The US Environmental Protection Agency has dismissed a recent finding that atrazine, the second most widely-used herbicide in the United States, is “probably carcinogenic to humans” by the World Health Organization’s cancer review agency. 

Atrazine is used extensively in the US on crops like corn, sorghum, and sugarcane. However, over 60 countries have banned the chemical due to its endocrine-disrupting properties and tendency to contaminate groundwater. 

In a new classification, published in the January, 2026 issue of The Lancet Oncology, the International Agency for Research on Cancer (IARC) ranked atrazine as probably carcinogenic to humans based on what it described as “limited evidence” for cancer in humans and “sufficient evidence for cancer in experimental animals.”

The agency’s findings are independent assessments that guide national regulatory authorities worldwide in the promulgation of rules around chemicals used in agriculture, food systems and occupational settings.  

The IARC assessment was the first in nearly three decades. In 1998 the organization said the compound was not classifiable as to its carcinogenicity to humans. 

Since the IARC findings were initially released in late November, the US Environmental Protection Agency (EPA), as well as chemical producers in China and elsewhere, have pushed back against the new classification. Both used almost identical terms in describing the determination as “flawed” and “inconsistent with scientific consensus.”

Hormone disruptor, cancer risk of atrazine  

Atrazine’s cancer links include evidence of oxidative stress, hormone disruption, immune suppression, and actual tumor growth, IARC found in its abbreviated evaluation of atrazine and two other compounds.

In particular, IARC pointed to animal studies that showed tumor growth in the mammary glands and uterus of female rats, reduced estrogen and testosterone in both male and female rats, and induced cell death and division. The most recent study IARC cites, from 2024, also concluded that male rats also experience oxidative stress and severe hormone disruption.

In humans, only a handful of studies have been published so far. However, IARC noted that two case-control studies that reported “strong positive associations between exposure to atrazine…and translocation-positive NHL [non-Hodgkin’s Lymphoma]” among exposed farm workers. Links to other human cancers did not show “consistent positive findings.” 

Beyond its cancer inks, atrazine exposure is also associated with birth defects, reproductive harm, and hormone disruption, in papers not cited by IARC – an agency that focuses almost exclusively on cancer risks. 

US regulators and manufacturers push back  

In a 21 November statement, Syngenta, the agrochemical company that produces most of the US’s atrazine, said:  “[c]urrent scientific evidence clearly demonstrate[s] that atrazine is safe when applied in accordance with registered label instructions.” Syngenta is owned by the Chinese state-operated company Sinochem.

Speaking to Health Policy Watch just a few days after the Syngenta statement, EPA used identical terms, arguing that IARC’s findings “stir up fear” by also including “very hot beverages, red meat, working the night shift and hairdressing in the same classification.” 

“This announcement on atrazine is just another example of the World Health Organization International Agency for Research on Cancer (IARC) using a deeply flawed approach to its cancer assessments on multiple levels,” the EPA added. 

The EPA spokesperson complained, in particular, that the two-page article published so far is just a brief summary of the full review, which is only due to be published later this year, according to IARC.

“[F]or IARC to roll out these “findings” and unnecessarily stir up fear when IARC has said it will not be publishing any of its detailed science until late 2026 or 2027 is irresponsible and lacks transparency. Currently, there is no opportunity for anyone to meaningfully review how IARC has reached its conclusions,” said the EPA’s press office. 

“In contrast, atrazine has been extensively studied by EPA across multiple administrations, including having five meetings of independent Scientific Advisory Panels peer review the cancer potential of atrazine between 2009-2011,” EPA said. 

“As a reminder, IARC also has a long history of being severely misguided in its findings,” the EPA spokesperson said, citing past IARC findings on red meat and night shift work, among other factors, as possible or probable causes of cancer. 

In 2015 IARC concluded that red meat was a possible cause of cancer and processed meat is a probable cause. Those findings have since been echoed by the American Institute for Cancer Research, which in 2021 concluded “there is strong evidence that eating high amounts of red meat increases the risk of colorectal cancer.” 

The US Department of Agriculture’s most recent dietary guidelines also suggest reducing red meat consumption. The US CDC concluded in a 2021 report that there is “high confidence” that persistent night shift work that results in circadian disruption can cause human cancer.

Environmental groups welcome IARC re-evaluation 

Separate EPA reviews in 2003 and 2018 concluded that available evidence does not support a relationship between atrazine exposure and human cancers. IARC cites one new animal study from 2024, while re-evaluating older data in a new light.

For pesticide experts like Dr Jennifer Sass, a senior scientist at the Natural Resources Defense Council (NRDC), IARC’s re-evaluation was welcome – correcting industry bias in previous evaluations. . 

“With atrazine, there’s hormone disruption, there’s oxidative stress, there is indication of tumors in human and animal studies,” said Sass. “IARC gathered a solid expert array of people from different disciplines, including industry people. They didn’t second-guess or overinterpret the data. 

“The EPA needs to take another look at their assessment to account for the carcinogenicity,” said Sass. 

Canada, Australia and Brazil also use atrazine extensively

Atrazine, also known under brand names such as Aatrex®, Aatram®, Atratol®, and Gesaprim®  is the second most widely-used pesticide in the US after glyphosate – also known as Roundup®. Glyphosate also was designated as a probable carcinogen in humans by IARC in 2015, in a ruling that continues to trigger controversy between the US and trading partners that have banned the chemical.  

Across the US’s corn belt, up to 60 million pounds of atrazine is applied to treat US crops each year. 

Atrazine also is flushed from soil into streams or groundwater aquifers where it can contaminate drinking water supplies, according to the US Centers for Disease Control and Prevention. 

A 2017 analysis of EPA tap water data found that nearly 30 million Americans across 28 states have tap water that contains atrazine. Drinking water in these areas had atrazine levels three to seven times higher than the federal limit of three parts per billion during spikes due to run-off in spring and summer, according to another major research and advocacy non-profit, Environmental Working Group. 

“Because the EPA has been sweeping the evidence on cancer under the rug for the time it has been on the market for 50 years, they’re not monitoring for atrazine in the water. The drinking water standard should be zero, because it’s a carcinogen,” said Sass. 

But the US is not alone in its reliance on the chemical, which is typically applied to unplanted fields so as to kill any other weeds or plants before crops are planted. 

Canada, Australia, Brazil and China also use atrazine to increase crop yields. 

In the decades prior to IARC’s classification, 60 countries banned the use of atrazine. These include 27 European Union countries, most Middle Eastern countries, and even many African countries. However, in many lower-and-middle income countries where atrazine is banned, the chemical is often smuggled in illegally. 

The European Union banned atrazine in 2004 and most EU countries have since banned glyphosate as well. Mexico, which exports tens of billions of dollars of agricultural products to the US, had attempted to ban glyphosate.  But in 2024 it postponed the ban indefinitely  following pressure from industry and the US, citing the lack of an effective alternative.

As international assessments increasingly flag risks associated with such chemicals, the US remains an outlier among high-income countries in continued widespread use. 

“We were all told for a long time that weed killers aren’t harmful to people because their mechanism of toxicity targets photosynthesis. We all believed them,” said Sass.

“Because of that, we didn’t have proper food monitoring. We didn’t have proper drinking water monitoring. The people exposed to atrazine occupationally, whether by mixing it, loading it, applying it, manufacturing it, need to take the proper precautions because of how toxic it is.

EPA stance contrasts sharply with MAHA rhetoric about healthier foods 

EPA’s pro-pesticide stance under the new US administration of President Donald Trump contrasts sharply with the healthy foods rhetoric of the “Make America Healthy Again” (MAHA) movement, critics also said.   

“Despite the rhetoric of MAHA, there will be no robust review of the dangers of pesticides by the Trump Administration,” said Sylvia Wu, of the Washington DC-based Center for Food Safety.  “Instead, a toxic poison like atrazine will continue to contaminate our lands and waters, making our children sick for decades to come,” said Wu.

Wu was responding, in particular, to another recent US Fish and Wildlife’s (FWS) statement that atrazine does not pose an extinction risk to any US endangered species. 

The Center for Food Safety sued the EPA in 2020 when the agency moved to reapprove atrazine, along with a host of other pesticides and herbicides, as part of a routine evaluation. 

“The public trust has been steamrolled by agrochemical interests,” said Sass of the Natural Resources Defence Council. “The EPA has fast-tracked the approval of their poisons, which is not only a violation of the public’s trust, but also of the mission of the EPA.”

“Using modern farming methods, we do not need weed killers like atrazine. We simply don’t have to kill every weed to grow our crops. It. We don’t have to be so scorched earth in our farm practices,” Sass argued. 

“I look forward to the day that the US catches up to other countries in keeping American families safe from chemicals.”

Image Credits: Akshat Jhingran , Waldemar Brandt , AP/Sierra Club, USGS.