Particle deposition in the human lung as a function of microplastics’ shape, size, orientation, and type

The widespread use and poor management of single-use plastics have created a global pollution issue with emerging human health concerns. Environmental degradation of plastics produces micro- and nanometer-sized particles that may become airborne and inhaled. While some are removed by lung defenses, others persist and trigger inflammation or toxic effects, including reproductive harm, carcinogenicity, and mutagenicity. Because airborne microplastics are often fibrous, this study focuses on how size, shape, and orientation influence their deposition. Deposition fractions of microplastic fibers in different regions of the human lung were estimated using the International Commission on Radiological Protection (ICRP) deposition model, with adjustments for fiber geometry, density, and orientation through aerodynamic and volume-equivalent diameters. Fiber lengths of 10-50 µm and diameters of 0.75-5 µm, representative of airborne microplastics reported in environmental samples, were modeled under parallel, perpendicular, and random orientations to evaluate regional deposition patterns. From our modeling results, the maximum deposition fraction was approximately 0.87 in the nasopharyngeal region for fibers with aerodynamic diameters of ~5-7 µm, whereas in the alveolar region, the highest deposition fraction was 0.13 for fibers with diameters of 0.75 µm and lengths up to 35 µm, with these outcomes predicted under random orientation conditions. This study provides the first systematic modeling of lung deposition for fibrous microplastics as a function of size, shape, density, and orientation, offering novel equations and predictive curves that can be directly utilized in inhalation exposure and human health risk assessment.