Regional and population-scale trends in human inhalation exposure to airborne microplastics: Implications for health risk assessment

Growing evidence shows that breathing microplastics (MPs)-polluted air increases the risk of pulmonary health effects. However, a complete understanding of how inhaled MPs distribute within the human respiratory tract (HRT) remains insufficient. This study developed a physiologically-based kinetic HRT model to evaluate the deposition and clearance of MPs over time and at varying concentrations based on their aerodynamic diameter (AD). We quantified the contributions of AD-specific MPs to inhalation exposure trends using literature-based atmospheric MP pollution data from 2015 to 2022. Exposure assessments were conducted in data-rich settings, including megacities, urban-rural, and age-specific populations. Our analysis revealed that all suspended MPs had ADs less than 70 μm, with fragments, fibers, and spheres in decreasing order of prevalence. Modeling results demonstrated a pronounced variation (∼10¹⁰ magnitudes) in internal MP burdens across airway regions during long-term exposure. On average, inhaled MPs larger than 40 μm accumulated exclusively in extrathoracic and bronchi regions, whereas MPs with ADs of 0.1–5 μm were the primary contributors to internal burdens. We identified nasal airflow rate as the most sensitive factor influencing internal burdens of MPs larger than 1 μm. Furthermore, our findings showed that infants, children, and the elderly were more vulnerable to short-term exposure, whereas adolescents and adults were of greater concern with long-term exposure. These insights provide valuable guidance for policy decisions on targeting interventions to at-risk regions or susceptible populations.