Critical Evaluation of Cancer Risk from 2,4-D

2,4-D [(2,4-dichlorophenoxy)-acetic acid] is a widely used postemergent herbicide. It is structurally similar to indoleacetic acid (IAA), a naturally occurring plant hormone (see Fig.1). This similarity allows 2,4-D to mimic IAA and is the basis for its herbicidal action. The synthesis of 2,4-D was first reported in 1941 (ARC 1986I). In 1945, Dow Chemical Co. discovered that a 1:1 mixture of 2,4-D and 2,4,5-T [(2,4,5-trichlorophenoxy)-acetic acid] was a more effective herbicide than either of the two chemicals alone. The mixture was widely used thereafter and referred to as Agent Orange (Lilienfeld and Gallo 1989). Exposure to 2,4-D in the past may have more often been to a mixture of herbicides rather than 2,4-D alone. 2,4-D preparations before 1975 were often contaminated with TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) (IARC 1986;Johnson et al. 1992).