Breast milk is crucial for infant health, offering essential nutrients and immune protection. However, despite increasing exposure risks from nanoparticles (NPs), their potential infiltration into human breast milk remains poorly understood. This study provides a comprehensive chemical profile of NPs in human breast milk, analyzing their elemental composition, surface charge, hydrodynamic size, and crystallinity. NPs were detected in 42 out of 53 milk samples, with concentrations reaching up to 1.12 × 1011 particles/mL. These particles comprised nine elements, with O, Si, Fe, Cu, and Al being the most frequently detected across all samples. We establish a mechanistic axis for NP infiltration, involving penetration of the intestine/air-blood barriers, circulation in blood vessels, crossing the blood-milk barrier via transcytosis or immune cell-mediated transfer, and eventual accumulation in milk. Structure-activity relationship analysis reveals that smaller, neutral-charged NPs exhibit stronger infiltration capacity, offering potential for regulating NP behavior at biological barriers through engineering design. This study provides the chemical profiles of NPs in human breast milk and uncovers their infiltration pathways.

Center for Genetic Epidemiology and Genomics School of Public Health Suzhou Medical College Soochow University Suzhou Jiangsu 215123 China

Collaborative Innovation Center of Radiological Medicine of Jiangsu Higher Education Institutions Suzhou Medical College Soochow University Suzhou Jiangsu 215123 China

Nanotechnology Centre Centre for Energy and Environmental Technologies Vysoká škola báňská Technical University of Ostrava Ostrava 70800 Czech Republic

State Key Laboratory of Radiation Medicine and Protection School for Radiological and Interdisciplinary Sciences Suzhou Medical College Soochow University Suzhou Jiangsu 215123 China

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Identification of nanoparticle infiltration in human breast milk: Chemical profiles and trajectory pathways