Has the embattled debate over glyphosate – the active ingredient in the world’s most widely used weed killer– come to a conclusion? After seemingly endless back-and-forth over the substance’s safety, that’s what a new scoop by Reuters seems to have done.

For years, glyphosate has been considered a safe alternative to harsher herbicides, used in agriculture, gardening, and forestry. Domestic and international health bodies, including the European Food Safety Authority (EFSA), the US Environmental Protection Agency, Health Canada’s Pest Management Regulatory Agency, Japan’s Food Safety Commission, and New Zealand’s Environmental Protection Authority have all deemed it benign. In May, the European Commission proposed extending its approval of glyphosate by another 10 years. But this latest decision has been met with an outcry by green members of the European Parliament, who demanded to see the scientific studies on which EFSA, and later the Commission, had based their decisions.

Their opposition, and continued uncertainty around the safety of glyphosate more widely, was driven by an outlying assessment by the International Agency for Research on Cancer (IARC) in May 2015, which deemed glyphosate “probably carcinogenic.” IARC has published hundreds of such assessments – known as monographs – on substances ranging from highly carcinogenic chemicals like formaldehyde to widely consumed products like coffee and red meat. Thus far, only one substance has been assessed as “probably not carcinogenic to humans.” Given the fear-inducing nature of IARC’s assessments, it’s not surprising that they tend to make headlines and raise concern among members of the public and lawmakers. This has especially been the case when it comes to highly regulated, widely used products like glyphosate.

But now, new information has raised serious questions about IARC’s classification of glyphosate and about the quality of its methodologies more broadly. According to previously confidential court documents, Aaron Blair, a scientist who led IARC’s review panel on glyphosate, revealed in a deposition this spring that he had access to data suggesting that there was no link between glyphosate and cancer. However, IARC never considered the data before issuing its final decision in March 2015 – despite the fact that, as Blair admitted, the data would have altered its evaluation. Why? IARC’s approach to assessing substances for carcinogenicity deviates from other regulatory bodies significantly in that it only considers data that has been published in peer-reviewed scientific journals.

Before publishing its monograph on glyphosate, IARC considered about 1,000 published scientific reports. But most of them focused on laboratory studies of rodents and only a few centered on cohort studies in humans, in which a disease-free study population is first identified by the exposure of interest and followed in time until the disease occurs. Such studies are the most relevant to understanding the health implications for those who have the highest exposures to a given chemical. Blair’s unpublished data came from such a cohort study: the Agricultural Health Study, a large-scale examination by the US National Cancer Institute of the effects of pesticides on agricultural workers and their families. More than 89,000 farmers and their spouses in Iowa and North Carolina have been involved in the study since 1993. Critically, the study found that there was “no evidence of an association” between glyphosate exposure and cancer.

A well-known American epidemiologist and a top British statistician interviewed by Reuters both said that the data relating the glyphosate was significant and relevant, and that they didn’t see why it wasn’t published or included in IARC’s evaluation process. For his part, Blair said that the data – which was available two years prior to IARC’s assessment of glyphosate – had not yet been published because there was too much to fit into a scientific paper. The National Cancer Institute also said that the data had not been featured in a scientific journal due to “space constraints.”

Yet whether or not it should have been published is a less important question than why IARC didn’t consider it, published or not. Cohort studies like the Agricultural Health Study often deliver the most solid evidence when evaluating whether something like pesticide use has a long-term effect on health. Indeed, they’re the best option for examining the effect of suspected risk factors that cannot, for ethical reasons, be controlled experimentally on humans. And despite the fact that the data from the Agricultural Health Study had not yet been published in a peer-reviewed journal, and it’s common practice to privilege published data over unpublished, the IARC panel consisted of the very sort of experts who would review submissions for such journals anyway.

Incomprehensibly, IARC shows no signs of taking down the barriers it has put up to unpublished data, nor of revisiting its assessment of glyphosate. But the revelations about Blair have already opened up a new wave of scrutiny on IARC’s evaluation, which is set to influence the European Commission’s impending negotiations with member states over its decision to re-license glyphosate. Whether or not IARC changes its anachronistic evaluation procedures, the doors on the glyphosate debate might be nearly closed for good.

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