The Environmental Protection Agency first rated atrazine as a possible human carcinogen in 1988 based on evidence of mammary (breast tumors) in female laboratory rats fed high levels of atrazine for most of their lives.

In 1994 atrazine and two other triazine herbicides simazine and cyanazine were placed under “Special Review” by the Environmental Protection Agency because of concerns about their potential cancer-causing effects. The Environmental Protection Agency also expressed concern because of risks resulting from exposure through contaminated water and food, and exposures in workers during mixing, loading and application of this herbicide.

In December 1999, the Environmental Protection Agency released a preliminary cancer risk assessment, rating atrazine as a ‘probable human carcinogen’.