2,4-D was first used in the United States in the 1940s. Agent Orange, an herbicide used during the Vietnam War, contained both 2,4-D and 2,4,5-T. Dioxin, a by-product of 2,4,5-T, led to the ban of Agent Orange

Researchers have observed apparent links between exposure to 2,4-D and non-Hodgkin's lymphoma (a blood cancer) and sarcoma (a soft-tissue cancer). But both of these can be caused by a number of chemicals, including dioxin, which was frequently mixed into formulations of 2,4-D until the mid-1990s. Nevertheless, in 2015, the International
Agency for Research on Cancer declared 2,4-D a possible human carcinogen, based on evidence that it damages human cells and, in a number of studies, caused cancer in laboratory animals.


More conclusive is the proof that 2,4-D falls into a class of compounds called endocrine-disrupting chemicals, compounds that mimic or inhibit the body's hormones. Laboratory studies suggest that 2,4-D can impede the normal action of estrogen, androgen, and most conclusively, thyroid hormones. Dozens of epidemiological, animal, and laboratory studies have shown a link between 2,4-D and thyroid disorders. "That's really important when we're thinking about development," says Kristi Pullen, a staff scientist in NRDC's Health program. "Our thyroid works to ensure the proper timing and development of the brain."

There are reports that 2,4-D can decrease fertility and raise the risk of birth defects. But even though fetuses, infants, and children are at highest risk of these, no studies have looked directly at the effects of 2,4-D on those groups.

Despite concerns about potential health risks, in 2014 the U.S. Environmental Protection Agency approved the combined use of 2,4-D and the popular weed killer Roundup (also known as glyphosate, a whole other—and in many ways more worrying—story when it comes to health and the environment). Enlist Duo, as the combo is called, was already legal in several states. It is used mainly on big farms, where it is sprayed on genetically modified crops called Enlist soy and Enlist corn that have been engineered to be resistant to the poisons.

In other words, farmers can now douse their fields with high concentrations of the weed killer without worrying that it will also destroy their crops. Originally, plants genetically engineered to resist Roundup were sprayed with that herbicide alone. But when the weeds it was intended to kill also developed resistance, 2,4-D was added to make the mix more effective. As Pullen puts it, "These chemicals by themselves can be problematic, but when we start combining them with other toxic chemicals, we're just creating a new problem in order to solve another problem."