April 7, 2015 -- An international group of health experts is questioning the safety of a widely used pesticide that had long been considered not harmful by U.S. regulators.

The International Agency for Research on Cancer (IARC), a research arm of the World Health Organization, in March said that glyphosate is a “probable” cancer-causing substance, or carcinogen. It’s a key ingredient in hundreds of crop-control agents and weed killers, such as Bronco, Glifonox, KleenUp, Ranger Pro, Rodeo, Roundup, and Weedoff.

The finding -- challenged by Roundup maker Monsanto -- comes decades after the U.S. Environmental Protection Agency said glyphosate is safe for people. The agency reaffirmed that decision in 2012.

It is the most widely used pesticide worldwide, sprayed on everything from golf courses to home gardens. The chemical is mainly used in agriculture, and sprayed on genetically modified crops like soy, corn, and cotton. The plants are designed to resist the pesticide, which is used to kill weeds around them.

In a fact sheet, the EPA says workers or home gardeners might breathe it in or get it on their skin “during spraying, mixing, and cleanup.”

Aaron Blair, PhD, of the National Cancer Institute, and a lead researcher on the IARC’s study, says a panel of 17 scientists from around the world concluded that glyphosate might be dangerous. They looked at all publicly available and published studies on the chemical and its relationship to cancer. That included studies on people, animals, and laboratory cells. The panel didn’t consider reviews done by regulatory agencies.

In 3 out of the 4 human studies of American, Canadian, and Swedish agricultural workers, the IARC panel found a link between glyphosate and non-Hodgkin lymphoma, a cancer of the immune system. A fourth multi-year study of 80,000 American farmers showed no relationship, Blair says.

Evidence from animal studies also showed a link between glyphosate and rare kidney and pancreatic cancers. Cell studies showed abnormal changes to cell DNA when it was exposed to glyphosate. The combination of all the studies led the IARC to conclude that it is a “probable” carcinogen, Blair says.

“Probable means that there was enough evidence to say it is more than possible, but not enough evidence to say it is a carcinogen,” Blair says.

“It means you ought to be a little concerned about” glyphosate, he says.

He could not say if glyphosate would “probably” cause cancer, because that question is connected to the level and type of exposure to the chemical. Blair’s group doesn’t consider factors like those.

The company says the IARC finding contradicts decades of evidence about the chemical.

“We are outraged with this assessment,” Robert Fraley, PhD, Monsanto chief technology officer, says in a statement. “This conclusion is inconsistent with the decades of ongoing comprehensive safety reviews by leading regulatory authors around the world.”

Ray McAllister, PhD, senior director of regulatory policy for CropLife America, an advocacy group for agro-chemical makers, says the IARC didn’t consider many studies the industry has produced to prove glyphosate’s safety.

“A consumer shouldn’t be worried about” the chemical, McAllister says.

The EPA determines safe exposure levels of chemicals. The agency last assessed of glyphosate 3 years ago.

The main source of the chemical in drinking water is runoff from herbicide use, the agency says. But it doesn’t last long in H2O.

Drinking water with more than a little glyphosate in it (0.7 milligrams per liter) over many years could damage the kidneys and reproductive organs, the EPA says. But, the agency says, there’s not enough evidence to say whether or not having water with glyphosate in it over the course of your life has the potential to cause cancer.

The EPA says in a statement it will consider the IARC’s conclusion as part of a scheduled re-evaluation of the chemical, which happens every 15 years.

Other chemicals the IARC has named as “probable” carcinogens include acrylamide, the chemical that comes from cooking high-carbohydrate foods such as potato chips and French fries, nitrates in cured meats like bacon and hot dogs,and emissions from wood burned inside the home.

“Exposure to a known carcinogen like tobacco doesn’t mean you’ll get cancer,” says Curt DellaValle, a senior scientist at the Environmental Working Group, an advocacy group focused on the environment and public health. “It depends on a lot of things, like the amount of exposure and [a person’s] genetics.”

DellaValle’s organization joined with seven other environmental groups urging the EPA to “weigh heavily” the IARC’s conclusion during its re-evaluation of glyphosate.

Cancers can take a long time to develop, DellaValle says. “It can take 20 years of exposure before a study picks up evidence.”

IARC: “IARC Monographs Volume 112: evaluation of five organophosphate insecticides and herbicides.”

EPA, Technical Factsheet on: Glyphosate.

New York Times: “Weed Killer, Long Cleared, is Doubted.”

Aaron Blair, epidemiologist, National Cancer Institute.

Ray McAllister, PhD, senior director of regulatory policy, CropLife America.

EPA Web site, “Basic Information about Glyphosate in Drinking Water.”

IARC web site, “The Acrylamide Working Group.”

Environmental Working Group: “EWG’s Dirty Dozen Guide to Food Additives: Food Additives Linked to Health Concerns.”

Straif, K. The Lancet Oncology, December 2006.

Curt DellaValle, senior scientist, Environmental Working Group.

Environmental Working Group: “Public Interest Groups on WHO’s decision on Glyphosate.”

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