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 (∼10 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.