A widely used weedkiller was added to a World Health Organization list of potential carcinogens, just three months after the agency made a controversial assessment of glyphosate herbicide.
Based on limited evidence from laboratory animal studies and inadequate evidence in humans, 2,4-D is “possibly carcinogenic to humans,” the International Agency for Research on Cancer, a Lyon, France-based arm of the WHO, said Tuesday in a statement.
2,4-D has been used for 70 years to control weeds in wheat, corn and soybean fields as well as gardens and lawns. The U.S. sprayed about 36 million pounds (16 million kilograms) of the chemical in 2012, according to government estimates.
The weedkiller is a key part of Dow’s agricultural business, Dow AgroSciences. Dow says its new Enlist crops, which are genetically engineered to tolerate sprays of both 2,4-D and glyphosate, will help double earnings at the unit through the end of the decade.
The IARC’s classification is inconsistent with government findings in nearly 100 countries that have found 2,4-D is safe to use as directed, Dow AgroSciences said in a statement.
“No herbicide has been more thoroughly studied, and no national regulatory body in the world considers 2,4-D a carcinogen,” John Cuffe, global leader of regulatory affairs for Dow AgroSciences, said in the statement.
The Industry Task Force on 2,4-D, of which Dow is a member, said a different WHO unit that conducts pesticide-risk assessments has reviewed 2,4-D five times since 1963 and found no evidence of carcinogenicity.
The IARC prompted a strong response from Monsanto in March after saying that glyphosate, marketed by the company as Roundup, “probably” causes cancer. Monsanto called the classification “irresponsible” and the result of “cherry picking” data.
The IARC is creating “confusion and unnecessary fear” because regulatory agencies in more than 100 countries have concluded that 2,4-D and glyphosate don’t increase health risks when used as directed, the National Corn Growers Association and American Soybean Association said in a joint statement prior to the latest IARC report.
The IARC reviewed research on 2,4-D and two other pesticides in a week-long meeting earlier this month. It determined that 2,4-D is less of a cancer hazard than glyphosate, having failed to find strong or consistent increases in the risk of non-Hodgkin lymphoma or other cancers. In March, it said it found “limited evidence” that glyphosate can cause non-Hodgkin lymphoma and lung cancer.
Still, the IARC found strong epidemiological evidence that 2,4-D induces oxidative stress and moderate evidence that it suppresses the immune system. The herbicide joins hundreds of “possibly carcinogenic” materials and occupations identified by the IARC since 1987, from coffee to chloroform to dry cleaning and firefighting.
IARC doesn’t conduct cancer risk assessments because it doesn’t take into account how people are exposed to a given chemical. Rather, the agency says it tries to identify cancer hazards, or the potential to cause cancer once exposure occurs.