The International Agency for Research on Cancer this week released findings of its latest review of a group of common chemicals, specifically identifying the chemical 2,4-D as "possibly carcinogenic to humans," class 2B.
2,4-D is used in agriculture, forestry and general yard or ditch maintenance for controlling weeds in farm fields and along highways and rail lines.
The finding for 2,4-D in class 2B is based on "inadequate evidence" in humans and limited evidence in experimental animals that 2,4-D can cause cancer, IARC said.
There is strong evidence, however, that 2,4-D can cause oxidative stress and moderate evidence it can cause immunosuppression, the group said.
According to the 2,4-D Research Task Force, which is comprised of companies holding technical 2,4-D registrations, the chemical has been continuously studied since its registration in 1947.
Dr. Julie Goodman, epidemiologist, board certified toxicologist and consultant to the 2,4-D Task Force, stressed on a conference call Tuesday that the classification was simply a hazard assessment, not a risk assessment. This means that it's only "one piece of the puzzle" when it comes to impacting use of pesticides, she explained.
"IARC has assigned its 2B grouping to many other common products including aloe vera, coffee and pickled vegetables," she noted, adding that in findings like this, it's important to understand how the substances are used.
Goodman said she believed the finding should have listed 2,4-D in class three because only a few studies reviewed by the IARC showed cancer incidences in animals. "It should be consistent," she said, "not just one study here and there."
She stressed that the group found "inadequate evidence" of cancer in humans, and "no regulatory agency in the world considers 2,4-D to be a carcinogen," she said.
Dow Chemical also characterized the IARC classification as inconsistent with government findings in nearly 100 countries where 2,4-D has been approved for decades. More than 4,000 studies have vetted the chemical, the company said.
"[IARC] reviews an incomplete set of information to focus solely on whether a substance or activity could be a carcinogen, not whether it is a carcinogen when used under real-world circumstances," a company statement said.
As recently as October, 2014, Dow said the U.S. EPA affirmed that 2,4-D is not a carcinogen based on a 17-year evaluation of relevant health and safety data.
The finding concerns not only companies that market the product, but also end-users. Ken McCauley, Kansas grower and past president of the National Corn Growers Association, said the IARC review could be misinterpreted by the general public.
"Herbicides like 2,4-D are essential to modern farming, helping us produce more food, control weeds, use less resources and reduce our costs, which ultimately helps the consumer," McCauley said.
IARC additional findings
Along with 2,4-D, IARC also reviewed lindane (gamma-hexachlorocyclohexane) and DDT (dichlorodiphenyltrichloroethane), finding lindane as "carcinogenic to humans" (group 1) and DDT as "probably carcinogenic to humans" (class 2A).
IARC earlier this year reviewed the commonly used ag chemical glyphosate, also declaring it a class 2A "probably carcinogenic to humans" substance.
At the time, ag groups and companies defended the chemical, citing findings from EU and U.S. task forces on glyphosate that were in disagreement with the IARC classification.
While IARC said the glyphosate review was due to "availability of new research," Dr. Philip Miller, vice president of global regulatory affairs for Monsanto, said no new research or data was used in the review and the most relevant, scientific data was excluded from review.
The conclusion on glyphosate also was "not supported by scientific data," he said.
Correction: A previous version of this story incorrectly identified DDT as class 2B, not 2A.