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Is glyphosate (Roundup) dangerous?

"The overall weight of evidence indicates that ... glyphosate ...was not associated with genotoxic [gene changes] effects.... [G]lyphosate is unlikely to pose a carcinogenic risk to humans from exposure through the diet.

At a Glance

Glyphosate, a garden herbicide introduced by Monsanto under the trade name Roundup in 1974 and off patent since 2000, is often paired with herbicide tolerant genetically modified crops. It allows farmers to spray a planted field, generally before the crops have sprouted, killing weeds but not the crops that will grow there. GMO critics claim glyphosate is linked to autism, cancer, gluten allergies, ‘leaky gut’ syndrome and other disorders using correlation graphs or studies by well known advocacy scientists, mostly in marginal journals. Concerns about glyphosate’s possible health impacts increased in 2015 after the International Agency for Research on Cancer, a research arm of the World Health Organization, classified glyphosate as “probably carcinogenic,” using what is called a hazard evaluation. The IARC classification was widely circulated by anti-chemical and ant-GMO advocacy groups, which argued for bans or tighter restrictions.

Regulatory oversight agencies in the US, Europe and elsewhere reviewed and, for the most part, rejected IARC’s cancer designation, noting it was based on the questionable quality of the studies evaluated to make its conclusions and that it focused on whether glyphosate might cause cancer in workers exposed to extreme doses over extended periods of times, not whether traces of it in our food pose a danger. According to the Environmental Protection Agency and almost every independent regulatory oversight agency, glyphosate is of “relatively low oral and dermal acute toxicity” and is not carcinogenic as used.  A joint panel from the World Health Organization and the Food and Agriculture Organization of the United Nations issued an summary evaluation of glyphosate in May 2016, concluding it poses no cancer risks as encountered in food and does not impact our genes. The US EPA released a long-delayed report in September 2016, considered to be one of the most extensive ever undertaken on the herbicide, concluding glyphosate is “‘not likely to be carcinogenic to humans’ at the doses relevant to human health risk assessment….” The global science consensus remains that glyphosate is a comparatively mild herbicidal toxicant that does not cause cancer or pose serious health threats to the general population.

Science and Politics

Glyphosate is derived from an amino acid, glycine. It acts by suppressing an essential biochemical mechanism commonly found in plants, but not in animals. According to the Extension Toxicology Network, a joint pesticide information project by Cornell University, Michigan State University, Oregon State University and University of California-Davis, and funded by US Department of Agriculture, glyphosate is non-volatile, minimizing exposure through inhalation and undergoes little metabolism in the human body. If accidentally consumed, glyphosate is excreted mostly unchanged in feces and urine, so it doesn’t stay in the body and accumulate.

How toxic is glyphosate exactly? To examine toxicity, one must look at a chemical’s LD50 value–a standard measure of acute toxicity for chemicals, expressed in the amount of chemical (milligrams) per body weight (kg) that it took to kill fifty percent of a population of test animals. Caffeine is over ten times more toxic than glyphosate. With LD50 of 192 mg/kg, it would take 12,192 mg of caffeine to kill an average 140 lb human being. A typical 8 oz cup of coffee only contains 95 mg of caffeine, much lower than the dose required for acute toxicity. The same reasoning applies to glyphosate. Using similar calculations, it would take 12.5 oz of glyphosate to kill an average 140 lb human being. That means drinking three gallons of Roundup Original. Using scientific measures, glyphosate is less toxic than baking soda and salt.

But what about long-term exposure to glyphosate? Given its widespread use, there is a good chance that we are eating residues in our food. The EPA considered this in setting maximum safe levels of residues, called tolerances. EPA conducted a dietary risk assessment for glyphosate based on a worst-case risk scenario, that is, assuming that 100 percent of all possible acreage were treated, and that tolerance-level residues remained in/on all treated commodities. The agency concluded that there is no evidence of chronic dietary risk posed by glyphosate food use. A multi-university research project funded by the US Department of Agriculture concluded that glyphosate, if accidentally consumed, undergoes little metabolism, does not accumulate and is excreted mostly unchanged in feces and urine.

In an attempt to get out their message that glyphosate is dangerous, scientists critical of GMOs have turned to predator pay-for-play journals (where authors pay to have their work published) to publish work that would never be accepted in mainstream science journals. In 2013 Stephanie Seneff, an MIT computer analyst with no expertise in toxicology, published a data analysis that correlated glyphosate use with all sorts of diseases, from depression to Parkinson’s—claims that drew sharp criticism from scientists with expertise in genetics and healths.


They pointed out that there was no original research and correlation is not causation. Just because two things are happening at the same time doesn’t mean one of them caused the other—the rise in autism also correlates with the rise in organic food sales.

Seneff and other GMO critics, including Jeffrey Smith, a former yogic instructor who heads the Institute for Responsible Technology, also allege that glyphosate disrupts gut microbes–a problem they relate to what has popularly become known as ‘leaky gut,” a syndrome that affects the intestinal walls and allows pathogenic contents of the intestines including E. coli, salmonella and botulism to enter the bloodstream.

The way glyphosate works is that it interrupts the shikimate pathway, a metabolic function in plants that allows them to create essential amino acids. When this path is interrupted, the plants die. Human cells don’t have a shikimate pathway so scientists and researchers believed that exposure to glyphosate would be harmless. The problem is that bacteria DO have a shikimate pathway and we have millions of good bacteria in our guts – our “gut flora.” These bacteria are essential to our health. Our gut isn’t just responsible for digestion, but also for our immune system. When glyphosate gets in our systems, it wrecks our gut and as a result our immune system.

The three studies often cited by Seneff, Smith and other well known anti-GMO opponents to support these claims provide no empirical evidence of microbial disruption in animals let alone humans.

Safety concerns were also raised by an oft-cited 2013 study by French geneticist Gilles-Éric Séralini, which featured pictures of rats with bodies twisted by cancer allegedly caused by exposure to glyphosate or GMO corn. The controversial study was retracted and then republished later without peer review. The study’s methodology was criticized by scientists and independent oversight agencies around the world, and was reviewed and dismissed by the National Academies its May 2016 report on genetically engineered crops.

Friends of the Earth Europe and Mom’s Across America claimed in 2014 that an informal test they funded found minute traces of glyphosate in breast milk and urine, causing a furor, with the story circulated even in such nominally mainstream blogs as Civil Eats. The results were challenged in a study led by Washington State University scientist and lactation expert, Michelle McGuire, who found no evidence that glyphosate accumulates in breast milk. Activists criticized the study–three Monsanto employees were authors. But McGuire’s team merely confirmed previous studies. And two subsequent German studies, including an independent report by scientists affiliated with the independent German Federal Institute for Risk Assessment (BfR)–which does risk assessments for the European Commission–published a separate study in 2016 found no traces of glyphosate.

In 2014, for the EC, the BfR reviewed hundreds of studies on glyphosate in a formal risk assessment, concluding: “the available data do not show carcinogenic or mutagenic properties of glyphosate nor that glyphosate is toxic to fertility, reproduction or embryonal/fetal development in laboratory animals.”

But concerns over the herbicide flared in 2015 with the release of an evaluation by a World Health Organization research sub-group, the International Agency for Research on Cancer in 2015. IARC did no original research. It did not review the hundreds of studies as did the EPA, USDA and BfR in making their conclusions; it considered a few dozen, eliminating all studies with financial links to industry or in which a researcher had professional associations with industry, as well as hundreds of independent studies. The panel concluded glyphosate was “probably carcinogenic” to agricultural workers, writing:

Limited evidence of carcinogenicity in humans for non-Hodgkin lymphoma. The evidence in humans is from studies of exposures, mostly agricultural, in the USA, Canada, and Sweden published since 2001. In addition, there is convincing evidence that glyphosate also can cause cancer in laboratory animals.

The claim of carcinogenicity, as limited and muddled as it was, generated global headlines, with Nature noting, “Widely used herbicide linked to cancer.” IARC’s declaratory statement was welcomed by GMO opponents, with blog headlines asking, “Why Monsanto’s Glyphosate Herbicide Should be Banned.” Many scientists and the regulatory community challenged IARC’s conclusions. No major international regulatory advisory agency had previously concluded glyphosate posed health risks. Among the most comprehensive prior analyses:

The mainstream scientific oversight agencies, including the BfR and the European Food Safety Committee (similar to the US EPA and FDA) criticized IARC for using poor methodology, confusing hazard with risk and selectively limiting the studies for evaluation to reach their conclusion. Conflict of interest charges were also raised because one of the scientists on the IARC committee was an advisor to a advocacy NGO, which is supposedly forbidden under IARC guidelines.

A joint panel from the World Health Organization and the Food and Agriculture Organization of the United Nations reviewed the IARC finings and issued its report on glyphosate in May 2016, concluding it poses no cancer risks as encountered in food and does not impact our genes. The toxicity was so low, the joint committee wrote, it was not necessary to establish a ARfD–an acute toxicity reference dose often used to regulate risk. Its also reviewed its impact on workers, noting that the only “high quality” study found no evidence of a cancer link.

In September 2016, the US Environmental Protection Agency issued what is considered the one of the most comprehensive reviews of the pertinent studies on glyphosate ever undertaken, authored by 13 prominent independent scientists, concluding:

…there is not strong support for the “suggestive evidence of carcinogenic potential” cancer classification descriptor based on the weight-of-evidence, which includes the fact that even small, non-statistically significant changes observed in animal carcinogenicity and epidemiological studies were contradicted by studies of equal or higher quality. The strongest support is for “not likely to be carcinogenic to humans” at the doses relevant to human health risk assessment for glyphosate.

glyphosate chart

University of Wyoming weed scientist Andrew Kniss put together a graphic of what each of the major studies on glyphosate has found related to its potential for causing cancer. The chart is an oversimplification, as Kniss has noted, but it illustrates that more than 20 studies have shown that glyphosate exposure reduces the cancer risk in some instances–which underscores why scientists look at “weight of evidence” to make carcinogenic calculations rather than depending on a small sample of studies, as IARC did. Regulatory scientists, reviewing this contradictory evidence, have concluded that glyphosate has no predictive impact as to whether human exposure at normal levels would result in cancer. 

Why is there such a discrepancy between the IARC hazard findings and risk assessments of the regulatory and mainstream science communities? Many people misunderstand how IARC and its findings are perceived in the global regulatory framework. For more than 40 years, the agency has assessed 989 substances and activities, ranging from arsenic to sunbathing hairdressing; it’s found only one was “probably not” likely to cause cancer in humans. But scientists and regulators have pushed back after the finding; almost any substance can be judged toxic, even water, if the ‘dose’ is extreme and the exposure time is long enough.

One of the basic principles of toxicology is that “the dose makes the poison.” Lots of things can cause cancer, but not everything that can cause cancer does. Even for common substances, if you are exposed to enough of it for a long period of time you will experience toxic health effects. Most scientists believe IARC’s hazard assessments are confusing to the public and the science community, and offer confusing regulatory guidance, noting for example that the sub-group also considers grapefruit juice and working the night shift to be as hazardous as glyphosate. Even more dangerous, in the organization’s opinion, are processed meat, sunlight, oral contraceptives, Chinese-style salted fish and alcohol–on the level of exposure to plutonium.

While IARC declared glyphosate a cancer hazard, its parent organization, the WHO, determined in three other reviews that the herbicide does not pose a cancer risk, and especially not in the minute amounts potentially consumed in food. IARC, in its own fine print, explains the difference between its hazard assessments and the risk assessments done by others:

… the Monographs Programme identifies cancer hazards even when risks are very low at current exposure levels, because new uses or unforeseen exposures could engender risks that are significantly higher.

The IARC study differed from science organization reviews in one other key way: it assessed commercial formulations that include a mixture of chemicals including glyphosate while most researchers have evaluated the chemical alone.

The Takeaway

Glyphosate is one of the world’s most studied chemicals. The IARC report, which focused on hazard and not risk, has been seized on by advocacy groups that oppose GMOs and target glyphosate as a proxy to oppose biotechnology. Almost the entire world science community challenges those views.

The US EPA and Health Canada, along with the European Food Safety Authority (EFSA) and the German BfR, addressed IARC’s cancer finding after its release.

A June 2015 re-review of glyphosate by Health Canada concluded:

An evaluation of available scientific information found that products containing glyphosate do not present unacceptable risks to human health or the environment when used according to the proposed label directions.

The EPA’s Carcinogenicity Peer Review Committee wrote a report in October (stamped ‘final,” it was posted on the web by the EPA in April 2016, then removed for unexplained reasons), sharply criticizing IARC’s methodology, concluding, “based on the weight-of- evidence, glyphosate is classified as “Not Likely to be Carcinogenic to Humans”. The EPA specifically rejected IARC’s claims that epidemiological studies raise questions of a likely cancer link.

The epidemiological evidence at this time does not support a causal relationship between glyphosate exposure and solid tumors. There is also no evidence to support a causal relationship between glyphosate exposure and the following non-solid tumors: leukemia, multiple myeloma, or Hodgkin lymphoma. The epidemiological evidence at this time is inconclusive for a causal or clear associative relationship between glyphosate and NHL.

In November 2015, the EFSA issued its evaluation of the IARC report:

Glyphosate is not proposed to be classified as carcinogenic under the EU regulation for classification, labelling and packaging of chemical substances. … neither the epidemiological data (i.e. on humans) nor the evidence from animal studies demonstrated causality between exposure to glyphosate and the development of cancer in humans.

The German BfR issued a FAQ on glyphosate in March 2016 again concluding: “based on current scientific knowledge, no carcinogenic risk to humans is to be expected from glyphosate if it is used in the proper manner for the intended purpose.”

A joint panel from the World Health Organization and the Food and Agriculture Organization of the United Nations issued a review of glyphosate in May 2016, concluding it poses no cancer risks as encountered in food and does not impact our genes. WHO has posted an online Q&A to address confusion over apparently conflicting evaluations, as four WHO agencies have concluded that glyphosate poses no significant risks while IARC raises hazard concerns.

The differences between IARC’s conclusions and the rest of the mainstream science community boils down to this: IARC carried out a “hazard assessment”, which evaluates whether a substance might pose a danger, and not whether it actually poses risks when used appropriately. There have been numerous reviews and hundreds of studies of the herbicide since its introduction that conclude it is safe when used as intended. Most scientists believe IARC has distorted the regulatory and public debate, unnecessarily scaring the public. IARC’s methods are poorly understood and do not serve the public well, according to Bob Tarone, a statistician formerly at America’s National Cancer Institute and now Biostatistics Director at the International Epidemiology Institute. He said of the way IARC works: “It’s not good for science, it’s not good for regulatory agencies. And for people? Well, they are just being confused.”

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