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A widely used farm chemical that is a key ingredient in a new herbicide developed by Dow AgroSciences "possibly" causes cancer in humans, a World Health Organization research unit has determined.
The classification of the weed killer, 2,4-dichlorophenoxyacetic acid, known as 2,4-D, was made by the WHO's International Agency for Research on Cancer (IARC).
The IARC said it reviewed the latest scientific literature and decided to classify 2,4-D as "possibly carcinogenic to humans." That is a step below the more definitive "probably carcinogenic" category but two steps above the "probably not carcinogenic" category.
IARC's findings on 2,4-D have been awaited by environmental and consumer groups that are lobbying U.S. regulators to tightly restrict its use, as well as by farm groups and others that defend 2,4-D as an important agent in food production that does not need more restrictions.
Since its introduction in 1945, 2,4-D has been widely used to control weeds in agriculture, forestry, and urban and residential settings.
In March, IARC said it had found another popular herbicide -glyphosate - was "probably carcinogenic to humans." Glyphosate, the world's most widely used weed killer, is the key ingredient in Monsanto Co's Roundup herbicide and other products.
IARC classifications do not carry regulatory requirements but can influence regulators, lawmakers and the public. Following the glyphosate classification, some companies and government officials moved to limit glyphosate use.
Dow AgroSciences, a unit of Dow Chemical Co, has had a particular interest in IARC's review. The company is using both glyphosate and 2,4-D in a herbicide it calls Enlist Duo that received U.S. approval last year. Enlist Duo is designed to be used with genetically engineered, herbicide-tolerant crops developed by Dow.
Dow said in a statement that IARC's classification was flawed and was "inconsistent with government findings in nearly 100 countries" that have affirmed the safety of 2,4-D when used as labeled.
IARC said it decided on the "possibly carcinogenic" classification because there was "inadequate evidence in humans and limited evidence in experimental animals" of ties between 2,4-D and cancer. It said that epidemiological studies provided "strong evidence that 2,4-D induces oxidative stress ... and moderate evidence that 2,4-D causes immunosuppression."
However, IARC said, "epidemiological studies did not find strong or consistent increases in risk of NHL (non-Hodgkin lymphoma) or other cancers in relation to 2,4-D exposure."
Dana Loomis, a deputy section head for IARC, said the most important studies reviewed showed mixed results, and a "sizable minority" judged the evidence as stronger than others did.
Among the research presented to IARC was an analysis funded by a Dow-backed task force that found no ties between 2,4-D and many cancers.
Some critics of 2,4-D had expected IARC to classify 2,4-D as "probably" cancer-causing. But the classification it did receive still underscores the dangers of 2,4-D and the need for a "much more rigorous regulatory process," the Pesticide Action Network North America said in a statement.
(Reporting by Carey Gillam; Editing by Steve Orlofsky and Leslie Adler)