12
Sep
Study Adds to Science Showing Elevated Toxicity Linked to Pesticide Mixtures
(Beyond Pesticides, September 12, 2025) A team of Argentinian researchers conducted a study published in Environmental Toxicology and Pharmacology of the combined effects of the herbicide glyphosate and the pyrethroid insecticide cypermethrin. The researchers observed significantly higher apoptosis in cells exposed to the mixtures than to the individual pesticides—a synergistic response. Apoptosis, also known as programmed cell death, is a standard way that tissues handle damaged cells to remove threats to their function.
The researchers sought to investigate the cellular toxicity of each chemical, individually and in combination, and assessed whether the effects of the mixture were additive or synergistic. Additive effects occur when two individual chemicals amplify the same sort of response, often because the chemicals have similar structures. Synergism can occur when chemicals have different mechanisms of action but work together to create more powerful effects.
Mixtures of pesticides are the least-studied area of research conducted for regulatory purposes. While regulators provide instructions to applicators regarding which pesticides can be applied together and combined in tank mixtures, there is no control over how pesticides travel through the environment once applied, as they flow through the air as spray drift, lodge in soils and water, and are incorporated into organisms via dermal, respiratory, and ingestion pathways. Herbicides, insecticides, and fungicides may affect different pests and have different mechanisms of action, but in combination, they can do even more profound damage to biological systems. Academic scientists are building up a body of research on pesticide mixtures that should trigger alarm – and action – in the regulatory community. See Beyond Pesticides’ archive on synergistic effects for more details.
Nor are chemicals’ harms necessarily confined to single organs or body systems. For example, Beyond Pesticides analyzed a review of reproductive health in August showing that pesticides and PFAS “disrupt the hypothalamic-pituitary-gonadal axis (HPG), impair ovarian function, and contribute to reproductive dysfunction through mechanisms such as oxidative stress, hormonal disruption, and epigenetic [gene expression or behavior] modifications,†according to the review authors. “This leads to menstrual irregularities, infertility, and pregnancy complications, as well as increases in the risk of reproductive system disorders such as endometriosis, polycystic ovary syndrome (PCOS), and ovarian cancer.â€
The effects of exposure to multiple pesticides are of dire significance for people in agricultural areas. The Midwest is a prime example. A recent report from Investigate Midwest details the consequences in southeastern Missouri. With the University of Missouri, the journalists analyze cancer rates and pesticide usage by county. Missouri, in general, has both very high pesticide use per square mile and very high cancer rates per 100,000 people. The investigators found that the state’s “bootheel†has the highest use of pesticides per square mile, and four of the bootheel counties have the highest cancer rates. Major crops are cotton, soybeans, and rice–crops grown in vast amounts with millions of pounds of pesticides. The investigators cited a study by Isain Zapata and colleagues, finding associations between pesticide use and leukemia, non-Hodgkin lymphoma, bladder, colon, lung, and pancreatic cancers; the strength of the associations was rivaled only by smoking. While specific pesticides were associated with specific cancers, neither the journalism investigation nor Zapata’s study attempted to determine the influence of combined exposures, highlighting the urgent need for more research.
The researchers studied the combined effects of cypermethrin and glyphosate because these pesticides are often applied to the same fields and both are found in soils, air, and water. There are some reports that in Argentina, the two pesticides are sometimes combined in tank mixtures or applied at the same time. Cypermethrin kills insects by interfering with neuron function and generating oxidative stress. It has also been found to increase numerous cellular biomarkers of DNA damage in at least three cell types. Glyphosate, the most-used pesticide in the world, is classified as a human carcinogen by the International Agency for Research on Cancer and is known to degrade kidney function, affect reproductive health, disrupt gut microbes, and more.
The authors wanted to see whether (and how) the cypermethrin-glyphosate mixture promotes cell death, and, if it damages but does not kill cells, how much it damages their genes. They used separate commercially available formulations of glyphosate and cypermethrin and formulated two mixtures with different ratios from scratch. They tested each formulation on a cell line called Hep-2, which is commonly used to study the effects of pesticides on the respiratory tract, and widely used in studies of viruses, cancer, pharmaceuticals, and genetic engineering.
A test of cytotoxicity and genotoxicity produced significant differences between control cells and all the treated groups – that is, treated groups showed a higher incidence of toxicity indicators like the occurrence of two nuclei, multiple micronuclei, and frequency of abnormal cell division. These types of aberrations are associated with apoptosis.
The researchers also point out that apoptosis is dose-dependent. At lower pesticide doses, cells’ genes may be damaged even though the cell does not die. This can result in long-term issues, including mutations and chromosomal breaks, which can in turn result in cancer.
Overall, the authors write, their study demonstrates “that agrochemical mixtures exert toxic effects on this cell line, and that in most cases, these effects are greater than expected for the individual substances alone.†This means that the old toxicological adage that “the dose makes the poison†is an inappropriate measure for assessing the effects of exposure to mixtures of pesticides. For mixtures, those effects are often more than the sum of their parts.
The fact that the U.S. Environmental Protection Agency (EPA) puts its stamp of approval on both glyphosate and cypermethrin and allows the use of these pesticides in combination identifies a serious deficiency. Both are known or suspected human carcinogens. Both harm insects (directly or indirectly), birds, fish, amphibians, aquatic invertebrates, humans, and likely many other organisms. The entire regulatory system is geared toward examining each chemical in isolation. But the real world is awash in uncontrolled combinations of chemicals. As mixtures are the least-studied form of pesticide exposure, those that are additive or synergistic likely represent the most urgent challenge posed by the real-world use of pesticides.
One of the foundational assumptions about synthetic pesticide use is that toxic chemicals can be controlled in the environment. After nearly a century of unbridled release of thousands of chemical compounds into the biosphere, there is abundant evidence that those chemicals move, combine, and degrade in unpredictable ways. They are not controllable in the ways that pesticide companies and regulators claim they are.
EPA continues to foster the massive glyphosate economy and refuses to acknowledge its harms, which are identified by international agencies and the independent peer-reviewed scientific literature. Regarding cypermethrin, egregiously, in its 2021 Interim Registration Review Decision, EPA “decided not to develop unique chemical-specific risk mitigation for the cypermethrins†and “concludes that the cypermethrins provide high benefits for controlling pests in indoor residential areas, outdoor urban areas, and in agricultural crop production…risks may remain to non-target organisms even after mitigation. Any remaining risks are outweighed by the benefits of the cypermethrins’ use†[emphasis added].
On September 8, Beyond Pesticides called on Congress to require EPA to recognize that pesticide exposures occur in the real world in the form of mixtures that often have synergistic effects. The organization’s analysis reviewed multiple lines of evidence, including that the neonicotinoid insecticide imidacloprid, along with infestation with a parasitic mite, synergistically increases honey bee deaths; plastics combined with petrochemical pesticides exacerbate health effects; the fungicide azoxystrobin, combined with natural fungal toxins, synergistically affects liver cancer cells. It is also noted that profound observers of nature and humanity, such as the poet John Donne and the naturalist John Muir, pointed out long ago that all life is interconnected; therefore, synergy is to be expected, not ignored. Here is a letter the public can send to Congress stressing the importance of acknowledging and studying the combined effects of pesticides, plastics, climate change, and other human-induced changes to the biosphere.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Sources:
Assessment of cell death and genotoxic potential of glyphosate and cypermethrin formulations, individually and in combination, in HEp-2 cells
Coalova et al.
Environmental Toxicology and Pharmacology 2025
https://www.sciencedirect.com/science/article/abs/pii/S1382668925001905
The unseen harvest: Pesticides, cancer and rural Missouri’s health crisis
Alex Cox, Adeleine Halsey, Kyla Pehr and Savvy Sleevar
Investigate Midwest August 18, 2025
https://investigatemidwest.org/2025/08/18/the-unseen-harvest-pesticides-cancer-and-rural-missouris-health-crisis/
Scientific Studies Identify EPA Deficiency in Evaluating Safety of Toxic Chemical Interactions
Beyond Pesticides, September 8, 2025
https://beyondpesticides.org/dailynewsblog/2025/09/scientific-studies-identify-epa-deficiency-in-evaluating-safety-of-toxic-chemical-interactions-calls-for-action/
Study Shows Importance of Testing Pesticide Mixtures to Determine Adverse Ecosystem Effects
Beyond Pesticides, June 6, 2024
https://beyondpesticides.org/dailynewsblog/2024/06/study-shows-importance-of-testing-pesticide-mixtures-to-determine-adverse-ecosystem-effects/
Children’s Health Threatened as Rates of Pediatric Cancers are Linked to Agricultural Pesticide Mixtures
Beyond Pesticides, March 4, 2025
https://beyondpesticides.org/dailynewsblog/category/statelocal/nebraska/
