The International Agency for Research on Cancer, a division of the World Health Organization, on Friday released classifications of five pesticides, most notably identifying glyphosate as "probably" carcinogenic to humans.
The group reviewed the five pesticides and insecticides – glyphosate, malathion, diazinon, etrachlorvinphos and parathion – because "substantial new data" is available on many pesticides that have widespread exposures, and review was recommended by an advisory group, IARC said.
Glyphosate, along with malathion and diazinon, were listed as "probably carcinogenic to humans," or group 2A. The category is used when there is limited evidence of carcinogenicity in humans, but sufficient evidence in experimental animals.
Limited evidence means a positive association has been observed between exposure to the agency and cancer has been noted, but other explanations for the observations can't be ruled out, IARC said.
The IARC report more specifically found "limited evidence of carcinogenicity" in humans for glyphosate, but said studies show it can cause DNA and chromosomal damage in human cells, despite returning negative results in tests using bacteria.
According to IARC, glyphosate has the highest global production of all herbicides. It is an active ingredient in the popular herbicide Roundup, developed by Monsanto.
Dr. Philip Miller, vice president of global regulatory affairs for Monsanto, said members of the EU and U.S. glyphosate task forces are in disagreement with the IARC classification.
According to a statement from Miller, the task forces determined that in the IARC study there is no new research or data that was used; the most relevant, scientific data was excluded from review; the conclusion is not supported by scientific data; and there is no link between glyphosate and an increase in cancer when the full data set is included in a rigorous review.
"We don’t know how IARC could reach a conclusion that is such a dramatic departure from the conclusion reached by all regulatory agencies around the globe," Miller said. "We have issued an urgent request for appropriate personnel of the WHO to sit down with the global glyphosate taskforces and other regulatory agencies to account for the scientific studies used in their analysis and, equally as important, to account for those scientific studies that were disregarded."
Miller said the IARC review "does not meet the standards used by respected agencies around the world."
In glyphosate's original registration document, the U.S. EPA lists the pesticide as showing "evidence of non-carcinogenicity for humans – based on the lack of convincing evidence of carcinogenicity in adequate studies."
There are 70 other items in group 2A, according to the IARC. Group 2A is preceded by group 1, carcinogenic to humans, and is followed by groups 2B, possibly carcinogenic to humans; 3, not classifiable as to its carcinogenicity to humans; and 4, probably not carcinogenic to humans.
IARC's review was completed by 17 scientists from 11 countries March 3-10, 2015.