A major new study has raised a troubling question for global health: what if chemicals considered “safe” on paper are quietly reshaping the body in ways that lead to cancer years later?

Researchers reporting in Nature Health have found that long-term exposure to common agricultural pesticides—none officially classified as cancer-causing—may increase cancer risk by as much as 150 per cent when people are exposed to them in real-world combinations.

The study, led by teams from Institut Pasteur, IRD, the University of Toulouse and Peru’s National Institute of Neoplastic Diseases, points to a hidden and largely unrecognised phase of disease. Not a tumour. Not even early cancer. Something subtler. And far more unsettling.

Cells, the scientists say, may be losing their identity.

Instead of simply damaging cells, repeated exposure to mixtures of pesticides appears to “confuse” them. In plain terms, cells begin to forget what they are meant to do. This breakdown is especially visible in the liver, an organ that processes chemicals entering the body. Researchers describe it as a kind of early warning system—a molecular sentinel—showing silent changes long before illness is detected.

The work focused on Peru, a country where intensive farming, varied climates and deep social inequalities intersect. Many Indigenous and rural communities there are exposed to multiple pesticides at once—on average, around 12 at the same time, often through food, water and air.

To understand the risk, scientists did something unusual. They stepped outside the lab.

“We first modelled the dispersion of pesticides in the environment over a six-year period, from 2014 to 2019, which allowed us to create a high-resolution map and identify areas with the highest risk of exposure,” explains Jorge Honles, PhD, in epidemiology at the University of Toulouse.

The team tracked 31 commonly used agricultural chemicals across the country. None are currently listed by the World Health Organization as known human carcinogens.

They then compared this environmental map with medical records from more than 150,000 cancer patients diagnosed between 2007 and 2020. The overlap was striking. Areas with higher pesticide exposure showed sharply higher cancer rates.

In laboratories at Institut Pasteur, the biological picture became clearer. Under the microscope, liver cells exposed to these chemical mixtures showed early disruption in the genes that help cells maintain their function. It was not dramatic cell death. It was quieter. The cells were still alive—but no longer fully themselves.

“This is the first time we have been able to link pesticide exposure, on a national scale, to biological changes suggesting an increased risk of cancer,” explains Stéphane Bertani, a researcher in molecular biology at the French National Research Institute for Sustainable Development (IRD), at the PHARMA-DEV laboratory (IRD/University of Toulouse).

The findings challenge a core assumption in modern toxicology—that chemicals can be judged safe when tested one by one. In reality, people are exposed to mixtures every day. Food carries traces. Water carries more. The air adds another layer.

This “real-world cocktail” may be far more harmful than the sum of its parts.

The study also raises concern about regulatory blind spots. If dozens of individually “safe” chemicals can combine to produce serious biological effects, current safety thresholds may be incomplete.

There is another factor. Climate. Extreme weather events such as El Niño, floods and heatwaves are changing how pesticides spread. Warmer temperatures can increase chemical volatility. Flooding can carry residues into water systems. Exposure patterns are shifting—and so may disease risk.

For families in farming regions, the implications are immediate, even if invisible. There are no early symptoms. No warning signs. Just slow biological change.

The researchers say this could open a new frontier in medicine—environmental biomonitoring, where subtle changes in cell function are tracked years before cancer develops.

It also points to a bigger shift in thinking. Cancer may no longer be only a disease of cells. It may be a disease of systems—of agriculture, climate and policy.

We may be entering an era of planetary oncology, where preventing cancer begins far beyond the hospital, in the soil, the water, and the chemicals we accept as safe.