What Biology Receives: PFAS, Microplastics, and Contact Dose at Living Interfaces.

Per- and polyfluoroalkyl substances (PFAS) detected in water, sorbed to microplastic or nanoplastic particles (MNPs), or retained within microbial biomass represent biologically distinct exposure states. Conventional metrics such as water or serum concentration, total chemical burden, particle count, and microbiome composition remain essential for exposure description, but they do not identify the fraction that reached a biological interface, its physicochemical state, its residence time, or the scale of the resulting response. Published evidence shows that PFAS sorption varies with particle and environmental properties, that biological coatings can modify MNP surfaces, and that microbes can respond to or accumulate exposure-relevant fluorinated chemistry. The strongest integrated evidence currently comes from gastrointestinal systems; extension of the same logic to other exposure routes remains to be tested. This Critical Perspective proposes two complementary interpretive constructs. Contact dose at a living interface describes what reached a biologically active boundary or compartment in an available or retained state. Perturbation scale places the resulting response against background variation, adaptive stress, early injury, disease-relevant biology, or a recognized toxicological insult. These constructs are intended to complement, not replace, conventional exposure metrics. A claim-aligned research agenda should connect chemical and particle exposure state with biological desorption, interface contact, microbial exposure fate, coherent host-response pathways, and endpoint-matched comparators. Adoption of this approach could improve the consistency, comparability, and biological interpretation of PFAS-MNP-microbiome studies.