A farmer sprays glyphosate across his cornfield. Photograph by Seth Perlman/AP

Wouldn’t it be a relief to discover that we could lower the risk of developing many types of cancer simply by banning a product that has for decades been sprayed on crops throughout the world? By many recent accounts, then, it’s time to rejoice. Late last month, the International Agency for Research on Cancer, an arm of the World Health Organization, issued a report that classified glyphosate, the active ingredient in the herbicide Roundup, as a “probable” cause of cancer. Glyphosate is the most heavily used herbicide on earth. In 2012, at least 283.5 million pounds were sprayed on American farmlands, according to the U.S. Geological Survey.

Although the patent has now expired, Roundup was developed in 1974 by Monsanto and is often used in conjunction with crops like corn and soybeans that the company genetically modified to resist it. This allows farmers to kill weeds but not their crops. Although the herbicide has many other uses, its association with G.M.O.s has caused food activists to condemn it for years. Even so, farmers, biologists, and home gardeners throughout the world use glyphosate.

The I.A.R.C. report should change none of that. The panel of seventeen scientists from around the world concluded that glyphosate could be dangerous. The organization evaluates data collected for previously published peer-reviewed studies—which is a valuable service—but it does not conduct its own research. “ ‘Probable’ means that there was enough evidence to say it is more than possible, but not enough evidence to say it is a carcinogen,” Aaron Blair, a lead researcher on the I.A.R.C.’s study, said. Blair, a scientist emeritus at the National Cancer Institute, has studied the effects of pesticides for years. “It means you ought to be a little concerned about” glyphosate, he said.

But how concerned should we be, exactly? Scores of studies have been carried out over the past forty years and they have found no connection between glyphosate and cancer. Its use has been approved by regulatory agencies, including the Environmental Protection Agency, throughout the world. Moreover, the I.A.R.C. does not include draft analyses in its assessments because, being drafts, they are subject to change. For that reason, the group omitted the conclusions of a comprehensive new study by the German Federal Institute for Risk Assessment. The organization reviewed hundreds of toxicological studies and nearly a thousand published reports. It found, based on available data, neither “carcinogenic or mutagenic properties of glyphosate nor that glyphosate is toxic to fertility, reproduction or embryonal/fetal development in laboratory animals.” (The one study that strongly contradicts that consensus, published by Gilles-Éric Séralini, has been retracted and subjected to intense criticism for its lack of rigor and for the ethics of his research methods.)

The lack of scientific support has hardly deterred the most strident critics of G.M.O.s from blaming glyphosate for diseases ranging from cancer to autism. The I.A.R.C. monograph, published last month in Lancet Oncology, will only be the latest weapon in the highly emotional and often irrational war against the use of G.M.O.s. After the study was released, the Times food writer Mark Bittman, in a column titled “Stop Making Us Guinea Pigs,” suggested that the herbicide should be removed from the market, at least until “Monsanto can prove” that it is safe.

But how do you prove that a substance is safe? Neither Monsanto nor anybody else will ever be able to do that, just as nobody will be able to say that your child’s next vaccination could not possibly make him sick. As with almost all cases that require complex assessments of scientific issues, to determine the risks and the benefits, we need to look at the data. And the data are, again, clear: after nearly forty years of use, no study has demonstrated a link between glyphosate and cancer in humans.

That doesn’t mean we ought to stop investigating glyphosate’s potential effects–both immediate and cumulative—on crops, farmers, humans, and animals. And it certainly doesn’t mean that we should continue to use glyphosate, or any other chemical, with complete abandon. Relying too heavily on glyphosate, as many farmers do, causes tremendous damage to the crops they are trying to grow, as well as to their immediate environment.

To understand what this report actually means, we need to look at its context. As the University of Michigan’s Andrew Maynard puts it in this excellent explanatory (https://www.youtube.com/watch?v=CbBkB81ySxQ), the I.A.R.C. classification “doesn’t indicate how likely” glyphosate is to cause cancer. “It is the equivalent of saying a rock could kill you but not pointing out that it probably needs to be dropped on your head from a great height first.” (This video first appeared at the end of a post about the I.A.R.C. report that was written for Grist by the highly reliable Nathanael Johnson.)

The list of potential and probable carcinogens is extremely long, and it includes things like fried foods, skewed patterns of sleep caused by working at night, burning fires, and many products we eat and use every day. Nowhere in its report does the I.A.R.C. indicate that the term “probable carcinogen” means that you will probably get cancer from using glyphosate, and nowhere does it suggest that the product ought to be withdrawn from the market. It should also be noted that people use glyphosate for good reasons. If there are risks there are also benefits: glyphosate is far less toxic than the pesticides it normally replaces, and it greatly reduces soil erosion by limiting the need to plow fields.

“There are over 60 genotoxicity studies on glyphosate with none showing results that they should cause alarm relating to any likely human exposure,’’ Sir Colin Berry, an emeritus professor of pathology at Queen Mary University of London, said when asked to comment on the recent study. “For human epidemiological studies there are 7 cohort and 14 case control studies, none of which support carcinogenicity. “

None of that suggests we should ignore this study. Yet we also have to be aware of blindly invoking the “precautionary principle,’’ as many activists argue that we should. In theory, the principle is hard to condemn: it essentially says that we ought not to develop and use a technology unless it is considered safe. Safety is a slippery concept, though. Many people ski voluntarily—an activity with its own dangers. Although hundreds of people drown in their in their own bathtubs, we still take baths. A strict interpretation of the precautionary principle would require us to stop driving because thousands of Americans die each year on the highways. More than that, the principle would have prevented the development of antibiotics, vaccines, X-rays, elevators, airplanes, and many other signature elements of modern life.

One of the first maxims taught in medical school is that “the dose makes the poison.” The products we use every day, whether it’s glyphosate or the soap we use to wash our hands, always come with risks. But most of those products have benefits, too. And we need to ask ourselves what presents the greater danger: using products like glyphosate or not developing them at all.