2,4-D was first used in the United States in the 1940s under the name Agent Orange, an herbicide used during the Vietnam War, which contained 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.

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. 2,4-D sticks around in the environment. Depending on the formulation, it can drift through the air from the fields where it is sprayed or be tracked inside homes by pets or children. By the EPA’s own measure, 2,4-D has already been detected in groundwater and surface water, as well as in drinking water. Australian scientists reported in 2012 that it was found in more than 90 percent of samples taken from agricultural catchments bordering the Great Barrier Reef—bad news for many fish, for whom the herbicide can be toxic. It can also poison small mammals, including dogs who can ingest it after eating grass treated with 2,4-D (3).