On September 19, 2018, the California Department of Pesticide Regulation (DPR) proposed a regulation to designate chlorpyrifos as a toxic air contaminant (TAC). DPR states that this proposal is being presented “after an extensive period of scientific and public review.” The proposed rule is based on a final evaluation issued in July 2018, in which DPR’s Human Health Assessment (HHA) Branch determined that chlorpyrifos meets the quantitative criteria for designation as a TAC. To make that determination, DPR utilized an inhalation reference concentration (RfC) based on new animal studies with chlorpyrifos that reported neurodevelopmental effects at exposure levels well below the threshold for acetylcholinesterase (AChE) inhibition. More information on DPR’s final TAC evaluation is available in our blog item "California DPR Releases Final Toxic Air Contaminant Evaluation for Chlorpyrifos." In August 2018, DPR posted the Scientific Review Panel on TAC’s findings on chlorpyrifos and the Director’s Proposed Determination Concerning Chlorpyrifos as a TAC.
The U.S. Environmental Protection Agency (EPA) previously issued a determination that the default 10X safety factor for infants and children established by the Food Quality Protection Act (FQPA) should be retained for chlorpyrifos. This determination was based primarily on epidemiology studies that purported to show adverse neurodevelopmental outcomes in humans at exposure levels below the threshold for AChE inhibition, but the methodology used in these epidemiology studies has been harshly criticized by the pesticide industry. In contrast, the DPR TAC proposal is predicated on a determination that new animal studies with chlorpyrifos report neurodevelopmental effects below the threshold for AChE inhibition, and DPR views the epidemiology studies utilized by EPA to make its FQPA determination as providing corroboration for the animal data. At this juncture, it is not clear how EPA will characterize the new animal data concerning chlorpyrifos. In any case, questions are likely to remain concerning EPA’s use of data concerning chorpyrifos to establish the FQPA safety factor for other organophosphate (OP) pesticides.