Containers of Monsanto’s herbicide Roundup line the shelves at a garden shop near Paris. In June, France’s environment and energy minister announced that the government would ban sales of the product, saying it could be harmful to humans. (Charles Platiau/Reuters)

It’s hard to talk soberly about glyphosate, the active ingredient in Roundup. Roundup, as anyone following the GMO brouhaha knows, is the herbicide that genetically modified crops have been designed to tolerate. Glyphosate tolerance is a trait that allows farmers to spray the GM crops and kill the weeds but not the corn or soy. And so it’s inevitable that glyphosate is all wound up in GMOs, and the debate is commensurately heated.

The controversy amped up earlier this year when the International Agency for Research on Cancer, a World Health Organization agency, declared glyphosate a probable human carcinogen. Predictably, opponents of GMOs made hay over the report, urging consumer caution and regulatory attention. Equally predictably, GMO supporters questioned both the agenda of the agency and the quality of the assessment. Monsanto has commissioned a panel to review it.

It’s important to note that the IARC didn’t do new research (that’s not its job) but evaluated existing research. Other organizations, evaluating the same research, have reached different conclusions. Although the Environmental Protection Agency’s assessment of glyphosate, done in 1991, is woefully out of date (a new assessment is due this year), the agency last year took a fresh look specifically at cancer, in which it “reviewed over 55 epidemiological studies conducted on the possible cancer and non-cancer effects of glyphosate” and concluded that “this body of research does not provide evidence to show that glyphosate causes cancer. . . . This is the same conclusion reached in 2004 by the United Nations’ Food and Agriculture Organization and affirmed this year by Germany’s pesticide regulatory officials.”

I asked toxicologist David Eastmond, professor and chairman of the department of cell biology and neuroscience at the University of California at Riverside, to sort this out. There have been hundreds of studies on glyphosate, he said, and “with that number of studies, you’re always going to find some which give you positive results, tests that would suggest it could be toxic under certain conditions, and you can run with that.”

As for the IARC report, he says it was the best judgment of a group of “thoughtful, diligent scientists.” Another group, Eastmond notes, might have come to a different conclusion. As the EPA did. (Eastmond, and every other scientist I spoke with, noted that until recent questions arose, glyphosate had been noted for its safety.)

A farmer harvests corn in Washington, Ill. Corn and soybeans are the country’s largest GMO crops, together accounting for about half our crop acreage. (Fred Zwicky/Associated Press)

The IARC is charged with identifying substances that can cause cancer, but not the levels of exposure that are risky. Although glyphosate is a probable carcinogen, alcohol consumption, according to the IARC, is an unqualified, definite carcinogen. So is leather dust. So is Chinese-style salted fish. The question about all of these is whether we’re exposed to a high enough level to put us at risk.

Unfortunately, dosage is an issue we humans tend to ignore. According to David Ropeik, author of “How Risky Is It, Really? Why Our Fears Don’t Always Match the Facts,” exposure levels aren’t what scare us. “We are more afraid of threats that are human-made than those which are natural,” he says. As well as “threats we can’t detect with our senses,” “risks that are imposed on us” and “threats generated from sources we don’t trust.”

Glyphosate is four for four: made by humans, undetectable, hard to avoid and generated by Big Agriculture. Alcohol consumption? Batting zero. Pass the Riesling, which will go nicely with Chinese-style salted fish.

But that thing we tend not to pay attention to — dosage — turns out to be the thing that can go a long way toward reconciling the disparate assessments of the danger of glyphosate. Keith Solomon, an environmental toxicologist and professor emeritus at the University of Guelph in Ontario who has studied glyphosate, points out that if the herbicide is a carcinogen, “it has to be a very potent one to pose a risk, because the exposure that humans have is relatively small.”

That same principle applies to another concern about the herbicide, that it may affect our gut biome. Solomon says glyphosate can certainly affect microorganisms, but not at our exposure levels. He cites the malaria parasite as an example. Glyphosate inhibits the parasite’s growth, but, unfortunately, “it’s not useful as an anti-malarial drug because the doses needed are so huge.” (And, in an interesting twist, a byproduct of glyphosate’s metabolism by microorganisms, a compound called AMPA, is being investigated as a cancer treatment.)

Eastmond, too, stresses that the amount of exposure is key. If there is a risk, it’s people who are regularly exposed to a larger amount of glyphosate — farm workers, for example — who run it. Both Solomon and Eastmond say exposure to the residue in food poses little risk.

What glyphosate does to humans isn’t the only issue, of course. There’s also what it does to the environment, and there have been both good and bad consequences of herbicide-tolerant crops.

First is the way in which herbicide use has changed. GMO opponents say the planting of GM crops has led to increased spraying. If we look at pounds of herbicide per acre, that’s true of soybeans but not of corn (corn and soy being the two largest GMO crops, together making up about half our crop acreage and going primarily into animal feed, biofuel, and processed food). GMO supporters point out that, even when more pounds are being used, glyphosate is replacing more-toxic herbicides. That’s true of both corn and soy, although herbicides other than glyphosate are now rising again in soybeans.

So, yes, GMOs have increased glyphosate use and have increased the pounds of herbicide per acre in some crops. And, yes, glyphosate has displaced more-toxic herbicides. Whether total herbicide use has increased or decreased depends on how you account for a less-toxic chemical replacing a more-toxic one, and the arcane details of those accounting schemes don’t seem to me the most important thing to argue about.

One definite positive here is that herbicide tolerance has enabled farmers to reduce tillage, traditionally used to uproot and kill weeds. Because tilling disturbs the soil, it facilitates erosion and runoff of nutrients and chemicals. Reduce tilling, and you reduce those problems and retain more water in the bargain.

One definite negative is that herbicide tolerance has hastened the development of glyphosate-resistant weeds. Not only does that mean farmers have to turn to other herbicides or, possibly, to tillage, it means that the widespread deployment of genetically modified crops has undermined the effectiveness of a very effective, relatively safe herbicide.

When Monsanto first introduced glyphosate-tolerant crops, the company was dismissive of worries about resistant weeds. Its 1993 application to the Agriculture Department says that “It is highly unlikely that weed resistance to glyphosate will become a problem as a result of the commercialization of glyphosate-tolerant soybeans.” This despite the fact that, according to Eric Sachs, a Monsanto scientist, the possibility of resistance was being raised internally. “Some of us thought a more cautious approach was warranted,” he says. “You tell a farmer, ‘You should be adding other mechanisms of action. You should be mixing herbicides.’ They would say, ‘But Roundup works. Why should I add costs to my weed management program?’”

Seed corn genetically modified to be “Roundup Ready” awaits planting at a farm in Princeton, Ill. (Daniel Acker/BLOOMBERG)

Fast forward 20 years, and we have no regulations limiting the extent to which crops resistant to one herbicide can be planted, and we have growing weed resistance problems to show for it.

The most destructive consequence of herbicide tolerance, though, is consumer hostility in a conversation about GMOs that is dominated by that trait. How many times have you read that dousing crops with chemicals creates superweeds? We talk less about the other major GMO — Bt crops, engineered to produce the insect-killing Bacillus thuringiensis toxin — which have a more positive legacy; although some pests show signs of resistance, the crops have unequivocally decreased insecticide use. More important, the future of GM crops that could dramatically improve the food supply for those most in need (such as wilt-resistant bananas and more-nutritious cassava for farmers in Africa) has been called into question as some consumers and activist groups call for limiting or banning GM crops.

The problems with herbicide tolerance aren’t problems with genetic modification — after all, there’s plenty of non-GM herbicide tolerance. We have a baby here, and we have bathwater. We have to learn to tell the difference.

Haspel writes about food and science and farms oysters on Cape Cod. On Twitter: @TamarHaspel. She’ll join Wednesday’s Free Range chat at noon: