Size-dependent deposition of particles in the human lung at steady-state breathing

Abstract

Numerous efforts have been made to model particle deposition in the human lung and to characterize regional deposition based on a single breath. However, over successive breathing cycles, accurate estimates of regional particulate dosimetry for long-term PM exposures cannot be elucidated without considering transport and deposition of the retained particles especially those in 0.1– 1 μ m size range. We estimated the total and regional dosimetry of 0.01– 10 μ m particles after the lung attains a steady state during multiple breathing cycles by employing a semi-empirical model developed in previous work [Park, S. S., & Wexler, A. S. (2007a). Particle deposition in the pulmonary region of the human lung: A Semi-empirical model of single breath transport and deposition. Journal of Aerosol Science, 38(2), 228–245; Park, S. S., & Wexler, A. S. (2007b). Particle deposition in the pulmonary region of the human lung: Multiple breath aerosol transport and deposition. Journal of Aerosol Science, 38(5), 509–519]. To understand the influence of alveolar wall motion to deposition of different sized particles, local alveolar entering probabilities of 0.5 μ m particles suggested by Haber et al. [2003. Gravitational deposition in a rhythmically expanding and contracting alveolus. Journal of Applied Physiology, 95(2), 657–671] were used to estimate the enhanced gravitational deposition efficiencies of other particle sizes. Increased deposition fractions are predicted for 0.1– 10 μ m particles with Haber et al.'s gravitational efficiency during both single and multiple breathing cycles. There are only slight increases in the pulmonary deposition of ultrafine and 1– 10 μ m particles during multi-breaths compared to that on single breath because the vast majority deposit on the first breath leaving very few in retained air. As for fine particles (0.1– 1 μ m ), the multi-breath pulmonary deposition fraction is about two times higher than the results from single breath simulation due to the deposition of particles that were retained from the previous breath. Since fine-mode particles have the lowest deposition efficiency, considering combined effect of three major deposition mechanisms, the retained fractions at the end of exhalation are significantly higher (14–32%) than the ones during a single breath. Therefore, these retained fractions have to be considered for the estimation of particle dosimetry in humans to evaluate long-term PM exposure.