Effect of microgravity and hypergravity on deposition of 0.5- to 3-mu m-diameter aerosol in the human lung

We measured intrapulmonary deposition of 0.5-, 1-, 2-, and 3-mu m-diameter particles in four subjects on the ground (mu G) and during parabolic flights both in microgravity (mu G) and at similar to 1.6 G. Subjects breathed aerosols at a constant flow rate (0.4 l/s) and tidal volume (0.75 liter). At 1 G and similar to 1.6 G, deposition increased with increasing particle size. In mu G, differences in deposition as a function of particle size were almost abolished. Deposition was a nearly linear function of the G level for 2- and 3-mu m-diameter particles, whereas for 0.5- and 1.0-mu m-diameter particles, deposition increased less between mu G and 1 G than between 1 G and similar to 1.6 G. Comparison with numerical predictions showed good agreement for 1-, 2-, and 3-mu m-diameter particles at 1 and similar to 1.6 G, whereas the model consistently underestimated deposition in mu G. The higher deposition observed in mu G compared with model predictions might be explained by a larger deposition by diffusion because of a higher alveolar concentration of aerosol in mu G and to the nonreversibility of the flow, causing additional mixing of the aerosols.