Sweat-induced aggregation of nanoplastics with different sizes and functionalities: Implications for global and body-region variability in dermal penetration risks

Sweat-induced aggregation influences dermal penetration risks of nanoplastics. This study investigated the aggregation kinetics of unmodified (PS20 and PS50), amino-modified (APS50), and carboxyl-modified (CPS50) polystyrene nanoplastics (20-50 nm) in sweat standards and evaluated penetration risks using global sweat data. Negatively charged PS20, PS50, and CPS50 aggregated fastest (1.06-3.51 nm/s) in acidic sweat, while positively charged APS50 aggregated most rapidly (3.30 nm/s) in alkaline sweat. Elevated particle and sweat concentrations promoted aggregation. APS50 and CPS50 exhibited the highest heteroaggregation rates in acidic sweat at a 1:1 ratio. Urea and lactic acid stabilized PS50 and APS50, respectively, while L-histidine and inorganic constituents enhanced aggregation. Dermal penetration potential ranked CPS50 > APS50 > PS20 > PS50. Sweat-induced aggregation reduced stratum corneum penetration but not entry through hair follicles or sweat glands. Penetration probability (0-53.91 %) and flux (3.11 × 10 to 9.31 × 10 particle·s·m) for PS50 varied globally, with higher values observed in Europe, South America, and Oceania. APS50 displayed higher critical diffusion concentrations (50-115 mM), indicating greater penetration potential. Both PS50 and APS50 posed the highest penetration risks in the calf region. These findings underscore the role of sweat-induced nanoplastic aggregation in modulating dermal penetration risks across global regions and body areas.