Epidemiological studies examining cancer mortality in two series of workers exposed to dieldrin provide no conclusive evidence of carcinogenicity in humans. Possible increases in liver, biliary, and rectal cancer were suggested in some of the later studies. Several studies in mice have shown that oral exposure to dieldrin caused an increase in the incidence of malignant liver tumours. However, studies in rats have been either equivocal or flawed.
Toxicokinetic data do not indicate that any different response would be expected following exposures by these routes. Accumulating evidence indicates that dieldrin is nongenotoxic tumour promoter acting through speciesspecific susceptibility of the mouse to induction of oxidative stress and inhibition of gap junctional communication.
The Department of Health and Human Services (DHHS) has not listed dieldrin in the U.S. National Toxicology Program Carcinogen List. The International Agency for Research on Cancer (IARC) determined that dieldrin is unclassifiable because the carcinogenicity data are incomplete or ambiguous. According to the U.S. EPA Carcinogen List there is a sufficient evidence of dieldrin carcinogenicity from animal studies with inadequate or no data from epidemiologic studies in humans, there for it is classified as a Category B2, Probable human carcinogen.