The Most Widely Used Herbicide in the United States Could Cause Cancer in Humans, Says a World Health Organization Study

Roundup, produced by the US-based agrochemical behemoth Monsanto, is one of the most widely used herbicides in the world and is doused on millions of acres of crops each year in the United States alone. Regulatory agencies around the world, including the US Environmental Protection Agency (EPA), have repeatedly called it safe to use.

But a controversial new study of its main ingredient, glyphosate, has called this conclusion into question and renewed calls by public health and environmental advocacy organizations to ban the chemical.

Earlier this month, the World Health Organization's International Agency for Research on Cancer (IARC) convened 17 scientists from 11 countries to evaluate existing science on five pesticides, including glyphosate, which was first developed by Monsanto in 1969. Laboratory studies showed an increase in some cancers, including rare cancers of the arteries and livers, in lab mice exposed to glyphosate. The lab animals were exposed to higher levels of the herbicide than what humans could encounter. But epidemiological studies of humans also showed a cancer risk, linking exposure to the chemical with an increased risk of non-Hodgkin's lymphoma.

That led the group to declare glyphosate "probably carcinogenic," the second-highest designation given out by IARC behind "carcinogenic to humans."

"It would be a different category if there was literally no human data," Aaron Blair, who chaired the IARC working group evaluating the pesticides, told VICE News. "But there was some human data, which says, okay, the animal doses are usually higher but still we have a little bit of a signal in humans."

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Monsanto has challenged the report, arguing that the group ignored scientific studies showing that glyphosate is safe. "We don't know how IARC could reach a conclusion that is such a dramatic departure from the conclusion reached by all regulatory agencies around the globe," said Philip Miller, a vice president with Monsanto.

The report, published Friday in the journal The Lancet Oncology, summarizes findings that will be released in detail in a comprehensive IARC report expected in about a year.

"Until that's out, it's hard to see how they reached this conclusion and if they interpreted the information correctly or not," Keith Solomon, a toxicologist at the University of Guelph in Ontario whose research was cited by IARC, told VICE News. "Interpretation is different from one person to another, from one group to another."

In the United States, the EPA is responsible for establishing levels of chemical use that are safe for both human health and the environment. The agency is currently in the midst of reassessing glyphosate, as part of a federally mandated evaluation that occurs every 15 years. In the 1980s, the EPA classified the chemical as a possible human carcinogen, but in 1991 reduced its status to "evidence of non-carcinogenicity for humans" after reevaluating lab studies on mice.

"We will give full consideration to the IARC study and all the other information we have before we reach a final decision," an EPA spokesperson told VICE News.

The European Union concluded in January that glyphosate is "unlikely" to be carcinogenic to humans.

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Glyphosate is used in more than 750 products worldwide, according to the IARC report. Its use has grown worldwide as companies like Monsanto create genetically modified crops that are resistant to the herbicide. In this way, farmers can spray their fields with the chemical, killing unwanted weeds while remaining benign to Roundup ready corn, soy, wheat, or cotton. One hundred and fifty-seven million acres of corn and soybean crops in the United States were treated with glyphosate in 2013 — a nine-fold increase in less than a decade.

Runoff from crops treated with glyphosate can end up in drinking water supplies. At high levels of exposure over long periods of time, it may cause kidney or reproductive issues, according to the EPA.

"I haven't seen a regulatory agency yet around the world" that's called glyphosate a carcinogen, Solomon told VICE News. "They all look at it in a very detailed way. None of them have come out with the same conclusion."

Follow Laura Dattaro on Twitter: @ldattaro