The World Health Organization’s cancer research agency has classified atrazine – the second most widely used herbicide in the United States – as “probably carcinogenic to humans,” adding to growing concerns about toxic exposures in the nation’s farm belt.
The evaluation means the first and second most widely used herbicides in the U.S. – glyphosate and atrazine – are now both considered probable human carcinogens by the world’s leading independent cancer-hazard authority.
Atrazine is banned in the European Union and other countries due to health and environmental concerns, but remains widely used in the U.S., where it is a common contaminant in drinking water, according to the Environmental Protection Agency. Despite these concerns, U.S. regulators allow its continued use.
In its cancer classification, the International Agency for Research on Cancer (IARC) found “limited” evidence in humans that atrazine causes non-Hodgkin lymphoma, and “sufficient” evidence for cancer in experimental animals “based on an increase in the incidence of malignant neoplasms in female rats in multiple well-conducted studies,” the researchers wrote in Lancet Oncology.
They also noted, “There is ‘strong’ mechanistic evidence that atrazine exhibits key characteristics of carcinogens” – including that it causes oxidative stress and DNA damage in multiple organs, suppresses the immune system, causes inflammation, disrupts hormones and alters normal cell growth in several tissues in rodents.
Read more about atrazine in our fact sheet:
New cancer report raises concerns about most-used herbicides
The new assessment by the WHO’s cancer agency comes 10 years after the agency’s landmark finding that glyphosate, the world’s most heavily used herbicide, is also “probably carcinogenic to humans.” In that ruling, the panel found limited evidence of carcinogenicity in humans for non-Hodgkin lymphoma and “convincing evidence” of cancer in laboratory animals.
Both atrazine and glyphosate are also endocrine disruptors, meaning they can disrupt key hormone systems that regulate growth, development and metabolism. Studies also show that glyphosate-based pesticide formulations, similar to atrazine, can cause oxidative stress and DNA damage – well-established mechanisms that increase cancer risk.
Both atrazine and glyphosate are largely produced by companies outside the U.S.
Both herbicides are also largely produced by companies outside the United States. Syngenta, owned by ChemChina, produces most of the atrazine used in the U.S., while Bayer, based in Germany, is the dominant producer of glyphosate.
In its report last week, the IARC panel also said that alachlor – an herbicide used primarily on corn, soybeans and peanuts – is also “probably carcinogenic to humans.”
Rising cancer rates in corn country
The cancer designation for atrazine comes amid reports of rising cancer rates across the U.S. Corn Belt, highlighted in a recent investigation by the Washington Post.
The Post analysis found that cancer diagnoses among young adults in six corn-growing states – Iowa, Nebraska, Illinois, Minnesota, Indiana and Kansas – are rising faster than in the country as a whole. Young people in those states have significantly higher rates of several cancers, including kidney cancer, the Post reported. Studies have linked both atrazine and glyphosate to kidney cancer.
Mounting evidence links agrichemicals to higher cancer risks. A 2025 study of children with cancer in Nebraska – which has among the highest childhood cancer rates in the country – found that being exposed to multiple pesticides raises the risk of childhood cancer more than exposure to a single pesticide. The EPA evaluates pesticides one at a time and does not assess the real-world risks of multiple pesticide exposures.
EPA regulators also rely heavily on industry data; in the case of glyphosate, Monsanto manipulated the data in several ways. Regulators typically assess just one health risk at a time, overlooking the multiple, interacting health effects tied to widely used pesticides.
Industry attacks on IARC cancer researchers
As evidence grows linking pesticides to cancer, pesticide companies have repeatedly tried to undermine the WHO agency that has worked for five decades to identify cancer hazards in order to inform policies that could prevent cancer.
Internal corporate documents and emails show how industry scientists privately strategized to discredit IARC and the scientists who served on its panel after the agency’s 2015 classification of glyphosate as a probable human carcinogen.
In the wake of the ruling, Monsanto organized an “unprecedented and harsh strategy” to discredit the IARC cancer experts, wrote Jonathan Samet, dean of the Colorado School of Public Health. Journalists at Le Monde described the effort as an attempt to “destroy” the WHO cancer agency “by any means possible.”
More than 100,000 people – farmers, groundskeepers, and their families – ultimately sued Monsanto (now owned by Bayer) alleging that glyphosate-based Roundup herbicides caused their non-Hodgkin lymphoma. Bayer is now lobbying for state and federal laws that would shield them from cancer lawsuits.
The glyphosate-defense campaign illustrates how chemical companies work to discredit experts even as evidence mounts about the cancer risk of the pesticides widely sprayed on crops in the U.S.
Syngenta used similar hardball tactics to protect atrazine. As Rachel Aviv reported in the New Yorker in 2014, Syngenta employees developed an internal playbook to “discredit” UC Berkeley biologist Tyrone Hayes, whose research showed that low levels of atrazine feminized male frogs and disrupted their hormones.
Syngenta had hired Hayes to research its herbicide and then tried to suppress his work when they didn’t like the findings. Internal documents reported in the New Yorker reveal how company employees planned to monitor and investigate Hayes, and pressure journals to retract his work.
