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California Says Key Ingredient in Roundup Weed Killer Can Cause Cancer

One of the most popular herbicides in the world can cause cancer, California health officials say, and they might demand warnings saying so.

That herbicide, glyphosate, will be added to California's list of chemicals that can cause cancer, the state's Office of Environmental Health Hazard Assessment said this week. Glyphosate is the key ingredient in weed killers such as Roundup.

California keeps a list of carcinogenic chemicals because of a law commonly called Proposition 65, which "requires businesses to provide warnings to Californians about significant exposures to chemicals that cause cancer, birth defects or other reproductive harm."

Monsanto, the maker of Roundup, insists glyphosate is not carcinogenic.

The decision to add glyphosate to that list stemmed from the International Agency for Research on Cancer's assessment that glyphosate is "probably carcinogenic to humans," said Sam Delson, deputy director of OEHHA.

But Monsanto, the maker of Roundup, is fighting back.