Atrazine is a widely used herbicide registered in the United States for controlling broadleaf and grassy weeds, with the highest agricultural use on corn, sorghum, and sugarcane. Farmers, pesticide applicators, groundskeepers, and others who mix, load, spray, or clean up atrazine products may experience more direct exposure than the general public.
The scientific debate over atrazine and cancer changed significantly in November 2025, when the International Agency for Research on Cancer (IARC) classified atrazine as “probably carcinogenic to humans” (Group 2A). The strongest human evidence involved a molecular category of non-Hodgkin lymphoma (NHL) marked by a chromosomal change called t(14;18).
IARC Concludes Atrazine is a Probable Human Carcinogen
IARC’s 2025 working group consisted of 22 scientists from 12 countries. It evaluated human epidemiology, animal studies, and mechanistic evidence.
Non-Hodgkin lymphoma is a group of blood cancers, not a single disease. IARC reported that the strongest human evidence involved NHL with a genetic change called t(14;18).
The group classified atrazine as a probable carcinogen based on three findings:
- Limited evidence in humans. IARC identified positive associations with t(14;18)-positive NHL, while concluding that the evidence for all NHL combined was mixed.
- Sufficient evidence in experimental animals. Multiple well-conducted studies reported increased malignant tumors in female rats.
- Strong mechanistic evidence in experimental systems. The reviewed evidence showed that atrazine can induce oxidative stress and suppress immune function, among other cancer-related effects.
“Limited evidence” is a technical IARC category. It means a positive association has been observed and a causal interpretation is credible, but the evidence does not meet IARC’s threshold for “sufficient” human evidence. A Group 2A classification identifies a carcinogenic hazard; it does not determine whether atrazine caused any one person’s cancer.
What Human Studies Found
Allegations in atrazine lawsuits are based on decades of epidemiologic research. Several key studies reported associations with NHL overall, younger-onset NHL, or t(14;18)-positive disease.
Early Midwest Studies
In 1993, National Cancer Institute researchers pooled three case-control studies from Nebraska, Iowa, Minnesota, and Kansas. Among 993 white men with NHL and 2,918 population-based controls, atrazine use was associated with an odds ratio of 1.4 (95% CI: 1.1–1.8).
A 2003 pooled reanalysis of the same National Cancer Institute case-control datasets used a different statistical approach to evaluate 47 pesticides together. Atrazine was associated with increased NHL risk after accounting for other pesticides (OR 1.6; 95% CI: 1.1–2.5).
The t(14;18) Findings
The t(14;18) change is found in certain types of NHL and can be identified through testing of tumor tissue.
A 2001 molecular-subtype study tested archived biopsies from 182 men with NHL in Iowa and Minnesota. Sixty-eight cases, or 37%, were t(14;18)-positive. Atrazine exposure was associated with t(14;18)-positive NHL (OR 1.7; 95% CI: 1.0–2.8), but not with t(14;18)-negative NHL. The authors called a causal relationship plausible but emphasized that the finding needed confirmation in a larger study.
A 2006 Nebraska study likewise reported that pesticide use—including herbicide and triazine use—was associated with t(14;18)-positive NHL but not t(14;18)-negative NHL. IARC later combined the two molecular-subtype studies and reported a meta-odds ratio of 1.99 (95% CI: 1.13–3.53) for t(14;18)-positive NHL.
Agricultural Health Study
The Agricultural Health Study (AHS) is a long-running prospective study of licensed pesticide applicators and their spouses in Iowa and North Carolina. Its findings have changed as follow-up time and case numbers increased.
A 2011 AHS analysis found no association between atrazine use and NHL or NHL subtypes; relative risks were near 1.0 across the main exposure measures. In a 2024 update, researchers again found no overall association with NHL. Among applicators diagnosed before age 50, however, ever-use of atrazine was associated with NHL (RR 2.43; 95% CI: 1.10–5.38; 45 cases), and researchers reported an exposure-response trend for B-cell NHL in that age group. The authors described these age-specific findings as suggestive and said further research was needed.
How Atrazine Could Affect Lymphocytes
Atrazine lawsuit allegations rely on laboratory and animal research to describe several biologically plausible pathways.
- Immune suppression. Studies have found effects on natural killer-cell activity, cell-mediated immunity, dendritic-cell maturation, leukocyte counts, and host resistance in human-cell or animal models. Because NHL develops in immune-system cells, IARC considered immunosuppression relevant to the evaluation.
- Oxidative stress. IARC found strong evidence in experimental systems that atrazine induces oxidative stress, including reactive oxygen species and oxidative damage. Oxidative stress can damage cellular components and contribute to carcinogenic processes.
- DNA and chromosomal damage. Experimental studies have reported DNA strand breaks and other genetic damage in mammalian cells, including cultured human lymphocytes. These findings are mechanistic evidence, not direct measurements of cancer risk in atrazine users.
What Atrazine Non-Hodgkin Lymphoma Lawsuits Allege
Atrazine lawsuits against Syngenta are generally based on the following allegations:
- Design defect: Atrazine-based pesticides were defectively designed and that feasible alternatives could have reduced applicator exposure, including different formulations, closed-transfer systems, and unit-dose packaging.
- Failure to warn outside the label: Syngenta did not communicate the asserted NHL risk through advertising, sales representatives, websites, product-stewardship programs, or applicator training.
- Negligence: inadequate testing of formulated atrazine products, inadequate post-market surveillance of end users, and failure to investigate epidemiologic signals involving NHL.
Plaintiffs generally allege Syngenta attempted to influence public discussion of atrazine science and discredit unfavorable research. Independent reporting based on unsealed court records previously documented a campaign directed at atrazine critics, including University of California, Berkeley scientist Tyrone Hayes.
The above are allegations in a lawsuit. They have not been proven or decided by a court.
Talk to an Atrazine Lawyer
Wisner Baum understands what it takes to litigate complex cases against agrichemical companies. Our attorneys co-represented Dewayne "Lee" Johnson, a school groundskeeper diagnosed with non-Hodgkin lymphoma, in the first Roundup cancer lawsuit to go to trial. That case put the science on herbicides and NHL in front of a jury, which awarded Mr. Johnson $289 million (later reduced). We later represented Alva and Alberta Pilliod in another Roundup cancer case, which culminated in a jury verdict worth $2.055 billion (later reduced).
Our toxic tort attorneys are now applying our experience to atrazine.
If you or someone in your family developed non-Hodgkin lymphoma after regularly mixing, loading, or spraying atrazine products, or working around them on a farm or job site, you may have legal options. Call (310) 207-3233 or fill out our online form for a free, confidential case evaluation.
