The cancer-research arm of the World Health Organization last week announced that glyphosate, the world’s most widely used herbicide, is probably carcinogenic to humans. But the assessment, by the International Agency for Research on Cancer (IARC) in Lyon, France, has been followed by an immediate backlash from industry groups.
On March 23, Robb Fraley, chief technology officer at the agrochemical company Monsanto in St Louis, Missouri, which sells much of the world’s glyphosate, accused the IARC of “cherry picking” data. “We are outraged with this assessment,” he said in a statement. Nature explains the controversy.
What does the IARC report say?
The IARC regularly reviews the carcinogenicity of industrial chemicals, foodstuffs and even jobs. On March 20, a panel of international experts convened by the agency reported the findings of a review of five agricultural chemicals in a class known as organophosphates. A summary of the study was published in The Lancet Oncology.
Two of the pesticides — tetrachlorvinphos and parathion — were rated as “possibly carcinogenic to humans”, or category 2B. Three — malathion, diazinon and glyphosate — were rated as “probably carcinogenic to humans”, labelled category 2A.
Why should I care about glyphosate? Glyphosate is the world’s most widely produced herbicide, by volume. It is used extensively in agriculture and is also found in garden products in many countries. The chemical is an ingredient in Monsanto's weedkiller product Roundup, and glyphosate has become more popular with the increasing market share of crops that are genetically engineered to be tolerant to the herbicide.
What evidence is there for a link between glyphosate and cancer? The IARC review notes that there is limited evidence for a link to cancer in humans. Although several studies have shown that people who work with the herbicide seem to be at increased risk of a cancer type called non-Hodgkin lymphoma, the report notes that a separate huge US study, the Agricultural Health Study, found no link to non-Hodgkin lymphomas. That study followed thousands of farmers and looked at whether they had increased risk of cancer.
But other evidence, including from animal studies, led the IARC to its ‘probably carcinogenic’ classification. Glyphosate has been linked to tumours in mice and rats — and there is also what the IARC classifies as ‘mechanistic evidence’, such as DNA damage to human cells from exposure to glyphosate.
Kathryn Guyton, a senior toxicologist in the monographs programme at the IARC and one of the authors of the study, says, “In the case of glyphosate, because the evidence in experimental animals was sufficient and the evidence in humans was limited, that would put the agent into group 2A.”
But not everyone agrees?
An industry group of agrochemical companies called the Glyphosate Task Force said that the agency’s evaluation “demonstrates serious deficiencies in terms of methodological approach and the overall conclusion is inconsistent with the results of all regulatory reviews concerning glyphosate’s safety profile”.