Human Sweat-Mediated Aggregation of Nanoplastics as a Driver of Dermal Exposure

Nanoplastics inevitably interact with human sweat upon dermal contact, where their aggregation behavior governs the subsequent migration and penetration risks. However, the driving factors and underlying mechanisms have remained poorly understood. Herein, polyethylene nanoplastics (PENPs), the main type of nanoplastics detected in skin care products and airborne particles, were studied in batch experiments with data analyzed by machine learning. The nanoplastic concentration level (49.8%), photoaging (17.1%), and organic constituents (8.7%, mainly l-histidine) were identified as dominant factors governing aggregation. Increasing PENP concentration promoted aggregation, thereby reducing penetration possibility through sweat glands and hair follicle pores. In contrast, photoaging suppressed aggregation and elevated the penetration possibility of photoaged PENPs (APENPs). l-Histidine adsorbed on PENPs via electrostatic attraction and hydrophobic interactions enhanced their dispersion and penetration possibility, whereas l-histidine promoted the aggregation of APENPs through cooperative LH-mediated electrostatic bridging among APENPs, thus reducing their penetration possibility. Moreover, model predictions indicated that higher penetration possibility in females, overweight individuals, trunk skin regions, and populations in China and Europe was attributed to higher dispersion due to lower particle concentration, stronger photoaging, and higher lactic acid in sweat. These findings contribute to understanding realistic exposure of PENPs and their molecular mechanisms of aggregation in sweat, thus providing useful insights into human health risks from dermal exposures.