Intrathoracic deposition

Experimental data, from several investigators who measured intrathoracic particle deposition during mouth breathing, were summarized by Stahlhofen et al. (184). Figure 12 shows these data as a function of the aerodynamic particle diameter. Because large particles are effectively collected by the extrathoracic airways, intrathoracic deposition does not increase or decrease monotonically with particle diameter, but exhibits maximal values for particles with diameters between 4 and 8 pm. It appears to be lowest for particles smaller than 1 pm. 

Inhaled particles deposited in the intrathoracic airways are eleared by several mechanisms, resulting in two major clearance pathways partiele transport and absorption of dissolved material. Particle transport is directed either toward the larynx, from which they are swallowed into the gastrointestinal tract (GIT) or toward the lymphatic system. Material cleared by absorption is eliminated mainly by the blood. 

Measurements of mucociliary clearance in lungs of healthy human subjects were performed by inhaling radiolabeled aerosol particles. Retention of these particles was measured either with a gamma-camera or other gamma-ray detectors from outside the lungs. The fast-cleared fraction of the retention function was used as an indicator of mucociliary clearance. The half-time of the mucociliary clearance in healthy humans was 4(fast) = 2-4 hr, and this fast phase of clearance was flnished after about 6-12 hr. Thus, many authors have used 24-hr clearance as a measure of airway deposition or deposition in the tracheobronchial tree. However, this is true only if all particles are cleared from intrathoracic airways by mucociliary clearance within 24 hr. If particles are retained in airways after 24 hr, they carmot be distinguished from those deposited in the peripheral lungs from where particle transport is well known to be slow (see Sec. III). 

---