(CNN) Glyphosate, an herbicide that remains the world's most ubiquitous weed killer, raises the cancer risk of those exposed to it by 41%, a new analysis says.
Researchers from the University of Washington evaluated existing studies into the chemical -- found in weed killers including Monsanto's popular Roundup -- and concluded that it significantly increases the risk of non-Hodgkin lymphoma (NHL), a cancer of the immune system.
"All of the meta-analyses conducted to date, including our own, consistently report the same key finding: exposure to GBHs (glyphosate-based herbicides) are associated with an increased risk of NHL," the authors wrote in a study published in the journal Mutation Research.
The potential carcinogenic properties of glyphosate are the subject of widespread scientific debate. The US Environmental Protection Agency said in a 2017 draft risk assessment that the herbicide "is not likely to be carcinogenic to humans," while the European Food Safety Authority maintains a similar stance. Bayer, which acquired Monsanto in 2018, said the same year that glyphosate is a "safe and efficient weed control tool."
In 2015, however, the World Health Organization's International Agency for Research on Cancer classified glyphosate as "probably carcinogenic to humans." Moreover, the chemical has triggered multiple lawsuits from people who believe that exposure to the herbicide caused their non-Hodgkin's lymphoma. In 2017, CNN reported that more than 800 people were suing Monsanto; by the following year, that figure was in the thousands.
One high-profile case against Monsanto was that of Dewayne Johnson, a former school groundskeeper diagnosed with terminal non-Hodgkin lymphoma in 2014. In August 2018, a judge ordered Monsanto to pay Johnson $289 million in damages, an award subsequently reduced to approximately $78 million after Monsanto appealed.
The authors of the University of Washington report analyzed all published studies on the impact of glyphosate on humans. Co-author and doctoral student Rachel Shaffer said in a statement: "This research provides the most up-to-date analysis of glyphosate and its link with Non-Hodgkin Lymphoma, incorporating a 2018 study of more than 54,000 people who work as licensed pesticide applicators." The scientists also assessed studies on animals.
Focusing on data relating to people with the "highest exposure" to the herbicide, the researchers concluded that a "compelling link" exists between glyphosate exposure and a greater risk of developing non-Hodgkin lymphoma. Senior author Lianne Sheppard, professor in biostatistics and environmental and occupational health sciences, said she was "convinced" of the carcinogenic properties of the chemical.
In a statement, Bayer called the new analysis a "statistical manipulation" with "serious methodological flaws," adding that it "provides no scientifically valid evidence that contradicts the conclusions of the extensive body of science demonstrating that glyphosate-based herbicides are not carcinogenic."
The authors of the new study acknowledged some limitations of their analysis, noting that "only limited published data" was available. Moreover, they wrote, studies they evaluated varied in the population groups they targeted: specifically, the glyphosate exposure levels of the participants differed between reports.
The available studies also neglected to assess the impact of the "green burndown" farming method, which sees glyphosate herbicides added to crops before they are harvested. Glyphosate residue has probably increased since the introduction of this method in the mid-2000s, the researchers wrote.
Francis Martin, a biosciences professor at the University of Central Lancashire, told CNN he welcomed the University of Washington report. He called the debate over the safety of glyphosate "important," explaining that "glyphosate is used as a general purpose herbicide so there will be exposure in the general population."
However, he noted that the report was limited by the small number of existing studies on the subject, though he stressed that the authors were "honestly self-reflective on the limitations of the analyses."
"[The report] highlights the need for new, well-designed and robust studies at appropriate exposure levels," Martin said, adding, "The number of robust studies in the literature examining this question is pathetically small."