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

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 × 10 11 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.