The weed-killing chemical glyphosate, which Monsanto uses in its Roundup herbicide, has been getting increased attention for its potential link to cancer.

Several gardeners and weed-whackers in the US who were exposed to glyphosate have won billions of dollars in lawsuits against Monsanto.

Glyphosate has been shown to cause harm in large doses. The World Health Organization's International Agency for Research on Cancer says it "probably" causes cancer. But t he EPA and Bayer (the company that now owns Monsanto) maintain that glyphosate does not cause cancer in humans.

B ayer and the US government may not be considering all types of exposure in their analyses, according to a study published in the journal Environmental Sciences Europe

Bayer announced this month that the company is committing $5.6 billion to the research and development of potential glyphosate alternatives.

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Gardeners Alva and Alberta Pilliod say they used Monsanto's Roundup spray to keep weeds off their driveway for more than two decades, applying the herbicide while wearing flip flops, shorts, and tank tops. Now, they both have Non-Hodgkin's Lymphoma (NHL). In a lawsuit, they were awarded $2.05 billion in damages in May.

School groundskeeper Dewayne Johnson, now in his 40s, also used to spray Roundup up to 30 times every summer. He has the same blood cancer as the Pilliods, and won nearly $80 million in a suit last year.

A third man, Edwin Hardeman, used Roundup for more than 25 years to keep weeds off his oak trees. He got NHL, too. A jury in California ruled in March that Roundup was a "substantial factor" in his diagnosis.

More than 13,000 similar lawsuits have popped up in the US. Almost all are being filed by gardeners, groundskeepers, and other professional weed-whackers who allege that their consistent, repeated use of Roundup gave them cancer. One of the latest suits against Monsanto came from 44-year-old groundskeeper Jeffrey Sabraski, who alleges he sprayed Roundup several times per week, wearing nothing but shorts and t-shirts, and also developed NHL, as the Minneapolis Star Tribune reported.

Roundup is the most widely-used herbicide in the world, and its key chemical is called glyphosate. The plaintiffs in this recent surge of court cases say that Monsanto didn't sufficiently warn them about the potential health risks associated with glyphosate or tell them how to protect themselves from exposure.

On Friday, Bayer — the company that now owns Monsanto — announced that it will devote 5 billion Euros ($5.6 billion) to researching potential glyphosate alternatives over the next 10 years.

But teasing out the health risks of glyphosate isn't easy. The World Health Organization's International Agency for Research on Cancer (IARC) says the chemical is "probably" carcinogenic to humans. But the US Environmental Protection Agency (EPA), and Bayer both say glyphosate isn't linked to cancer in any meaningful way.

"EPA continues to find that there are no risks to public health when glyphosate is used in accordance with its current label and that glyphosate is not a carcinogen," the agency says on its website. Canadian health authorities support the US' stance, and several other major regulatory bodies around the world also say it's unlikely that glyphosate is carcinogenic to humans, including the European Food Safety Authority (EFSA).

Read More: 33 of the most dangerous things science has strongly linked to cancer

However, peeling back the curtain on the scientific studies that have looked at glyphosate thus far can tell us a lot about how the chemical could indeed be dangerous when people and animals are exposed to it in the wrong ways.

Argentinian former farm worker Fabian Tomasi used to work supplying herbicides for plane-spraying and did not use protection. Pablo Aharonian/AFP/Getty Images

What Monsanto says about glyphosate

Glyphosate, which has been in use around the world since the 1970s, blocks an enzyme pathway that helps plants grow. The herbicide's patent expired in 2000, and Indian and Chinese farmers are now the weed-killer's biggest buyers.

It's tricky for epidemiologists to say conclusively that something causes cancer, so what we know so far — from thousands of studies conducted in rats, mice, and humans — is that the pesticide might be linked with cancer, or it might not. Glyphosate might make cancer more likely, since there's evidence it can encourage DNA damage. But more research is still needed to learn how it affects the human body.

Monsanto maintains that there's no good evidence that glyphosate causes cancer. A 2017 study of more than 54,000 pesticide sprayers in Iowa and North Carolina (mostly farmers) found no statistically significant association between glyphosate and cancer. (The results did suggest that chemical sprayers with the highest exposure to glyphosate had a slightly higher risk of developing acute myeloid leukemia, but the difference was so small that more research is needed.) Large-scale studies of French, Norwegian, and North American farmers have found glyphosate to be safe, too. Plus, chemical makers registering their pesticides with the EPA are required to provide data that demonstrates their products are safe.

So as long as consumers follow the instructions on the label, Monsanto says, glyphosate-based herbicides are safe.

"At the end of the day, whether you're in the court of law, regulatory agencies, or court of public opinion, it's the science that should matter here," a Bayer spokesman said in an email to Business Insider. "And the extensive body of science over 40 years, including several recent human epidemiology studies, shows that glyphosate-based herbicides are not associated with NHL. Customers who know these products best continue to rely on them."

In the Pilliod's trial, environmental scientist Robert Phalen testified that Alva Pilliod would have had to apply Roundup at least 95,000 times in order to absorb a teaspoon of the product into his skin. (This Robert Phalen is not the same scientist who said modern air may be "a little too clean," though the two did once author a textbook together.)

What the EPA says about glyphosate

The EPA has also consistently said that consumers have nothing to worry about when it comes to glyphosate.

"In 2017, EPA published comprehensive ecological and human health risk assessments for glyphosate. No human health risks were identified," the agency said in April. However, the EPA cautioned that "potential ecological risks were identified for terrestrial and aquatic plants, birds, and mammals, primarily from exposure to spray drift."

Humans, of course, are terrestrial mammals, too.

"The evidence is not as strong so as to be absolutely sure" Manolis Kogevinas, of the Barcelona Institute for Global Health, told Business Insider. "But yeah, there's definitely a chance that they will develop cancer."

The IARC has a different stance

In March 2015, the IARC convened a group of 17 experts from 11 countries to determine the potential human health dangers of glyphosate, along with four other pesticides and herbicides. The international, non-partisan committee concluded that glyphosate is "probably" carcinogenic to humans, based on studies in humans and lab animals.

That's why glyphosate is now a "Group 2A" carcinogen, according to IARC. That categorization means the chemical is considered to be a contributor to cancer, but not on the level of well known cancer-causers like formaldehyde and UV radiation.

But the IARC also has a complicated history with glyphosate research. Reuters reported in 2017 that the agency had cut sections out of a draft report that suggested glyphosate might not contribute to cancer. In a follow-up letter to Congress, IARC director Christopher Wild wrote that the group revised that draft because some of the data came from "a review article co-authored by a Monsanto scientist."

He added that the information from that article "was insufficient for independent evaluation of the individual studies and the conclusions reached by the Monsanto scientist."

Why these groups came to different conclusions

A tightly reviewed and vetted paper published in the journal Environmental Sciences Europe earlier this year looked into why the EPA and IARC appear to disagree about the risks of glyphosate.

The study found that while the EPA and the IARC looked at some of the same studies, the EPA relied on more "unpublished regulatory studies," many of them industry-funded. The IARC, on the other hand, looked at "mostly peer-reviewed studies," including three in humans that suggested glyphosate might be toxic.

That might help explain why 99% of the glyphosate studies the EPA considered found that the chemical was not a cancer-causer, while 70% of the studies the WHO looked at suggested it probably is.

"It's kind of easy to understand why EPA would say, 'Well, geez, you know, it must be okay,'" study author Charles Benbrook, an agricultural economist who now serves as an expert witness for plaintiffs involved in Roundup litigation, told Business Insider. "It was okay because they were looking at a bunch of negative studies that Monsanto [did]. Monsanto repeated essentially the same study like 30 times."

Benbrook alleges that the reason so many big payouts are being won in court is that Monsanto didn't properly warn people who used Roundup at home or on a small scale that they should wear protective clothing when they applied the weed-killer.

"It's not farmers out in the field with their large, modern spray rigs, where the operator is inside a steel and glass cab with a sophisticated air filtration system that essentially eliminates exposure," Benbrook said. "Some people are spraying Roundup 6 or 8 hours a day for five days a week with this kind of handheld equipment."

Monsanto disputes Benbrook's statements and points to hundreds of glyphosate studies in EPA databases that were not Monsanto-funded.

Christine Sheppard in her backyard garden in Oceanside, California. Sheppard said she sprayed Roundup for years to control weeds on her coffee farm in Hawaii. In 2003, she was diagnosed with non-Hodgkin's lymphoma and given six months to live. Now 68, she is in remission but experiences severe pain in her hands and legs from her cancer treatment and has a weak immune system. AP Photo/Gregory Bull

Monsanto tweaked its Roundup formula in Europe

The US Food and Drug Administration (FDA) recommends that pets stay off lawns and avoid plants that have just been sprayed with glyphosate, since animals can develop digestive or intestinal issues if they touch the substance before it dries. In people, glyphosate has also recently been linked to a higher level of severity in cases of fatty liver disease.

Beyond glyphosate, there's another compound in Roundup: soap-like detergents called surfactants that help the chemical penetrate the leaves of plants. Benbrook thinks Monsanto should be doing more to warn people about how surfactants work and how to protect themselves.

"It's the surfactants that are carrying the glyphosate first through the skin and inside the body, and then inside of cells where damage can be done to DNA," he said.

In Europe, Benbrook noted, surfactant chemicals have been made less toxic for consumers. In the US, however, Monsanto has not changed Roundup formulas in the same way.

"When juries learn that Monsanto is making a product that's at least 10 times, maybe 100 times safer all across Europe?" he said. "That's not acceptable. That's one of the reasons that I think we can expect a continuation of these mammoth punitive damage awards that have occurred in the first three trials."

Bayer said that allegation is one "we don't believe is supported by the extensive body of science," and noted that Monsanto tweaked the surfactants in products sold in Europe "due to market preferences or country-specific requirements in the region."

The company added: "Regulators have specifically assessed the safety of the class of surfactants used in glyphosate-based herbicides. In 2009, the US EPA concluded that these surfactants are not carcinogenic."

Argentine environmentalist Sofia Gatica tries to stop the spraying at a soybean field in Dique Chico, Argentina, on January 20, 2018. Diego Lima/AFP/Getty Images

Traces of glyphosate in food don't seem to hurt people

As lawsuits about glyphosate have entered the spotlight, some people have expressed concerns about the trace amounts of the chemical in our food.

The quantity of glyphosate in a bowl of Cheerios or a glass of wine are probably not going to kill anyone, though.

"Using the internationally accepted limits, an average adult male would have to drink over 1,000 liters of wine a day to reach any level of risk," pharmacologist Ian Musgrave recently wrote in The Conversation.

Kogevinas likewise doesn't think the chemical's presence in food is much to worry about.

"If you ask me, is the glyphosate the problem in my food? No, it is not," he said. But he noted that he prefers to buy organic produce to avoid pesticides more generally.

Kogevinas lives in Barcelona, where Roundup formulations adhere to the European standards, but that did not affect his decision about whether or not to buy it: He has opted never to use glyphosate in his own garden.

Update: This story was originally published on June 1, 2019. It has been updated with the latest news about Bayer's research, as well as with additional information about the study in Environmental Sciences Europe, the EFSA's stance on glyphosate, and the Pilliods' trial.