Microplastics and nanoplastics in humans: exposure pathways, biological mechanisms, and implications for health risk assessment

Microplastics (MPs) and nanoplastics (NPs) are ubiquitous contaminants, but their implications for human internal exposure, biological effects, and health risk assessment remain incompletely defined. Although many reviews summarize environmental occurrence and ecological impacts, fewer integrate human exposure pathways with mechanistic toxicological evidence and safety-relevant uncertainties. This review synthesizes evidence on inhalation, ingestion, and dermal exposure, emphasizing particle size, barrier interactions, translocation, and distribution. We examine experimental and clinical evidence describing oxidative stress, inflammatory signaling, epithelial barrier dysfunction, gut dysbiosis, immune activation, and endocrine interference associated with MP and NP exposure. Particular attention is given to distinguishing MPs from NPs in terms of cellular uptake, biodistribution, analytical detectability, and toxicodynamic behavior. We also discuss limitations in exposure assessment, including contamination bias, analytical detection limits, and harmonized reporting standards. Rather than focusing on environmental prevalence, this review highlights gaps in exposure quantification, dose-response relationships, chronic low-dose exposure, vulnerable populations, and co-exposure to plastic additives or adsorbed contaminants. An organ-specific risk perspective is integrated, emphasizing respiratory, digestive, immune, and systemic effects, as well as particle size, polymer type, exposure concentration, and human dose-response uncertainty. By synthesizing toxicological, analytical, and public health evidence, this review defines research priorities for risk assessment.