Effect of Settling Velocity on Particle Recovery from Human Conducting Airways after Breath Holding

Particle recovery was measured after bolus inhalation of aerosols into shallow lung depths. Measurements were carried out with aerosol particles of different sizes (aerodynamic diameters between 0.8 and 4 μm). The assumption that during periods of breath holding these particles were deposited in the airways by gravitational settling only was disproved. Cardiogenic mixing as previously discussed [Scheuch and Stahlhofen, Exp. Lung Res. 17, 763–787 (1991)] was also found to be involved. Larger particles are less subject to cardiogenic mixing because they have higher settling velocities than smaller ones.

Measurements of effective airway diameters, which are based on the assumption of gravitational settling during breath holding, therefore depend on particle size. In volumetric lung depths of less than about 40 cm³ no significant effect of particle size on recovery could be measured.