Secondary organophosphate esters: A review of environmental source, occurrence, and human exposure

Organophosphate esters (OPEs), a group of synthetic chemicals widely used as flame retardants and plasticizers, have garnered significant international attention due to their adverse effects on the environment and human health. Traditionally, environmental OPEs are thought to originate via direct emissions. Recent evidence suggests that OPEs also have an important indirect source: The transformation of organophosphite antioxidants (another group of mass-produced commercial chemicals) to OPEs via atmospheric chemical reactions. This indirect source can lead to the formation of secondary OPEs (SOPEs) such as tris(2,4-di-tert-butylphenyl) phosphate (TDtBPP), which are widely distributed in the global environment and have distinct physiochemical and toxic properties compared with the well-studied primary OPEs. Therefore, there is an urgent need to obtain a strong fundamental knowledge of SOPEs. This review summarizes the current understanding of the sources, environmental occurrence, human exposure pathways, and environmental hazards of SOPEs. They have been detected in various environmental matrices such as air, soil, and indoor dust, as well as in consumer products such as face masks and foodstuffs. Notably, the reported SOPE concentrations are higher than most primary OPEs. Human exposure pathways related to SOPEs include dietary intake, dust ingestion, hand-to-mouth contact, dermal absorption, and air inhalation. Additionally, risk evaluation indicates that SOPEs are more persistent in the environment and in organisms, and may pose a higher risk than the primary OPEs. Finally, by summarizing the current advances and remaining challenges for the investigation of SOPEs, we propose future research directions regarding their environmental monitoring needs, transformation chemistry, environmental impact, and health effect.