A US court case that linked Roundup to cancer has prompted more calls for a ban on the popular herbicide - but should it?

Like a hardy thistle that won't die, debate surrounding cancer and the key chemical in one of the world's favourite weedkillers continues to sprout.

Two years after argument had apparently been settled here in New Zealand, a San Francisco jury has ordered Monsanto to pay $439 million to a former school ground keeper, on the basis its product had contributed to his cancer.

The lawsuit from Dewayne Johnson claimed Roundup caused his non-Hodgkin's lymphoma, which Monsanto denied.

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The company insisted hundreds of studies have established that the active ingredient in Roundup - glyphosate - was safe and planned to appeal the decision.

That wasn't enough to reassure many here - and one group, GE Free NZ, went as far as stating the ruling had "brought out the dark underbelly of Monsanto".

New Zealand scientists have read about the case with interest - notably because it was based on some relatively new evidence.

In 2016, the International Agency for Research on Cancer (IARC) re-classified glyphosate as a "probable human carcinogen" - a decision based on an extensive review of available data, including human studies.

However, the impact of this finding was being debated widely, partly due to the involvement of large corporations, University of Otago toxicologist Dr Belinda Cridge said.

But there were also questions about the IARC's work itself - when it looked at a chemical's carcinogenic potential, it didn't generally conduct a full risk assessment, or judge where and how contact with the chemical might occur.

Cridge pointed out that even red meat consumption had been classified as a probable carcinogen, something which also highlighted that wider factors were critical to determining full risk.

Still, she said, the underlying finding of the IARC stood - glyphosate may cause cancer, but under the right conditions and exposures.

As for the US case, it cited that additives in the Roundup, beyond the active glyphosate compound, may have had a synergistic effect to cause the cancer.

Such effects occurred when two chemicals which are relatively benign separately, acted together to make a small effect much worse.

"The toxicology of mixtures such as this is something that toxicologists are only just starting to really understand in any detail," Cridge said.

"It is very difficult to model and track all possible interactions. This means that there is a very real possibility that adjuvants in the Roundup mixture accelerated any carcinogenic effects but to the best of my knowledge this is hypothesised rather than proven.

"It is a very real possibility but has not been conclusively demonstrated using laboratory or epidemiological studies."

In any case, Johnson's lawyers didn't need to demonstrate conclusively that glyphosate caused the cancer - only that it was a plausible contributing factor.

Also, Monsanto was unable to prove that glyphosate definitely did not cause the cancer.

"There is still no proof either way but the success of the prosecution will encourage others to seek remuneration using the IARC classification as evidence."

So what does that mean for New Zealand?

Despite being widely used by gardeners and farmers alike, there had never been such a case here.

Federated Farmers spokesman Andrew Hoggard said he'd be concerned if Roundup was banned, as it was effective for spraying out paddocks between pastures, and there were no alternatives as useful.
Rigorous testing had also shown that concerns about glyphosate residue in milk products were unfounded.

Yet, on the back of the ruling, Associate Environment Minister Eugenie Sage asked the Environmental Protection Authority (EPA) to consider adding Roundup to its hazardous substance reassessment list in the light of the decision.

The EPA remained of the view that Roundup was safe to use.

After the IARC's report came out, the agency commissioned Dr Wayne Temple, a toxicologist and former director of the New Zealand National Poisons Centre, to undertake a review of the evidence relating to the possible carcinogenicity of glyphosate.

Temple found glyphosate was unlikely to be carcinogenic to humans or genotoxic - damaging to genetic material or DNA - and should not be classified as a mutagen or carcinogen under New Zealand law.

This conclusion was formed on the back of human and animal studies, including many not looked at by the IARC, and the findings were peer-reviewed by experts.

Not all toxicologists were happy with it though.

The University of Canterbury's Dr Ian Shaw said Temple's review didn't appear to have covered the possibility of non-genotoxic carcinogenesis, where some chemicals that caused cancer didn't directly alter genes.

"This was particularly surprising because experiments have shown that glyphosate can interact with a cell receptor that stimulates some cells to grow – this is the way some non-genotoxic carcinogens work," Shaw said.

"In view of this, and other aspects of Temple's report, I found the EPA's decision lacked scientific rigour."

Still, scientists argued the US ruling shouldn't prompt a knee-jerk ban of Roundup here.

"Herbicide use is seldom exposure to just one specific product - and the dose, duration, type, and frequency of exposure is relevant to any potential risk," said Associate Professor Brian Cox, a cancer epidemiologist at the University of Otago.

"A sudden reaction to one case in one US law court, that has not yet gone to the appeal court, is not an appropriate method of developing health policy in New Zealand."

But it was still appropriate New Zealand should keep watch on overseas evidence, he said.
Cridge argued Roundup remained one of the safer options currently available.

"My standard advice is for people to not use chemicals where they don't need to, know what chemicals you are using and be rigorous about safety equipment," she said.

"This applies to all the chemicals we use from home cleaners to industrial chemicals in the workplace to agrochemicals such as Roundup."