischemic heart disease, adverse pregnancy outcomes, neurodevelopmental delays, and immune dysfunction.

In every country assessed, non-cancer risks also rose sharply under the CO₂+Temp scenario. Hazard quotients for health issues like heart disease and diabetes reached worrying levels.

In Vietnam, for instance, the hazard quotient for these conditions reached 12 – far above the safety threshold of 1. The pattern was similar for fetal and infant health effects and cognitive development concerns.

“From a health perspective, the toxicological effects of chronic iAs exposure are well established; and include cancers of the lung, bladder, and skin, as well as ischemic heart disease,” said Dr. Ziska.

Climate patterns and rice arsenic levels

While the FACE experiments simulated future climate conditions, natural seasonal variations from 2014 to 2023 added another layer of evidence.

Arsenic levels in rice tracked closely with temperature changes across those years, even under current CO₂ levels. This real-world correlation supports the experimental data and suggests that climate-induced arsenic risks may already be escalating.

Furthermore, the presence of organic arsenic compounds like dimethylarsenic acid (DMA) also increased under warmer, CO₂-enriched conditions. Although these compounds were not found in highly toxic forms like DMMTA in this study, their rise raises concerns for future rice safety and yield.

Urgent global action needed

“Based on our findings, we believe there are several actions that could help reduce arsenic exposure in the future,” noted Dr. Ziska.

He and his co-authors recommend urgent attention to agricultural adaptation. That includes breeding rice strains that naturally accumulate less arsenic, managing irrigation to reduce soil anaerobic conditions, and improving milling processes to lower arsenic content in edible rice.

Policy gaps remain a critical issue. Most countries lack enforceable standards for arsenic in rice. For example, under the simulated climate scenarios, over half the rice samples exceeded China’s proposed 200 ppb safety threshold for inorganic arsenic.

Meanwhile, many nations either lack such standards entirely or rely on voluntary guidelines.

Rice safety in a changing world

“Our study underscores the urgent need for action to reduce arsenic exposure in rice, especially as climate change continues to affect global food security,” said Dr. Ziska.

Rice is too important to ignore. It sustains billions and shapes cultures. But as this study reveals, it can also quietly carry a growing threat when climate stress meets overlooked soil dynamics.

The good news? The problem is not irreversible. With the right investment in science, policy, and farming practices, we can protect rice as a safe and trusted food for future generations.

The study is published in the journal The Lancet Planetary Health.

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