Beyond Total PFAS: Isomer Composition Matters for Water Quality and Human Exposure Studies.

Per- and polyfluoroalkyl substances (PFAS) produced by electrochemical fluorination are complex mixtures of linear and branched isomers that differ in physicochemical properties, environmental fate, bioaccumulation potential, and biological activity. Increasing evidence shows that not only linear but also branched PFAS isomers are prevalent in the environment. Yet, current monitoring frameworks and regulatory approaches largely rely on total PFAS isomer metrics, masking the distinct behaviors and toxicities of individual isomers and introducing uncertainty into exposure and risk assessments. In this Perspective, we examine the current knowledge of (1) the distribution and accumulation of PFAS isomers across environmental compartments, (2) analytical challenges associated with isomer separation, identification, and reporting, and (3) critical gaps in understanding the environmental and biological processes governing isomer-specific toxicity and fate. We argue that reliance on the sum of PFAS isomers rather than individual isomers may bias PFAS quantification, weaken exposure-response assessments, and limit the development of scientifically robust, isomer-informed risk assessment and regulation. Advancing PFAS risk evaluation will require a shift from aggregate measurements to isomer-resolved science and policy. To support this shift, we propose integrating advanced analytical techniques with in silico, in vivo, in vitro, and biomonitoring approaches to better characterize isomer-specific exposure, fate, and toxicity. Such efforts will improve risk characterization and provide a stronger scientific foundation for regulatory frameworks that more accurately reflect real-world PFAS exposures and better protect environmental and human health.