In Roundup case, federal judge vets the experts for testimony

A federal lawsuit alleging Monsanto’s top-selling weed killer, Roundup, causes cancer is at a pivotal moment as the presiding judge deliberates on which scientific experts will be permitted to testify before a jury. A verdict preventing key experts from linking Roundup’s main ingredient, glyphosate, with non-Hodgkin’s lymphoma could deliver a disastrous blow to the case filed by farmers, landscapers, and consumers suffering from cancer.

During a weeklong videotaped hearing, dubbed “science week,” federal Judge Vince Chhabria, of the Northern District of California Court, heard testimony from dozens of scientists—statisticians, toxicologists, oncologists, and epidemiologists—who discussed scientific findings related to whether glyphosate causes cancer. The judge said his role would be acting as a gatekeeper to evaluate whether experts used valid methodologies to arrive at their conclusions, qualifying them to offer testimony.  

The lawsuit began in 2015, shortly after the World Health Organization’s International Agency for Research on Cancer ruled that glyphosate is a probable human carcinogen, countering determinations by regulatory agencies in the United States and other countries. Since then, more than 300 people have sued Monsanto in the federal case now centralized in California, claiming that Roundup caused them to develop cancer. Additionally, thousands of people have filed similar suits against the company in state courts in Arizona, New York, Delaware, Missouri, Nebraska, and elsewhere. Plaintiffs estimate 3,500 suits nationwide, the first of which is scheduled to go to trial on June 18 in San Francisco.

Monsanto has fiercely fought the cancer allegations. “Glyphosate has a long history of safe use based on evaluations spanning four decades,” company spokesperson Scott Partridge said. “The overwhelming conclusion of experts worldwide has been that glyphosate, when used according to label directions, does not present an unreasonable risk of adverse effects to humans, wildlife or the environment.”

The company’s argument rests on the National Institute for Health’s Agricultural Study examining 54,000 farmers in Iowa and North Carolina over multiple decades. Much of the testimony during the week revolved around that study, which was interpreted in varying ways.

The plaintiffs’ first witness, Beate Ritz, an epidemiologist at University of California Los Angeles, said the agricultural study had several shortcomings. She displayed a map showing that between 1994 and 2014, glyphosate exposure in the state of Iowa had become masked by the chemicals’ omnipresence in the environment—so many people were exposed, she said, that the Iowa study was irrelevant. Thus, “in 2014 Iowa, it would not even be possible to estimate any risk,” she said.

Later, a Monsanto witness, Harvard epidemiologist Lorelei Mucci, would flip Ritz’s argument upside down, saying that the supposed flaw was actually a “real strength,” because “if something were to be associated with cancer, what you’d expect is a lot more exposure to it would be associated with even stronger risk.” Additionally, Mucci said the plaintiffs weren’t considering the risks posed to farmers by exposure to other pesticides, especially those independently linked with cancer, a point that the judge himself questioned multiple times throughout the hearings.  

Judge Chhabria asked Ritz specifically if it is her opinion that “Roundup is currently causing NHL in the exposure levels that human beings are experiencing today, or is it that Roundup is carcinogenic, and therefore it’s capable of causing NHL in the abstract, or somewhere in between?” Ritz replied that it’s probably the latter. “We know that the toxicology is in the dose,” and farmers have the highest dose.

Plaintiff witness Dennis Weisenburger, a physician and pathologist specializing in non-Hodgkin’s lymphoma, testified that studies of both animals and humans exposed to glyphosate showed links between the chemical and the cancer. “There is a body of evidence that is pretty compelling that glyphosate and the formulations are genotoxic in living cells,” he told the judge. Again, Chhabria expressed concern that studies making such connections were not taking into account exposure to other pesticides.  

Near the conclusion of the hearings, the judge gave no indication of which way he would rule, though he did express some frustration about the field of epidemiology. “I do have a difficult time understanding how an epidemiologist in the face of all the evidence that we saw and heard last week” can conclude that glyphosate is “in fact causing” non-Hodgkin lymphoma in human beings, he said. “The evidence that glyphosate is currently causing NHL” at current exposure levels is “pretty sparse,” he added.

It is not yet clear which witnesses will be permitted to testify if and when the federal suit goes to trial. (The state suits follow a different standard for admitting expert testimony, which is why the first of those trials is scheduled to begin this summer.) Plaintiff attorney Pedram Essandiary, with the California firm Baum, Hedlund, Aristei, and Goldman said, “It’s hard to speculate on how the judge will rule, but whatever the outcome, we are going to have hurdles to cross.”  

FERN wrote an in-depth article about the glyphosate controversy in an article with The Nation magazine in 2017.

Rene Ebersole writes and edits articles about science, health, and the environment for such outlets as National Geographic, Popular Science, Prevention, Outside, Modern Farmer, and Audubon, in addition to FERN.