As tens of thousands of cases against the makers of the weedkiller Roundup mount up in the United States, a class action is about to test the product's standing in Australia.

John Fenton is getting ready to take on the world's largest agricultural chemical company, Monsanto.

The 57-year-old from Robe in South Australia is in remission after being diagnosed with non-Hodgkin lymphoma (NHL) in 2008.

"There's not a day goes by that you don't think about it; every ache and pain in your body, you sort of know what it is," he said.

Like hundreds of others in Australia, he believes his cancer was caused by glyphosate, the active ingredient in Roundup.

The weedkiller revolutionised agriculture and backyard gardening in the 1970s. ( ABC News: Jennifer Ingall )

Mr Fenton grew up on a farm and owned a weed-spraying business for six years in south-west Victoria.

"We were using pesticides, insecticides, but our predominant chemical was Roundup," he said.

"That's what we used pretty much every day of the week when the weather was right."

He is now the lead plaintiff in a class action against Monsanto, which is owned by Bayer.

"I just think it's a cause worth fighting for, and I'm not doing it as chasing money ... I'm just chasing what I lost."

US litigation prompts Australian cases

On the NSW central coast, Heather Slade said she heard about the cases in the United States and wanted to know if a class action was happening in Australia.

"Because I thought, well, I'd like to be part of it," she said.

Ms Slade started using Roundup on her farm in the 1970s and was diagnosed with NHL in 2011.

"I was just forever spraying either in the garden or on the gravel areas or through the orchard, so I did that for the 12, 13 years that we were on the farm regularly," she said.

"There was nothing at all saying to use protective gear and that sort of thing — everyone just sprayed."

Heather Slade believes her cancer was caused by using Roundup and wants it taken off shelves. ( ABC News: Anthony Scully )

Company denies cancer claims

The class action alleges that Monsanto knew or ought to have known that Roundup was inherently unsafe and the company failed to provide suitable directions for use.

US courts have found against Monsanto in large part because they did not properly warn people to wear protective clothing when they applied the weedkiller.

The company refutes any claims Roundup causes cancer and is appealing those cases.

At the same time, Bayer is looking to settle the mass-tort litigation in the United States, with an estimated 75,000 claimants, for as much as $US10 billion.

There are reports that as part of that settlement, the company could remove its products from retail and only sell directly to farmers.

Heather Slade sprayed Roundup on her farm in WA during the 1970s and '80s. ( ABC News: Anthony Scully )

Bayer would not comment on those reports but told the ABC it continued to defend the company in litigation.

"We believe the allegations in this case lack merit given the extensive body of science and the conclusions of regulators worldwide, including the Australian Pesticides and Veterinary Medicines Authority, which supports the safety of glyphosate-based herbicides. "We are defending these cases and are confident that if they are decided on science, the company will prevail."

Roundup is the most commonly used herbicide in the world. (File photo) ( ABC News: Neale Maude )

Class action expert Peter Cashman said any settlement in the US would not impact whether or not the case went to trial in Australia.

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"It's not unusual for corporations to shove out large amounts of money to resolve US cases but wage war worldwide, in Canada, Europe or in Australia," Dr Cashman said.

"The fact that there is a settlement under consideration, which is likely, in my view, to resolve most of the US cases, it doesn't necessarily follow that the company will be equally keen to resolve the Australian litigation."

Anger over labelling

Tamworth hobby farmer Rob McIlveen was diagnosed with NHL last year and also blames his use of Roundup.

"Remembering how it was presented to me when I bought my first bottle of Roundup, say, the early 90s, was, 'This is the you-beaut stuff and toxicity is minimal' — I felt cheated," he said.

"I've been quite angry about it, to think that I've got lymphoma, undergoing treatment still, and not sure what the future is."

Rob McIlveen believes product safety warnings weren't strong enough when he was using Roundup. ( ABC News: Jess Davis )

Mr McIlveen said Roundup was marketed as safer than table salt and coffee, and wished there had been more instructions to cover up on the label.

"What I'm looking for is recognition that they've treated me like an idiot and I want compensation for the pain and the suffering," he said.

"Most of the discussion is on how effective it is on the weeds; there's very little discussion how detrimental it is on human beings."

What scientific evidence is there?

Dr Cashman said it would be difficult to establish causation in the Australian class action.

"These cases do give rise to some fairly vexed questions about proof of causation and the epidemiology evidence, although it's reasonably favourable, is still highly contested," he said.

"The plaintiff needs to establish not only that exposure to this product can cause injury, they have to prove that their particular injury was caused by their individual exposure, and that can be a difficult burden to meet in many cases."

The class action alleges that Monsanto misled consumers. ( ABC News: Jennifer Ingall )

The US court cases relied in part on the findings of the International Agency for Research on Cancer (IARC), which is part of the World Health Organisation.

In 2015, it found glyphosate was a probable carcinogen and that there was evidence to suggest exposure to the chemical increased the risk of NHL.

Tim Driscoll, a professor of epidemiology and occupational medicine at the University of Sydney, said the evidence was strong but not definitive.

"We need to take seriously what IARC says, and even if they didn't say that, we need to take seriously what the evidence says," Dr Driscoll said.

"If you don't need to use it, don't use it. But if you do need to use it, OK, but use as little as possible and make sure that you're well protected."

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Vested interests

The IARC report has been criticised by Monsanto and divided the scientific community.

Ivan Kennedy, an emeritus professor in agricultural and environmental chemistry, has been studying glyphosate for decades and disagreed with IARC's findings.

"From my point of view, there's no evidence about carcinogenicity. I think we need to be careful about such things," he said.

"It's [IARC] not taking into account the likely exposures to the chemical, and even people working with glyphosate every day take precautions that are given on the label and don't get much exposure."

But Dr Driscoll said criticism of IARC's report was unwarranted and that it mainly came from those with vested interests.

"IARC is just looking to see: 'Does the substance increase the risk of cancer, yes or no? And then it's up to us as a society to decide," he said.

"Well given that, do we still want to use it or not? And in what circumstances do we still want to use it or not?"

"There are potential risks and there are benefits, and we need to weigh those up and decide, well, as a society, do we get enough of the benefits to take the risks?"

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