At a meeting in Lyon, France, this week 24 scientists representing the World Health Organization’s International Agency for Research on Cancer (IARC) will analyze scientific findings regarding suspected links between cancer in humans and the herbicide active ingredient 2,4-D.

The discussion over the possible links comes less than three months after a separate group of IARC scientists in March unanimously decided to classify glyphosate, the key ingredient in Monsanto's Roundup weed killer, as “probably carcinogenic to humans.”

As some public officials and consumers subsequently called for bans on the herbicide, Monsanto demanded a retraction of the WHO group’s classification.

If 2,4-D is implicated, Dow’s plans to launch its new genetically manipulated Enlist Duo corn and soybean franchise, containing both glyphosate and 2,4-D, this year could be doubly challenged.

The US Environmental Protection Agency (EPA) in 2014 approved the Dow herbicide with restrictions, due to concern over its effects on the insect population. Among other things, Dow is required to track and report on weed resistance to the product.

Additionally, EPA has issued a 30-foot in-field "no spray" buffer zone for application areas and banned Enlist Duo from being used when wind speeds are above 15 miles per hour.

Because of its approval of the Dow product, EPA is being sued by the US Natural Resource Defense Council (NRDC), which fears the herbicide will threaten the Monarch butterfly. Some environmental campaigners also have identified glyphosate as a threat to the butterfly.

Enlist Duo, developed by Dow as an answer to the severe weed resistance problems that are limiting crop production around the US, is also eyed critically because of its role as a component of the Vietnam War defoliant Agent Orange.

There is just as strong, or even a stronger case for links to cancer on 2,4-D than there was for glyphosate, Michael Hansen, a senior scientist at Consumers Union who has served on an advisory committee of the US Department of Agriculture as well as an expert on WHO consultation projects, commented to the news agency Reuters.

Dow will have representatives at the meeting. A 2,4-D task force backed by the US chemical giant funded an analysis of 14 studies that it said refutes concerns about the chemical. “No national regulatory authority in the world considers 2,4-D a carcinogen,” Dow asserted.

However, Maria Leon, who co-authored the analysis by IARC last year, said there were indications that farm exposure to 2,4-D was associated with a 40% higher risk of Non-Hodgkins Lymphoma. (dw)