Critical Evaluation of Chlorpyrifos’ Breast Cancer Risk

Abstract Chlorpyrifos is an organophosphate pesticide extensively used in agricultural and non-agricultural settings. It has been widely used as a termiticide in homes and buildings. There are reports that demonstrate the potential for widespread non-occupational exposure and for children to come in contact with this insecticide. There has been no cancer risk classification for chlorpyrifos by the US Environmental Protection Agency (EPA), the National Toxicology Program (NTP) or the International Agency for Research on Cancer (IARC). For this report, we have used a modification of IARC approach to conduct a detailed evaluation of any direct or related evidence of cancer risk from exposure to chlorpyrifos, with a focus on breast cancer risk. We have critically evaluated all the available human, experimental animal and cancer-related molecular and mechanistic studies on chlorpyrifos. Chemical and up-to-date regulatory information is included, as well as a discussion of chlorpyrifos’ environmental fate and potential for human exposure. Evidence available so far does not indicate that chlorpyrifos increases breast cancer risk. It should be noted that this conclusion is based on the limited scientific evidence currently available and we have identified several research gaps in this evaluation. Thus, we propose that chlorpyrifos be classified in Group 3. In the Program on Breast cancer and Environmental Risk Factor's (BCERF) breast cancer risk classification scheme, this group represents chemicals that are not classifiable for breast cancer risk in humans.