Pesticide Patrol

A new report says the most widely used weed killer probably causes cancer.

April 02, 2015

Photo: Chris Quintana

Farmers the world over spray their fields with Roundup. The herbicide is an effective, efficient weed killer—at least for a while, before those pesky plants evolve a resistance and become “superweeds.” On top of that, the active ingredient, glyphosate, has been linked to amphibian deaths, monarch butterfly declines, and, most recently, to cancer in humans.

The U.S. Environmental Protection Agency listed glyphosate as a “possible” carcinogen in 1985 but deemed the chemical safe for use six years later. Just last week, however, a report from the World Health Organization said glyphosate probably does cause cancer in humans. Oops.

The EPA has already been reevaluating the effect glyphosate may have on human health (a review required every 15 years) and plans to publish its draft risk assessment later this year. The agency is also making moves to tackle that superweed problem, announcing Tuesday that it will propose restrictions on glyphosate, which will likely include weed-monitoring, farmer education, and remediation plans.

For its part, the WHO’s International Agency for Research on Cancer routinely evaluates agents for potential carcinogenicity. In the last year, the WHO evaluated five organophosphate pesticides: tetrachlorvinphos, parathion, malathion, diazinon, and glyphosate. The researchers found that three of these—glyphosate, malathion, and diazinon—“probably” cause cancer. (The other two “possibly” cause cancer, one step down on the IARC’s list of classifications.)

“Our assessment doesn’t come with any recommendations,” says Katherine Guyton, a WHO toxicologist. She says it’s simply a hazard assessment that governments can choose to act on by labeling Roundup and its generic equivalents accordingly or by investigating the chemical further to protect farmers and field workers. (The WHO’s full report comes out later this year; the Lancet published a two-page summary last week.)

Photo: USGS

The WHO has looked at nearly a thousand agents over the last 15 years, relying on publicly available research to determine whether they are harmful. So far, it has classified 116 agents as known carcinogens, including tobacco smoke and sunshine.

The researchers, however, don’t have enough information to evaluate most of the substances. “It’s not that nothing is going on,” says cancer epidemiologist Aaron Blair, who helped prepare the WHO report. “It’s that scientific investigation and inquiry takes time.” Most companies, of course, would rather not wait for the answer, and in the United States, products like Roundup go to market before we know whether they’re safe.

Unsurprisingly, Monsanto, the company that makes Roundup, is urging the WHO to reverse its decision. (The company did say that it would work with the EPA on the weed-management plan.)

Glyphosate has been on the market since 1970. For their report, Guyton and her team looked into both animal and human studies. In several laboratory studies, rats and mice that were exposed to glyphosate developed unusual tumors. Research also showed that the DNA within human cells became damaged after being dosed with glyphosate. The WHO scientists also found some evidence that it may cause non-Hodgkin lymphoma in humans.

The WHO determination surprised environmental epidemiologist Jane Hoppin, who was involved in the Agriculture Health Study that investigated the condition of more than 60,000 U.S. farmers. More than a hundred studies have resulted from that research, including some on glyphosate that found the pesticide did not increase cancer risk. Still, Hoppin says, as we uncover more about the chemicals we use, we’ll understand more about their effect on us and the environment.

Farmers in the United States applied more than 280 million pounds of glyphosate on crops in 2012, the most recent year for which data is available. That could mean field workers are being exposed to more than twice as much as they were in 2002. If the stuff causes cancer…well, the idea is to kill weeds, not people.

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

onEarth Story

Plus, Sonny Perdue doesn’t understand cancer science, and offshore drillers are writing their own rules.


Famous for their elegant colors and transcontinental feats of migration, these beloved pollinators are also in free fall, as habitat loss and heavy use of herbicides jeopardize their future.

Policy Primer

Shown to be toxic to kids, chlorpyrifos is nevertheless still being sprayed on crops across the country—and making its way into our bodies. So why has the EPA refused to ban it?


This toxic herbicide comes with known health risks, but it’s still being used on crops, in parks, and maybe even in your own backyard.

onEarth Story

New research shows a certain type of milkweed can harm monarchs—but we still need to give these butterflies something to snack on.

Personal Action

Be a good neighbor to struggling pollinators by turning your backyard into a welcome pit stop.

On the Front Lines

Organic farmer Jim Cochran proves that growing nontoxic strawberries isn’t just possible, it’s profitable.

onEarth Story

Rising CO2 levels could upset the delicate relationship between the butterflies and their parasites.

Join Us

When you sign up you'll become a member of NRDC's Activist Network. We will keep you informed with the latest alerts and progress reports.