As a third-generation cotton farmer in Bakersfield, California, John Barton estimates that he sprayed thousands of gallons of the herbicide Roundup over the course of his 30-year working life.

“My family were farming 1,000 acres of cotton, so we’d be out in the fields spraying it, and we’d get our pants wet, our shoes wet, our socks wet, and if the wind changed it would blow in our face,” Barton tells me. “We did that spring, summer and fall for most of my life. There was really no regulation at the time that we were spraying Roundup; no one was offered any protection. But I didn’t think anything of it, as they kept telling us how safe it was.”

By ‘they’, Barton is referring to Monsanto, the corporation that produces Roundup. Monsanto, which was acquired by the German pharmaceutical giant Bayer last year, is currently facing more than 9,000 lawsuits across the US from plaintiffs, mostly former gardeners and agricultural workers who believe that Roundup exposure caused their cancer.

Last summer, former school groundskeeper Dewayne Johnson, who is terminally ill with non-Hodgkin lymphoma, won a landmark victory against the company when jurors ruled that Monsanto had failed to warn him of the health risks posed by Roundup. In the latest trial, which recently got under way in San Francisco, Edwin Hardeman, who suffers from an aggressive form of non-Hodgkin lymphoma, is pursuing a similar verdict. Like Johnson and Hardeman, Barton has also developed non-Hodgkin lymphoma and is preparing to take legal action.

Dewayne Johnson won a landmark trial against Monsanto in 2018, after developing non-Hodgkin lymphoma. Photograph: Reuters

“There’s not really a lot of history of cancer in my family,” he says. “I’ve been healthy all my life, so when I was diagnosed in 2015 it surprised me that all of a sudden I had this disease. Now, as a father, I’m worried that I’ve exposed my sons, who are also farmers, to the same cancer.”

But while Johnson’s legal triumph sent reverberations across the world last year, the very question of whether a weedkiller could be responsible for a person’s cancer remains a divisive and highly charged topic across both the scientific and political worlds.

In 2015, the International Agency for Research on Cancer (IARC) ruled that glyphosate – the active chemical within Roundup and many other popular weedkillers – was “probably carcinogenic”. However, numerous other international agencies, including the European Chemical Agency and European Food Safety Authority (Efsa), continue to declare glyphosate as safe, and there are many scientific studies which have found no association with cancer.

An estimated 6.1 billion kilos of glyphosate-based weedkillers were sprayed across gardens and fields worldwide between 2005 and 2014 (the most recent point at which data has been collected). That is more than any other herbicide, so understanding the true impact on human health is vital.

So, what do we know, and why is there so much uncertainty?

‘Where are all the bodies?’ The inconclusive data

The reason glyphosate was thought to be completely safe for many years is that it works by inhibiting an enzyme pathway behind plant growth, which does not exist in humans. Since the introduction of Roundup-resistant GM food crops – genetically engineered to resist glyphosate – in the mid-1990s, farmers in the US have been able to use it in large quantities to get rid of weeds selectively, while in the UK it is used as the weedkiller of choice, outside of the growing season.

But in the past two decades, some research has suggested that glyphosate may not be as benign as once thought. Last month, a high-profile collaborative study by three US universities reported that individuals with particularly high exposures to glyphosate-based herbicides, for instance those spraying it, could have a 41% increased relative risk of developing non-Hodgkin lymphoma.

“The lifetime risk of developing NHL is usually around 1 in 50, so what this means is that in populations who are exposed to the very highest levels of glyphosate, it moves to around 1 in 35,” explains Michael Davoren, a molecular toxicology researcher at the University of California. “But the bulk of the risk, as with any cancer, is still going to be due to other factors, including in part strings of ‘bad luck’ mutations in a given set of cells.”

Multiple theories have been voiced as to why this increased risk might arise, such as the idea that glyphosate may mimic the behaviour of certain hormones. One study, by researchers in Thailand, suggested that by doing so, even low levels of glyphosate could increase the rate of breast cancer cell growth in petri dishes.

Edwin Hardeman, who has non-Hodgkin lymphoma, is suing Monsanto for failing to warn him about the risk of Roundup. Photograph: Courtesy Andrus Wagstaff PC

However, the trouble is, for every research paper that purports to show a link between glyphosate-based herbicides and cancer, there is another which finds the exact opposite. This hasn’t been helped by the fact that many of the studies may not have been entirely objective. “A lot of the studies backing glyphosate have been funded by entities in a position to profit from the continuing sales,” Davoren says. “And many of those which point towards significant risks are funded by groups who are either engaged in lawsuits against the makers of glyphosate, or are in the position to benefit from sales of glyphosate alternatives. So it gets very, very tricky.”

But even some of the largest independent population-based studies have failed to find any sort of definitive proof. Last year, a two-decade-long analysis of data of nearly 45,000 farmworkers who applied glyphosate-based herbicides to their crops, conducted by the US National Institute of Health, showed no association with non-Hodgkin lymphoma or overall cancer risk.

“This is the strongest argument that Monsanto has,” says Deborah Kurrasch, a neuroscientist at the University of Calgary who has been researching glyphosate for several years. “If it’s so damn bad, then where are all the bodies? The scientific evidence, as it stands right now, is not at all conclusive.”

But one of the factors that have left commentators suspicious of the potential toxicity of these herbicides has been incidents of combative corporate behaviour. In the latest trial, Monsanto has caused eyebrows to raise by obtaining a ban preventing attorneys for the plaintiffs from presenting information regarding its alleged influence on research.

The regulators versus the politicians

An Avaaz protest in Brussels, at the EU’s plans to relicense glyphosate – the licence was extended in 2017 for five years. Photograph: John Thys/AFP/Getty Images

There is no question that the glyphosate debate has become highly politicised in recent years. Despite the limited evidence linking glyphosate to health risks, a European Citizens Initiative petition against its use in agriculture still garnered 1.3 million signatures, with the European Union’s 2017 decision to license it for another five years sparking mass protests across the continent.

In addition to cancer, environmental activists have claimed links between herbicide exposure and everything from coeliac disease to autism, while on the other side of the fence, regulatory agencies blame an ongoing anti-GM agenda for driving public sentiment against this small molecule. If glyphosate is banned, campaigners will have struck another severe blow against GM crop production.

A crop sprayer in Doebern, Germany: Germany, Italy and France are keen to ban glyphosate-based herbicides. Photograph: Florian Gaertner/Photothek via Getty Images

“My personal perception is that glyphosate has become a symbol for the use of chemicals in agriculture and the way we produce food in Europe,” says Dr Bernhard Url, executive director of Efsa. “When science meets values, things become complicated. So when politicians are confronted with the opinion of Efsa that glyphosate is safe, they say, ‘No, I don’t want to hear that glyphosate is not carcinogenic because it doesn’t fit into my world view. I want a world without agrochemicals and if you, Efsa, tell us that glyphosate is safe to be used, you must be corrupt.’”

A 2016 study which found a 1,000% rise in the levels of glyphosate in our urine in the past two decades – suggesting that increasing amounts of glyphosate is passing through our diet – provoked further outrage. Except it isn’t really clear whether that has any consequences at all for our health. An Efsa letter, published in the journal Nature, pointed out that glyphosate residues found in Italian pasta or German beer would only exceed known risk thresholds if someone were to consume their entire body weight’s worth of those products in a single day.

To try to understand any potential mechanisms for how glyphosate could be doing something untoward in our bodies, increasing numbers of studies have been conducted in cell lines, rodents, zebrafish and even worms, some of which have suggested that it could have the potential to disrupt basic biological processes such as mitochondrial function. “If anything needs to be looked at, it’s whether glyphosate has some toxicity at a metabolic level,” Kurrasch says. “If you look at a variety of central nervous system (CNS) disorders, all of those have been linked to mitochondrial dysfunction.”

However, so far no link has been found between glyphosate exposure and CNS disorders in humans. The same is true for theories which speculate as to whether glyphosate passing through our gut may perturb the microbiome, inhibiting beneficial bacteria, and so promoting the growth of inflammation-inducing pathogenic bacteria. These theories link glyphosate to inflammatory disorders such as intestinal cancer, yet to date, no such associations have been found in population studies.

The glyphosate debate has even moved to the insect world. While glyphosate has been known for many years to pose health risks to fish, and as such, its use near water is strictly regulated, a paper last year claimed that it could pose a risk to bees. The study found that glyphosate levels in flowers could affect the bee microbiome, potentially affecting their health. However, given that the study used just 15 bees per group when comparing glyphosate exposed bees and non-glyphosate exposed bees, this also remains somewhat tenuous.

The backlash

With the legal battles over glyphosate’s alleged link to non-Hodgkin lymphoma set to continue over the coming years, scientists and regulatory bodies alike agree that the only way to try to come to a common consensus about whether it poses a genuine threat is to transparently share their data.

According to Davoren, “The only way this debate is going to be settled is with a large amount of further research built on a philosophy of open data, where everyone says, ‘OK, this is what we found, here is the exact way we did it, here is our raw data, and everybody take a look to be sure that you see there’s no bias.’”

However, such is the political pressure surrounding the use of glyphosate that many strongly suspect it will begin to be phased out, regardless of the scientific conclusions, in the near future. French authorities banned the sale of a form of Roundup earlier this year. President Macron has vowed to outlaw glyphosate-based herbicides altogether by 2021, and both Germany and Italy are reportedly considering following suit.

Following Brexit, there is also the potential that the UK, too, will change its current stance on the use of glyphosate in agriculture. A 2017 House of Commons briefing paper on glyphosate suggested that ministers at the Department for Environment, Food and Rural Affairs may well take a different approach from the EU.

Efsa is pressing for further discussions about the potential consequences on farming and the food industry of banning glyphosate before drastic measures are taken, but whether its call is heeded remains to be seen.

“There needs to be meaningful discussions about this on a political stage,” Url says. “Do we want to use agrochemicals in Europe or not, and if so, under which conditions? What would a world without glyphosate and herbicides mean for agriculture and biodiversity, food prices, consumers? And what are the risks and benefits?”

But for Barton and the many plaintiffs, there remains no doubt in their minds that the high levels of glyphosate exposure, which they encountered throughout their working lives, have contributed to their illnesses.

“There was never a warning on that product to be careful when you use it, that you need to be protected, because there could be a danger,” Barton says. “I believe Monsanto put profit above people, and they’ve got away with it for all these years.”

Everyday dangers

International Agency for Research on Cancer (IARC) is funded by the World Health Organization and its research is regarded as the benchmark for determining what agents may be cancer-causing. Some examples of its classifications below…

Group 1 carcinogens

Carcinogenic to humans. 120 agents, including:

• alcoholic drinks

• asbestos

• diesel engine exhaust emissions

• indoor tanning

• tobacco

• x-rays

Group 2A

Probably carcinogenic to humans. 82 agents, including:

• red meat

• indoor emissions from wood-burning stoves

• glyphosate

• shiftwork that involves circadian disruption

• petroleum refining (occupational exposures in)

• frying – emissions from high temperature

Group 2B

Possibly carcinogenic to humans. 311 agents, including:

• dry cleaning (occupational exposures in)

• firefighting (occupational exposures in)

• aloe vera

• bracken fern

• ginkgo biloba extract

• lead

• This article was amended on 21 March 2019. An earlier version implied that a study examined just 15 bees. To clarify: The study used just 15 bees per group when comparing glyphosate exposed bees and non-glyphosate exposed bees.