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Microplastics are now ubiquitous contaminants in the atmosphere which have raised
substantial public health issues. While extensive research has focused on particle
deposition during inhalation, the exhalation phase, a critical component of the
complete respiratory cycle, remains comparatively less explored. The study aims at
addressing this research gap by conducting a comprehensive computational
investigation into the deposition dynamics of microplastic particles during exhalation
within an idealized human tracheobronchial airway model from generations G3–G6.
A three-dimensional airway geometry was constructed via Weibel's morphometric
data, adjusted for a 50-year-old adult. Computational Fluid Dynamics (CFD)
simulations are performed using the Reynolds-Averaged Navier-Stokes (RANS)
approach with the (SST) 𝑘-𝜔 turbulence model. The Discrete Phase Model (DPM)
was employed to track trajectories of spherical microplastic particles 2-22 µm under
four exhalation flow rates 125, 300, 500, and 1000 ml/s, representing varying
breathing intensities from resting to heavy exercise. The results demonstrate an
inverse relationship between exhalation flow rate and overall deposition efficiency
(DE). Lower flow rates resulted in the highest DE, as particles had greater residence
time for gravitational sedimentation and were less influenced by turbulent dispersion.
In contrast, higher flow rates generated significant turbulent kinetic energy, which
enhanced particle mixing and reduced net deposition. Furthermore, deposition was
strongly governed by inertial impaction, as evidenced by a positive correlation with
the Stokes number. Spatial analysis revealed that while high flow rates created intense
deposition hotspots at major bifurcations, the cumulative particle deposition isXIV
significantly lower than at gentler flow rates, where deposition is more widespread.
This study concludes that gentle exhalation poses a greater risk for microplastic
retention in the lower bronchial airways. The findings challenge the assumption that
higher airflow invariably leads to increased deposition and provide crucial insights
into the mechanisms of particle exposure during the exhalation phase.