Glyphosate herbicide linked to cancer - IARC World Health Organization assessment

The International Agency for Research on Cancer (IARC) - the cancer evaluation arm of the World Health Organization - convened a meeting of 17 scientific experts from 11 countries to assess whether certain pesticides, including glyphosate, caused cancer in humans (Meeting 112, March 2015). The outcome of that meeting is that glyphosate "probably" causes cancer in people (IARC Group 2A). This is particularly alarming for a chemical that is used in over 750 herbicide products and applied to fields in the US at over 250 million pounds annually (USGS 2012 data). The IARC review underscores the need for the Environmental Protection Agency (EPA) to examine and act on all we've learned about glyphosate's dangers in the two decades since it was last approved for use.

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IARC's decision to classify glyphosate as "probably" carcinogenic to people (Group 2A) was made unanimously, after reviewing hundreds of scientific studies, based on three lines of evidence:

  • "sufficient" evidence of cancer in mice and rats that were fed glyphosate over a several years (see reports by EPA 1991 and the WHO 2004);
  • "strong" evidence from mechanistic or cellular studies that explain how glyphosate may cause cancer;
  • And, "limited" evidence from epidemiologic studies of people, particularly pesticide applicators and farmworkers.

The details of the IARC decision are reported in the Lancet Oncology in summary, with the full length Monograph expected by early 2016. More details of the evidence are provided below.

A meta-analysis of all the relevant epidemiologic studies was conducted by IARC staff scientists in 2014 that reported an overall lack of data, particularly in low- and middle-income regions of the world where the health of agriculture workers is poorly documented, if at all. Farmers, farmworkers, and pesticide applicators are the unfortunate 'canaries in the coal mine' when it comes to pesticide poisoning and chronic diseases like cancer, because they are living in close range to the highest concentrations of agrochemicals. For example, the U.S. National Cancer Institute has a large Agriculture Health Study that has reported a higher risk of prostate cancer in farmers compared to the general population.

And cancer isn't the only health risk from pesticides. Increasing evidence suggests that pre-natal and early-life exposures to pesticides increase the risk of birth defects. For example, the war-era organophosphate pesticides have been shown to cause learning and behavioral impairments in kids born to both urban and city-dwelling mom's that were exposed to high levels during pregnancy (see summary of studies in my blog here). It is notable that the other pesticides that the IARC experts classified as "possible" (Group 2B) or "probable" (Group 2A) human carcinogens are organophosphate pesticides.

Farmers and their families are on the front line for pesticide health risks, and they will need support to identify other options and move away from glyphosate and other harmful pesticides. These options include moving towards integrated pest management, use of cover crops, and crop rotation practices to reduce their herbicide use.

NRDC has already filed two lawsuits and a petition with EPA to restrict the use of glyphosate-containing herbicides because of their devastating impact on monarch butterflies. The soaring use on genetically modified crops of glyphosate, the most widely used herbicide in the world, has wiped out much of the milkweed, a native wildflower that monarchs need to survive. The finding by IARC that glyphosate probably can cause cancer in humans only adds urgency to the need for EPA to launch an immediate review of the herbicide's toxicity. It was last assessed in 1993, well before the massive surge in glyphosate use and the disturbing new evidence of its probable health impact.

EPA's review should consider whether glyphosate should be pulled from the market immediately to protect human health and the environment. A letter from NRDC, Environmental Working Group, and other environmental groups asks EPA to consider whether immediate use restrictions are necessary to protect human health and the environment.


Details of the IARC review of glyphosate:

The IARC Working Group of experts made an independent assessment of existing studies, including those sponsored by Monsanto, the glyphosate manufacturer. Monsanto representatives attended the 8-day meeting as observers and given the opportunity to speak, but did not vote on the classification of glyphosate or other pesticides being reviewed by the Working Group experts.

The IARC Working Group reviewed ten laboratory studies on rodents, five on mice and five on rats, culled from the public literature and reports of the EPA (1991) and the WHO (2004). One mouse study reported a positive trend for heamangiosarcoma (WHO report) and another mouse study reported a rare tumor called a renal tubular carcinoma (EPA study) associated with glyphosate in long-term feeding studies. In both cases, effects were stronger in the males than females. The IARC Working Group of experts also reviewed five studies in rats: two reported significant increases in pancreatic islet-cell adenomas in male rats (EPA report); two studies did not find significant cancer increases; one study did not last long enough to draw any conclusions about cancer risks. The Working Group determined that there was a statistically significant trend in the occurrence (incidence) of haemangiosarcoma in the male mice, but not the female mice. In accordance with the pre-specified procedures and criteria of IARC, the Working Group concluded that the studies provided evidence of carcinogenicity in experimental animals.

The IARC experts also reviewed cellular studies and determined that there was "strong" evidence from mechanistic studies showing that glyphosate caused damage to the cell's genetic information (genotoxicity), which can lead to abnormal cell function and ultimately a cancerous cell. Studies also showed that glyphosate also caused oxidative stress in cells, which can lead to cellular damage and elevate the risk of a cell becoming cancerous. This mechanistic evidence - both genotoxicity and oxidative stress - provides a plausible explanation for how glyphosate may cause cancer, and therefore supports the evidence from the animal studies.

The IARC experts found that the evidence from epidemiologic studies provided some evidence of cancer, in particular elevated risk of non-Hodgkin lymphoma, but it was "limited" because the studies were either weakly positive or did not find a cancer risk at all.

Taken together, these three lines of evidence support the classification of glyphosate as a Group 2A chemical that "probably" causes cancer in exposed people, according to the pre-specified procedures and criteria of IARC.

More about glyphosate:

EPA first registered glyphosate for use in pesticides in 1974. As a non-selective herbicide, glyphosate does not discriminate between target and non-target plant species. Because of its damage to crops, glyphosate's use was initially limited. Since glyphosate was reregistered in 1993, however, the development of genetically-modified, glyphosate-resistant crops has facilitated a dramatic rise in the herbicide's application. As of 2012, over 250 million pounds of glyphosate were applied annually in the U.S. Having experienced an approximately ten-fold increase in use since its reregistration, glyphosate is now the most widely used herbicide in the United States.

Glyphosate is an herbicide that has been registered for many agricultural and non-agricultural uses. EPA has approved glyphosate's use on over 100 terrestrial food crops, including fruit, vegetable, and field crops. The agency has also registered glyphosate for use in non-crop settings, often to achieve total vegetation control. Non-crop areas to which glyphosate can be applied includes residential, industrial, forestry, greenhouse, ornamental, aquatic, and other sites. When applied at lower rates, glyphosate also functions as a plant growth regulator.

About the Authors

Jennifer Sass

Senior Scientist, Health program

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