SAN FRANCISCO (CN) - Continuing a week of expert testimony on the alleged link between cancer and the world’s most widely used pesticide, a scientist said Thursday he found no connection between the two in animal studies.
The fate of more than 300 lawsuits filed against Monsanto and consolidated in a multidistrict case in San Francisco hinges on whether plaintiffs’ experts are allowed to testify before a jury on studies that show a link between the herbicide and cancer.
The alleged connection between glyphosate, the active ingredient in Monsanto’s Roundup weed-killer and non-Hodgkin’s lymphoma, along with the science behind it, has been heavily disputed.
“In my overall opinion, I found glyphosate to not be a carcinogen in rats and mice,” said Dr. Thomas Rosol, a veterinary pathologist from Ohio State who specializes in toxicology.
Rosol said he studied 12 bioassays - five on mice and seven on rats - to determine the potency of glyphosate. A bioassay is a method of determining the potency of a substance.
“This is an enormous data set. I’ve not been involved in a study where I’ve looked at 12 bioassays for one chemical,” Rosol said. “I looked at each bioassays individually. None of them demonstrated to me evidence of carcinogenicity.”
He added: “It’s important to put this data set into perspective. This is one of the cleanest sets of data I have ever evaluated in terms of carcinogenicity and this is something I routinely do.”
The lawsuits were filed by farmers and landscapers who say exposure to Roundup’s active ingredient caused them to develop cancer of the lymph nodes, organs crucial for a properly functioning immune system.
In 2015, the World Health Organization’s International Agency for Research on Cancer concluded that glyphosate is “probably carcinogenic to humans.”
Monsanto presented two of its experts on Thursday: Dr. Rosol and Dr. Christopher Corcoran, a biostatistician who poked holes in the IARC study on which Dr. Christopher Portier, an expert in carcinogenicity and one of plaintiffs’ witnesses, was an adviser.
Corcoran said there were multiple problems with Portier’s study.
“He hasn’t used a consistent approach. It’s been kind of a moving goal post,” Corcoran said.
Corcoran also objected to Portier’s pooling of data, a statistical technique that combines results from different studies and clinical trials to get a clear answer.
Corcoran called the pool approach “seriously flawed” that can lead to “spurious associations.”
U.S. District Judge Vince Chhabria will decide what expert testimony a jury should be allowed to hear after the week’s events conclude on Friday. His charge is not to determine whether Roundup causes cancer, but whether the experts’ claims met the standard of having been tested, subjected to peer review and been widely accepted in the scientific community. Chhabria will also look at the competing studies’ rate of error and controlling standards.