Glyphosate, the main ingredient in Monsanto's widely used herbicide Roundup, will be added July 7 to California's list of chemicals known to the state to cause cancer, according to a Reuters report Tuesday. This news comes after the company's unsuccessful attempt to block the listing in trial court and requests for stay were denied by a state appellate court and California's Supreme Court.

California's Office of Environmental Health Hazard Assessment (OEHHA) announced the designation on Monday under the Safe Drinking Water and Toxic Enforcement Act of 1986, or Proposition 65.


Citing figures from the state's pesticide regulation department, the Los Angeles Times noted that glyphosate is the most widely used herbicide in California and is sprayed on more than 200 crops across 4 million acres across the Golden State.

The St. Louis-based agrochemical maker adamantly defended its star product, telling Reuters the OEHHA listing was "unwarranted on the basis of science and the law."

The company's appeal of the trial court's ruling is pending. "This is not the final step in the process, and it has no bearing on the merits of the case," Scott Partridge, Monsanto's vice president of global strategy, said. "We will continue to aggressively challenge this improper decision."

Under Prop 65, Monsanto and other companies that sell the chemical in the state will be required to add warning labels to packaging within one year from the listing date. Furthermore, warnings would also be required if glyphosate is sprayed at levels deemed unsafe by regulators, Reuters reported.

California's decision to add the substance on its cancer list was prompted by the World Health Organization International Agency for Research on Cancer's (IARC) finding in 2015 that the chemical is a "probable" human carcinogen.

Environmental groups welcomed the OEHHA's decision.

"California's decision makes it the national leader in protecting people from cancer-causing pesticides," said Nathan Donley, a senior scientist at the Center for Biological Diversity. "The U.S. EPA now needs to step up and acknowledge that the world's most transparent and science-based assessment has linked glyphosate to cancer."

In another blow to Monsanto, Olga V. Naidenko, a senior science advisor for children's environmental health at the Environmental Working Group, commented last week that OEHHA is preparing to issue a "No Significant Risk Level" of 1,100 micrograms per day for exposure to glyphosate—marking the first time any agency in the world has issued a health guideline for glyphosate based on cancer risk.

"The state's proposed safe level is more than 100 times lower than the U.S. Environmental Protection Agency's legal allowance for the average-sized American," Naidenko said. "By itself, that listing would be a big blow to Monsanto, because it would require cancer warning labels on containers of Roundup and on foods that have high residues of glyphosate. Monsanto is appealing the decision in state court, but in the meantime the OEHHA has moved forward in setting a so-called No Significant Risk Level of the amount of glyphosate people could safely consume each day."

Glyphosate has been at the center of a number of controversies in recent years, including a wave of lawsuits in which plaintiffs across the U.S. claim that they or their loved ones developed non-Hodgkin's lymphoma due to exposure to Monsanto's Roundup, pointing in part to the IARC's cancer classification.

A recent Reuters investigation, published earlier this month, raised questions about the IARC's assessment. The piece accused Dr. Aaron Blair, a top epidemiologist from the U.S. National Cancer Institute and a lead researcher on the IARC committee, for failing to share unpublished scientific data from the Agricultural Health Study (AHS) suggesting that the weedkiller was not linked to cancer.

However, scientists have since voiced concerns with what they are calling a "flawed" Reuters story.

Michael Hansen, senior scientist at Consumers Union, said the Reuters report "omits the fact that the data from the other epidemiology studies (all case control studies), and the meta-analyses, clearly show a statistically significant increase in non-Hodgkin's lymphoma with glyphosate exposure." Other concerns of the study include the failure to use an appropriate latency period for cancers, the control group having an elevated risk of non-Hodgkin's lymphoma and exposure misclassification.

The IARC also said it is sticking with its classification.