A long-term survey of tens of thousands of U.S. farmers found no significant link between cancer and exposure to glyphosate, best known as the main ingredient in Monsanto Co.'s Roundup herbicide.
But the study shines a light on previously neglected areas, and could lead to further inquiry into the link between glyphosate and an uncommon form of leukemia, one of the paper’s authors said.
The paper, published in the Journal of the National Cancer Institute Nov. 9, marks the first update in 12 years to the continuous national Agricultural Health Study (AHS) of farmers. The findings are consistent with the last paper from the AHS, which also found no correlation between the controversial weedkiller and cancer.
However, scientists did find a more than twofold increase in cases of acute myeloid leukemia (AML), a rare cancer of the blood, for the most exposed farmers compared with those who never sprayed glyphosate.
This finding is not statistically significant, Laura Beane Freeman, co-author of the paper, said because of the small number of AML cases. But it signals a need for further study of the matter.
“There’s definitely a need to look at glyphosate in other exposed populations,” Freeman, an epidemiologist with the National Cancer Institute, told Bloomberg Environment. “I think replication of this [AML] finding in other populations is probably the most important thing to do.”
The study results are the best example of how glyphosate affects farmers and their spouses in the real world, according to Scott Partridge, Monsanto’s vice president of global strategy.
“It is the gold standard. It conclusively demonstrates that there is no association between the exposure, use of glyphosate, and cancer,” Partridge told Bloomberg Environment.
Largest Study of Its Kind
The National Cancer Institute began tracking farmers and their families in 1993 to measure their exposure to a number of pesticides and other environmental factors, and the incidence of cancer. It’s the largest study of its kind in the world.
The latest paper counted and studied more than 54,000 farmers in North Carolina and Iowa. The institute released findings in 2005 from the survey that also found no strong link between glyphosate exposure and cancer.
The latest paper includes more than three times the cancer cases found in the 2005 results—7,290 compared to 2,088.
The paper also marks the first time the institute could analyze different types of leukemia, Freeman said.
The size of the study gives it importance, John Yu, an assistant professor in the University of Georgia’s Department of Environmental Health Science, told Bloomberg Environment. But it has some weaknesses.
The paper measures exposure only by the days and years a person was around glyphosate, but doesn’t take into account measures to protect oneself, such as wearing protective clothing. It also doesn’t mention exposure to other pesticides, which can be a confounding factor, Yu said.
News of the findings of the paper hit headlines months before its official release. In June, legal documents surfaced indicating that the senior scientist in charge of the AHS, Aaron Blair, thought the AHS findings could have led to a different outcome on a controversial scientific finding on glyphosate.
In 2015, the International Agency for Research on Cancer assessed the scientific literature on the weedkiller and found it to “probably” cause non-Hodgkins lymphoma, a cancer of the lymphatic system. IARC didn’t look at the latest AHS data to come to its determination, even though it was complete, because it hadn’t been published in a scientific journal.
The conclusion put the spotlight on a chemical that has been treated as a relatively safe herbicide by regulators around the world. Monsanto launched a campaign to discredit IARC’s monograph, or assessment of the substance.
IARC Had ‘Full Access’ To Earlier Findings
Recent reports also indicate that IARC favored certain studies that showed an association between glyphosate and cancer.
“It’s time now for IARC to withdraw that monograph,” Partridge of Monsanto said.
Freeman disagrees that the latest study would have moved the needle on IARC’s decision. The agency had access to the published 2005 report, which came to the same conclusion as the latest one: glyphosate exposure does not result in higher cancer rates.
“They had full access to the 2005 paper that showed no association,” Freeman said.
Although Monsanto is the original maker of glyphosate, dozens of companies now make it. As of 2010, more than 750 glyphosate products were sold in the U.S., and it was registered for use in more than 130 countries, according to the paper.
The current 15-year authorization to let farmers continue to use glyphosate in the 28-nation European Union expires Dec. 15.
But a sufficiently large bloc of countries opposed a compromise five-year reauthorization in a regulatory committee vote Nov. 9, meaning the decision will be kicked to another committee meeting before the end of November.