The World Health Organization’s cancer research agency announced yesterday it had placed the herbicide 2,4-D on its list of "possible" carcinogens, just months after the body gave another weedkiller — glyphosate — a similar designation.
According to a press release from the International Agency on the Research of Cancer, there is "strong evidence" that 2,4-D induces oxidative stress, a mechanism that can cause damage to human cells, and moderate evidence that 2,4-D suppresses the immune system, based on animal studies. On a scale of 1 to 4, with 1 indicating a strong cancer link, 2,4-D is in Group 2B, making it a "possible" carcinogen.
However, IARC’s review of epidemiological studies did not find strong or consistent increases in risk of non-Hodgkins lymphoma, a type of cancer of the lymph nodes, or other cancers in relation to 2,4-D exposure.
The findings are in line with the conclusions of U.S. EPA and other Canadian and European agencies, which have approved 2,4-D for use, and does not mean that 2,4-D will lead to cancer, said Julie Goodman, a consultant to the 2,4-D Task Force, which funds industry studies to support the registration of the chemical in the United States and Canada.
"It’s very important to put this in perspective," she said on a press call.
IARC also announced classifications for two other pesticides. There is "sufficient evidence" that the insecticide lindane causes cancer, they found. The well-known mosquito-killer, DDT, is also a "probable" carcinogen. But the designation for 2,4-D struck an especially strong chord, given that the chemical is an active ingredient in the controversial herbicide Enlist Duo that spurred a lawsuit from environmental groups earlier this year (Greenwire, April 21).
Enlist Duo, developed by Dow AgroSciences, has been approved by EPA for use in 15 states. It combines a choline salt of 2,4-D with glyphosate, better known by its commercial name Roundup, and has been commercialized to use with genetically modified corn and soybeans that can withstand applications of the herbicide. The herbicide was developed to provide farmers with an alternative to glyphosate alone, which has steadily become less effective as weeds evolve to resist applications. Environmental groups say the combination will lead to more pesticide use. The IARC classifications provide strong evidence that both glyphosate and 2,4-D are hazardous, they said.
"This warning from IARC comes at a critical crossroads for farming worldwide," Marcia Ishii-Eiteman, senior scientist with the Pesticide Action Network, said in a statement. "If we keep running on this pesticide treadmill, farmers, their families and rural residents will continue to be in harm’s way. It’s time to look to ecology-based solutions for weed management."
IARC classifies the strength of evidence that a product causes cancer on a scale of 1 to 4, where substances in Group 1 have "sufficient evidence of carcinogenicity in humans" and substances in Group 4 are "probably not carcinogenic to humans." For agents in Group 2B, where 2,4-D sits, there is "limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals." Glyphosate was placed in Group 2A, where there is "is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals" (Greenwire, April 2).