Chlorpyrifos (CPS), sold under many brandnames, is an organophosphate pesticide used to kill a number of pests including insects and worms.[6] It is used on crops, animals, and buildings.[6] It was introduced in 1965 by Dow Chemical Company. It acts on the nervous system of insects by inhibiting acetylcholinesterase. Chlorpyrifos ingestion, mostly due to suicide, results in more than 10,000 human deaths a year.[6]

Chlorpyrifos is considered moderately hazardous to humans by the World Health Organization.[7] Exposure surpassing recommended levels has been linked to neurological effects, persistent developmental disorders and autoimmune disorders. Exposure during pregnancy may harm the mental development of children, and most home use was banned in 2001 in the U.S.[8] In agriculture, it is "one of the most widely used organophosphate insecticides" in the United States, and before being phased out for residential use was one of the most used residential insecticides.[9] On March 29, 2017, EPA Administrator Scott Pruitt denied a petition to ban chlorpyrifos.[10]​