With spring comes warmer days and the rush to get the garden ready for planting. But before you reach for an herbicide to tackle those weeds, take a look at a report issued by the cancer research arm of the World Health Organization (WHO) this week, concluding that glyphosate, the herbicide used in Roundup, is "probably carcinogenic" to humans.

The report, which was compiled by the International Agency for Research on Cancer, evaluated the existing research on the herbicide and its effect on humans and lab animals. Researchers found that there was "convincing evidence" that glyphosate can cause cancer in laboratory animals. They also noted "limited evidence" of risk of non-Hodgkin's lymphoma among people who were exposed to glyphosate through their work and had traces of the chemical in their blood and urine.

"There was sufficient evidence in animals that glyphosate is carcinogenic and evidence that it can cause some of the steps towards cancer in cells, but only a limited number of studies in humans, so it was classified as a probable carcinogen," said professor Lin Fritschi, a co-author of the study, which was published in the Lancet online.

These reasons led the panel to conclude that glyphosate was "probably carcinogenic to humans," but the panel stopped short of naming the chemical a conclusive carcinogenic.

The agency looked at several insecticides and pesticides, rating them according to four levels of risk: known carcinogen, probable or possible carcinogen, not classifiable and probably not carcinogenic, according to the Chicago Tribune.

Monsanto, the manufacturer of glyphosate, rejected the panel's 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,” said Philip Miller, Monsanto’s vice president of global regulatory affairs, in a prepared statement.

Several other environmental health regulators from around the world, including the Environmental Protection Agency (EPA), have previously concluded that glyphosate has “low acute toxicity.”

Glyphosate is used in more than 750 agricultural, household and forestry products worldwide, and its use is only predicted to increase now that Monsanto has also released genetically modified "Roundup Ready" seeds that are resistant to it. Environmental advocates hope that this new report will force health regulators to take a closer look at Roundup and its possible effects on humans and the planet.

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