10
Dec
Atrazine Designated Probable Carcinogen by International Agency for Research on Cancer; EPA Defends It
(Beyond Pesticides, December 10, 2025) On November 21, 2025, the International Agency for Research on Cancer (IARC), a branch of the World Health Organization (WHO), designated the endocrine-disrupting herbicide atrazine (as well as the herbicide alachlor) as “probably carcinogenic to humans.”
Manufactured by the multinational, China-based pesticide corporation Syngenta, atrazine has been linked to various adverse health effects and runoff into waterways across the continental United States for years. Tyrone Hayes, PhD, researcher and professor of Integrative Biology at the University of California, Berkeley, who studies the endocrine-disrupting properties of atrazine and other chemicals, has said that atrazine induces cancer by turning on the enzyme aromatase. Dr. Hayes told conferees of Beyond Pesticides’ 31st National Pesticide Forum that: “[W]hat is concerning about aromatase expression and estrogen in mammals is breast cancer and prostate cancer. With regard to prostate cancer, there is an 8.4-fold increase in prostate cancer in men who work in atrazine factories and bag atrazine. There is at least one correlational study, which I didn’t publish, that shows women whose well water is contaminated with atrazine are more likely to develop breast cancer than women who live in the same community, but don’t drink the well water. (Kettles, M., et al. Environmental Health Perspectives. 1997 Nov; 105(11): 1222–1227) [I]t’s just a correlational study, but if you look at rats, testosterone goes down when you give them atrazine, and estrogen goes up. Syngenta’s own studies (1994) show that, if you give rats atrazine, there is an increase in breast cancer and mammary cancer.” (See Dr. Hayes’ talk here.)
Environmentalists have pointed to Dr. Hayes’ and others’ research for years in urging the U.S. Environmental Protection Agency (EPA) to ban atrazine, which the agency allows to be widely used in food production and landscape management. Beyond Pesticides called on EPA to ban the chemical in an action earlier this year. (See here.) The IARC designation of atrazine as a Group 2a carcinogen adds to the list of pesticide active ingredients previously designated as probable carcinogens, including but not limited to glyphosate in 2015 (see Daily News here), diazinon, and malathion.
IARC Designation
Published in their report in The Lancet Oncology, a group of nearly two dozen cancer researchers from 12 different nations identified various human epidemiological studies, in vitro studies, and other related assessments that link atrazine to DNA damage and oxidative stress.
“The Working Group evaluated both atrazine and alachlor as probably carcinogenic to humans (Group 2A) on the basis of the combinations of (i) limited evidence for cancer in humans and sufficient evidence for cancer in experimental animals; and (ii) limited evidence for cancer in humans and strong mechanistic evidence in experimental systems,” reads the IARC press release. The release continues: “For atrazine, positive associations have been observed for non-Hodgkin lymphoma that is positive for the chromosomal translocation (14;18). Positive associations have been observed between alachlor and cancer of the larynx.”
The researchers conclude that “factory and agricultural workers have the highest exposures, which can occur during pesticide production and agricultural or horticultural activities, primarily via inhalation and dermal absorption.” Meanwhile, the average consumer’s “exposure occurs primarily via ingestion of drinking-water and foodstuffs, and is typically estimated to be low.” The authors continue: “In the general population, residential turf contact can result in higher short-term incidental dermal and oral doses than from food or drinking-water.”
Regulatory Background
The triazine herbicide is the second most widely used herbicide in the U.S. after glyphosate, despite its ban in the European Union in 2004 and in over 40 countries worldwide. Many organizations, including Beyond Pesticides, have called for the chemical to be banned in the U.S. and joined in litigation, spearheaded by the Center for Food Safety, against EPA in 2020.
The agency’s 2020 interim registration review, which was extended in 2024, lacks important safeguards for children’s health and a monitoring program intended to protect groundwater from contamination. As is typical with EPA, the agency merely proposed minor label changes in an attempt to mitigate risks identified in its registration review. According to a release from the EPA, the decision was made not out of concerns related to human health and environmental protection, but to provide “regulatory certainty” for farmers and local officials. Although a hefty 200,000 lbs. of propazine were used each year, mainly on sorghum in Texas, Oklahoma, and Kansas, this amount pales in comparison to the over 70 million lbs. of atrazine used throughout the United States.
Beyond Pesticides submitted comments earlier this year (see here), highlighting concerns from previous comment periods in 2022, 2020, and 2016 (see here) regarding the ecological and human health impacts of continuous use and exposure to this toxic herbicide. While EPA assures the public that its Herbicide Strategy Framework will address mitigation concerns, Sara Grantham, science, regulatory, and advocacy manager at Beyond Pesticides, highlights the fallacy of this approach in the 2025 comments:
“The utilization of the mitigation menu website, however, is a process so cumbersome and convoluted that it cannot be taught, implemented, or enforced. The expectation is placed on applicators, who may not be fully versed in math or English, to use a point system to determine if enough mitigation measures can be applied to allow for the use of the pesticide in a use limitation area. Users complain of technological problems with the website, while also commenting on the complexities involved in the process.”
Known Adverse Health and Ecological Effects
Exposure to atrazine is widespread in the environment and linked to adverse health effects in humans. According to EPA, “Pesticide products containing atrazine are registered for use on several agricultural crops, [including] field corn, sweet corn, sorghum, and sugarcane, []wheat, macadamia nuts, and guava, as well as nonagricultural uses such as nursery/ornamental and turf,” emphasizing the various potential exposure pathways for consumers, workers, and farmers.
In terms of drift and exposure, atrazine has been found in various environments, including waterways. The latest Scientific Investigations Report for 2025 from the U.S. Geological Survey (USGS), entitled “National Water Quality Program: Multidecadal Change in Pesticide Concentrations Relative to Human Health Benchmarks in the Nation’s Groundwater,” finds moderate concentrations of five pesticides (including atrazine), with the highest percentages in agricultural wells. (See Daily News here.)
The Maryland Pesticide Education Network published a report in 2025 highlighting the ongoing stress to the Chesapeake Bay Watershed from pollutants, particularly pesticides such as atrazine. The Chesapeake watershed report notes that atrazine and metolachlor (also an estrogen/androgen disruptor and suspected human carcinogen) occur together frequently in the Chesapeake Bay. While many estrogen-related compounds in waterways are thought to come from wastewater treatment plants, agricultural runoff containing these pesticides can also trigger endocrine disruption. Among their possible victims are the world-famous Chesapeake Bay oysters, which are an important part of the Bay’s food web. Oysters filter millions of gallons of water a day. The report cites research showing that oysters lose their beneficial microbes when exposed to low concentrations of atrazine. This leads to opportunistic colonization by pathogenic microbes, posing a risk to the long-term survival of oysters and to the health of the bay generally. (See Daily News here.) Advocates continue to raise alarms given that nearly 13 million people get their drinking water from the watershed.
A similar dynamic has emerged in the Great Lakes region, with waterways that flow into the region experiencing year-round pesticide contamination that exceeds benchmarks meant to protect aquatic life, according to research published in Environmental Toxicology and Chemistry by scientists at the U.S. Geological Survey (USGS). Across every sampled tributary, pesticides were found. Accordingly, 96% (190 out of 198) of samples taken contained pesticides or their breakdown products. Herbicides represented the most frequently detected chemicals, with atrazine and its breakdown products (deethylatrazine and hydroxyatrazine) the most common of the bunch, detected in more than 75% of samples. (See Daily News here.)
Meanwhile, approximately four in ten private wells in the state of Wisconsin contain toxic pesticides and pesticide metabolites, according to findings released earlier this year from a 2023 survey, entitled Wisconsin Agricultural Chemicals in Wisconsin Groundwater, conducted by the Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP) in partnership with U.S. Department of Agriculture’s (USDA) National Agricultural Statistics Service (NASS). Atrazine is one of the 22 detected pesticides in this report. (See Daily News here.)
Pesticide exposure may also be close to home, or worse, inside dwellings! A 2025 study published in Environmental Science and Technology found that there are 47 current-use pesticides (CUPs)—products with active ingredients that are registered by EPA —detected in samples of indoor dust, drinking water, and urine from households in the state of Indiana. Thirty-three CUPs were detected in drinking water samples, with atrazine and its breakdown products making up “over 50% of the total herbicide concentrations” accounted for in this study. (See Daily News here.)
Advocates raise alarm bells given the continuation of publications highlighting specific risks associated with atrazine exposure, including disproportionate risk to women. Individuals living near chemical-intensive agricultural environments have a heightened risk of Alzheimer’s disease relative to the general population, according to a 2024 study published in Psychiatry Research. One of the pesticides of concern in this Spain-based study is atrazine. (See Daily News here.) A study published in Ecotoxicology and Environmental Safety finds that atrazine promotes breast cancer development through suppression of immune cell stimulation, and thus function, and upregulation of enzymes mediating tumor development. The study notes, “This study demonstrated that atrazine accelerated the cell cycle and encouraged the proliferation and invasion of breast cancer tumor cells. Furthermore, atrazine can reduce anti-tumor immunity by decreasing lymphocyte infiltration and modulating cytokine production inside the tumor microenvironment, thereby promoting tumor immune escape and breast cancer progression. To fully understand the mechanism underlying atrazine’s immunosuppression of breast cancers, further research is needed.” (See Daily News here.)
There are additional adverse health effects linked to atrazine exposure. A 2021 study published in Cellular Physiology and Biochemistry finds atrazine exposure worsens lung disease outcomes in individuals with idiopathic (spontaneous) pulmonary fibrosis (IPF), a group of incurable lung diseases involving damaged/scarred lung tissue. Furthermore, chemotherapeutic products used to treat lymphoma (immune system cell cancer), like bleomycin, can induce pulmonary fibrosis complications exacerbated by pesticide exposure. (See Daily News here.) Research from the Silent Spring Institute in 2020 identifies 28 registered pesticides, including atrazine, linked with the development of mammary gland tumors in animal studies. (See Daily News here.)
Call to Action
The connection between the climate crisis and synthetic chemical dependency has also been corroborated through peer-reviewed research. A new, simultaneous chemical identification method employed in 2020 found the presence of the weed killer atrazine and 200+ other hazardous chemicals in hydraulic fracturing (fracking) wastewater or produced water, according to collaborative research published in the Journal of Separation Science by scientists at the University of Toledo (UToledo) and the University of Texas at Arlington. Although produced water is a waste product of fracking, EPA allows many states to reuse produced water in agriculture and other industries or dispose of it into waterways. (See Daily News here.)
Rather than continue the whack-a-mole of addressing individual pesticide exposures like atrazine in your community, advocates across the country continue to call for a wholesale transition to organic pest management for agricultural and nonagricultural contexts, like parks and playing fields. Beyond Pesticides has developed and actively maintains the Keeping Organic Strong resource hub, a one-stop shop for you to learn about changes in organic regulations and how to take action.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: Center for Biological Diversity; U.S. Right to Know; The Lancet Oncology
