2019 Paper on Glyphosate Genotoxicity

IARC vs. EPA Genotoxicity Paper


How did the U.S. EPA and IARC reach diametrically opposed conclusions on the genotoxicity of glyphosate-based herbicides?“,  Environmental Sciences Europe, January 15, 2019, DOI: 10.1186/s12302-018-0184-7.

By: Charles Benbrook, PhD

Key Findings:

Genotoxicity studies cited in EPA’s 2016 “Glyphosate Issue Paper: Evaluation of Carcinogenic Potential” and in the glyphosate chapter of IARC’s 2017 carcinogenicity evaluation “Monograph 112: Some organophosphate insecticides and herbicides” are compared and contrasted.

Key findings are highlighted below. The graphics are available in different formats (see below).

Key Finding #1: 

While IARC referenced only peer-reviewed studies and reports available in the public literature, EPA relied heavily on unpublished regulatory studies commissioned by pesticide manufacturers. In fact, 95 of the 151 genotoxicity assays cited in EPA’s evaluation were from registrant studies (63%), while IARC cited 100% public literature sources.

There is also a stark difference in the outcomes of registrant-sponsored assays versus those in the public literature.  Of the 95 regulatory assays taken into account by EPA, only 1 reported a positive result, or just 1%.  Among the total 211 published studies (right circle in the graphic), 156 reported at least one positive result, or 74%!

Key Finding #2:

Another important difference is that EPA focused their analysis on glyphosate in its pure chemical form, or “glyphosate technical.”

IARC, on the other hand, placed considerable weight on 85 studies focused on formulated GBHs, the herbicide people actually use and are exposed to (“Glyphosate Herbicides” in the graphic). 79% of the GBH assays published in public literature reported one or more positive result.

While EPA did list studies on formulated glyphosate-based herbicides (GBHs) in an Appendix F of its report, EPA acknowledges it placed little to no weight on GBH assay results.

This difference is reflected in the overall percent of positive assays.  Just 24% of the 151 assays cited by EPA reported positive results, while 76% of those cited by IARC had at least one positive result.

Key Finding #3

EPA’s analysis was limited to typical dietary exposure to the general public as a result of  legal uses on food crops, and did not address occupational exposure and risks.

IARC’s assessment encompassed data from typical dietary, occupational, and elevated exposure scenarios.   Elevated exposure events caused by spills, a leaky hose or fitting, or wind are actually common for people who apply herbicides several days a week, for several hours, as part of their work. The highest exposures typically occur when herbicides are applied with a handheld or backpack sprayer, or an ATV or truck-mounted sprayer.

The equipment used to apply GBHs has a huge impact on applicator exposures, as does whether applicators use Personal Protective Equipment like gloves. The applicator driving the modern, large-scale sprayer pictured on the left might typically spray around 500 acres in an 8-hour day, apply 700 pounds of glyphosate technical, and be exposed to 3.5 milligrams (3,500 ug, or micrograms). The applicator using a backpack sprayer without gloves would apply only about 3 pounds of glyphosate in 8 hours, but would be exposed to around 175 milligrams of glyphosate, 50-times more than the driver of the spray rig. And that driver would spray more than 230-times more glyphosate in an 8 hour day. In virtually all high-exposure scenarios, wearing gloves makes a big difference, reducing exposures by around one-half to one-tenth of “no gloves” exposure levels. The estimated exposures in the scenarios in the graphic represent applications during which everything goes “by the book,” but in the real-world a variety of factors can, and frequently do dramatically increase exposure levels.

Key Finding #4 

A closer look at the assays referenced by EPA but not IARC, and by IARC but not EPA helps explain why EPA and IARC reached opposite conclusions.

EPA cited 109 total assays not included in the IARC report, 87% of which were regulatory studies commissioned by industry, and all but one was negative.

IARC included the results from 67 assays not included in EPA’s analysis, all of which were from peer-reviewed publications, and 82% of which had at least one positive result for genotoxicity.

Graphic Files:

Related EPA and IARC Resources:

EPA Documents:

IARC Documents:

Other Resources: