The decades-long debate over the health risks of a popular weed killer took another turn last month when an international agency classified glyphosate as a probable human carcinogen. Any regulatory effects of the determination by the World Health Organization, however, remain to be seen.

The WHO's International Agency for Research on Cancer listed glyphosate among "probable or possible carcinogens," the second-highest level of classification used by the testing agency. Glyphosate is widely used in industrial agriculture and in the Roundup line of weed killers, although the agency said use of Roundup or other products in home gardens posed little risk.

Monsanto, the agribusiness giant that makes both Roundup and genetically modified seeds engineered to resist glyphosate in farm fields, fired back with allegations that the WHO was "cherry-picking" data and had "an agenda-driven bias."

Philip Miller, the St. Louis company's vice president for global regulatory affairs, said the finding is "starkly at odds with every credible scientific body that has examined glyphosate safety." The National Corn Growers Association also criticized the finding.

Officials long considered glyphosate to be safer than most other pesticide options, and the WHO classification places the chemical in the same category of concern as working overnight shifts or at a hair salon.

Determining whether not a substance can cause cancer is often very difficult due to variations in both laboratory studies and patient risk factors.

The U.S. Environmental Protection Agency made a similar determination about glyphosate in 1985 only to rescind the finding six years later citing a lack of evidence. The EPA said it would factor the WHO finding into subsequent reviews.

More vocal critics have called for an outright ban on the chemical.

Monsanto officials, meanwhile, downplayed the impact of the WHO decision. President and COO Brett Begemann said the company expects "farmers will continue to use this and do not anticipate that there will be an impact on our sales."