Roughly one-sixth of croplands globally — 14 to 17 percent, or approximately 598 million acres — are contaminated by one or more toxic heavy metals like arsenic, cobalt, cadmium, copper, lead and nickel at levels exceeding human health and agricultural safety thresholds, a new study has found.
Heavy metals are elements that come from both natural and human-made sources. They do not break down in the environment and stay in soils for decades, allowing them to be absorbed by crops and make their way into the food chain. They accumulate in the body over time, leading to chronic diseases that may not appear for years.
“Soil provides the basis for nearly 95% of food consumed by human beings.” the authors of the findings wrote. “Often overlooked in the matter of soil quality is soil pollution by toxic heavy metals and metalloids… which reduces crop yields and results in unsafe food. Even though some metals such as cobalt (Co) and copper (Cu) are essential in small amounts for biological functioning, their bioaccumulation in organisms, including crops, can render them toxic in the human food chain.”
Heavy metals like arsenic, mercury, cadmium and lead are toxic at low concentrations, Jagannath Biswakarma, a senior research associate with the School of Earth Sciences and the Cabot Institute for the Environment at University of Bristol, wrote in The Conversation.
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Some of these metals are part of the natural geology, while others have been left behind by decades of agricultural and industrial activities, having settled into soils through fertilizers, factory emissions, mining or contaminated water.
“When crops grow, they draw nutrients from the soil and water – and sometimes, these contaminants too. Rice, for instance, is known for taking up arsenic from flooded paddies. Leafy greens can accumulate cadmium,” Biswakarma explained. “These metals do not change the taste or colour of food. But they change what it does inside the body.”
Long-term exposure to lead, arsenic and cadmium has been linked to kidney damage, cancer, developmental disorders in children and osteoporosis. Local diets that rely heavily on one staple crop like wheat or rice multiplies these health risks.
For the study, led by Deyi Hou, an associate professor with Tsinghua University’s School of Environment in China, the research team used machine learning and a database of more than 796,000 soil concentrations to assess global soil pollution for the toxic metals arsenic, lead, cadmium, chromium, copper, cobalt and nickel.
The team found that concentrations of cadmium in agricultural soil often exceeded the safety threshold.
In the findings, they describe a “metal-enriched corridor” that stretches from southern Europe to the Middle East and South Asia. In these areas, there is an overlap of agriculture with a history of industrial activity, mining and limited regulation.
The amount of cropland heavy metal contamination varies by region and is often shaped by land use history, water management and geology. The groundwater used to irrigate rice fields in Central and Southeast Asia naturally contains arsenic, which is deposited into the soil and absorbed by the rice.
“Fortunately, nature often provides defence. Recent research showed that certain types of iron minerals in the soil can convert arsenite – a toxic, mobile form of arsenic – into arsenate, a less harmful species that binds more tightly to iron minerals. This invisible soil chemistry represents a safety net,” Biswakarma wrote in The Conversation.
In Burkina Faso and other areas of West Africa, drinking and irrigation water contaminated with arsenic has also impacted croplands.
Croplands near small-scale mines in parts of South America such as the Amazon basin face additional risks, with informal gold mining and deforestation contributing to releases of mercury. Natural mercury sinks, forests store atmospheric mercury in soil and biomass. When trees are cut down, the stored mercury gets released into the environment, potentially affecting croplands and nearby water bodies while raising atmospheric levels of the toxic metal.
Croplands located close to legacy mining sites are often impacted by long-term contamination, but these sites have the potential to be remediated with appropriate technologies, and even transformed to become part of a circular economy.
“Soil contamination is not just a scientific issue. It’s a question of environmental justice. The communities most affected are often the least responsible for the pollution. They may farm on marginal lands near industry, irrigate with unsafe water, or lack access to testing and treatment,” Biswakarma wrote.
Biswakarma said these communities face food and water insecurity coupled with toxic exposure.
“There is no single fix. We’ll need reliable assessment of contaminated soils and groundwater, especially in vulnerable and smallholder farming systems. Reducing exposure requires cleaner agricultural inputs, improved irrigation, and better regulation of legacy industrial sites. Equally critical is empowering communities with access to information and tools that enable them to farm safely,” Biswakarma said.
The study, “Global soil pollution by toxic metals threatens agriculture and human health,” was published in the journal Science.
“Soils carry memory. They record every pollutant, every neglected regulation, every decision to cut corners. But soils also hold the potential to heal – if given the proper support,” Biswakarma added. “This is not about panic. It’s about responsibility… No country should unknowingly export toxicity in its grain, nor should any farmer be left without the tools to grow food safely.”
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