Do alternative PFAS reduce risk or shift it? A critical comparative review of persistence, mobility, bioaccumulation, toxicity, exposure, and safer substitution criteria.

Regulatory restrictions on legacy per- and polyfluoroalkyl substances (L-PFAS), including perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and related long-chain compounds, have accelerated the global adoption of alternative and emerging PFAS (A-PFAS, functional umbrella term rather than structural class). Many A-PFAS were introduced to reduce selected legacy concerns, particularly bioaccumulation and long human retention. However, lower bioaccumulation alone does not establish lower overall risk, because several A-PFAS remain highly persistent, mobile, treatment-resistant, or capable of transforming into persistent terminal products. Here, we present a critical comparative review and analysis of eleven L-PFAS and eighteen A-PFAS across physicochemical properties, environmental fate, global occurrence, bioaccumulation, trophic transfer, toxicity, and exposure, with particular attention to whether substitution reduces risk or shifts it among endpoints. The evidence indicates that PFAS substitution has often reduced some hazard dimensions while preserving or intensifying others, but the direction and confidence of risk shifting differ markedly among compounds. Risk shifting is demonstrated for several chlorinated, fluorotelomer-derived compounds, and perfluorohexanesulfonic acid (PFHxS) that show documented increases in persistence, mobility, bioaccumulation, trophic magnification, or transformation potential. Mixed evidence applies to most short- and ultra-short-chain perfluoroalkyl acids (PFAA) and the hexafluoropropylene oxide (HFPO) compounds, which reduce bioaccumulation or human retention but shift concern toward high mobility, treatment difficulty, diffuse exposure, or toxicity. The evidence is insufficient for several fluorotelomer-based precursors and N-(Methyl) nonafluorobutanesulfonamide (N-MeFBSA), for which bioaccumulation, toxicity, and exposure data remain too limited to characterize the direction of risk shifting. No compound met the criteria for reduced concern across all dimensions. These patterns indicate that many current substitution pathways shift risk from bioaccumulation and long human retention toward persistence, mobility, diffuse exposure, treatment difficulty, secondary-waste generation, and end-of-life uncertainty. Avoiding further regrettable substitution requires moving beyond molecule-by-molecule replacement toward function-based alternatives assessment, transparent production and use information, monitoring beyond conventional target PFAS, evaluation of treatment removability and destruction feasibility, end-of-life release control, and class-or subgroup-based management where justified.