The active ingredient in the world’s most widely-used Roundup herbicide has been classified as “probably” carcinogenic to humans by a branch of the World Health Organization. The agrochemical giant Monsanto, has immediately rejected the new conclusions.

The International Agency for Research on Cancer (IARC), in their latest study said that there was“convincing evidence” that glyphosate in Roundup can cause cancer in lab animals.

St. Louis-based Monsanto was not pleased with WHO conclusions, claiming that scientific data does not support their assumptions and urging the health watchdog to hold a meeting to explain the findings.

“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,” Philip Miller, Monsanto’s vice-president of global regulatory affairs, said in a brief statement released soon after the report was published.

The study, published Friday in the journal Lancet Oncology also said it found “limited evidence” that glyphosate was carcinogenic in humans for “non-Hodgkin lymphoma.” The conclusion of the research was based on studies of exposure to the chemical in the United States, Canada, and Sweden that date back to 2001.

According to the study, Glyphosate is used in more than 750 different herbicides in air dissemination during spraying, in water and in food. IARC said glyphosate was traced in the blood and urine of agricultural workers.

IARC has four levels of classifications for cancer agents. Glyphosate now falls under the second level of concern known as ‘probable or possible carcinogens.’ The other agents are classified either as carcinogens, ‘probably not carcinogenic’ or ‘not classifiable’.

Glyphosate, which was invented by Monsanto back in 1974, is a broad-spectrum herbicide used to kill weeds, especially annual broadleaf weeds and grasses known to compete with commercial crops.

In the US the herbicide is considered safe since 2013, when Monsanto received approval from the US Environmental Protection Agency (EPA) for increased tolerance levels for glyphosate. In its original assessment the US watchdog said glyphosate can “be used without unreasonable risks to people or the environment.” The EPA said it would consider IARC’s evaluation.

A German government evaluation conducted for the European Union last year also found the herbicide safe to use. “The available data do not show carcinogenic or mutagenic properties of glyphosate nor that glyphosate is toxic to fertility, reproduction or embryonal/fetal development in laboratory animal,” the German Federal Institute for Risk Assessment said.

Monsanto insists that “all labeled uses of glyphosate are safe for human health,” according to Miller.

Glyphosate is mainly used on genetically modified corn and soybeans, thus the general public is unlikely to face the greatest risk of exposure, according to the report.

However, “home use” is not the issue, said Kate Guyton of IARC.

“It’s agricultural use that will have the biggest impact. For the moment, it’s just something for people to be conscious of.”

Last month, a leading US environmental group, the Natural Resources Defense Council (NRDC), filed a lawsuit against the Environmental Protection Agency, accusing regulators of dismissing the dangers of glyphosate.

In a recent report by the Center for Food Safety, the heavy proliferation of Roundup was linked to a drastic 90-percent drop in the population of monarch butterflies in the US. Roundup has become a leading killer of Glyphosate-sensitive milkweed plants – the only spots where monarchs lay eggs, as the plant is the only food source for monarch larvae.

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