The World Health Organisation’s cancer research arm has found that broadleaf herbicide 2,4-D is ‘possibly’ carcinogenic to humans.

WHO’s International Agency for Research on Cancer updated its classification of the herbicide on Tuesday, having assessed the latest scientific literature on the chemical. 2,4-D is an active ingredient in new preparations recently released by Dow chemicals.

The publication comes three months after IARC found that widely used herbicide glyphosate is ‘probably’ carcinogenic to humans, a finding which manufacturer Monsanto said was “a dramatic departure from the conclusion reached by all regulatory agencies around the globe.”

Both 2,4-D and glyphosate are active ingredients in Dow AgroSceince’s new Enlist Duo preparation, which was approved by US regulators last year. The product has been designed for use on herbicide-tolerant GM crops

IARC’s ruling was made in light of some evidence linking 2,4-D to cancer in non-human animals, but a lack of definitive evidence of similar effect in humans.

Reacting to the classification, which was published in medical journal The Lancet Oncology, Dow said that “2,4-D has been widely used around the world to help protect crops, pastures and other areas from noxious and invasive weeds… for 70 years.”

John Cuffe, Regulatory Affairs spokesperson for the company said “No herbicide has been more thoroughly studied and no national regulatory body in the world considers 2,4-D a carcinogen. In fact, IARC stated that there is ‘inadequate evidence’ for human carcinogenicity. IARC’s findings on 2,4-D are not the last word even within the WHO, whose JMPR does not consider the handling and use of approved 2,4-D herbicides to pose a cancer risk.”

JMPR is the Joint Meeting on Pesticide Residues, whose evaluations support the establishment of world food standards.

Publishing their findings, the IARC experts said, “There is strong evidence that 2,4-D induces oxidative stress, a mechanism that can operate in humans, and moderate evidence that 2,4-D causes immunosuppression,” but added that “Epidemiological studies did not find strong or consistent increases in risk of [non-Hodgkin lymphoma - a group of blood cancers] or other cancers in relation to 2,4-D exposure.”