During investigation of the relationship of aerosol persistence in the human respiratory tract to the dimensions of the air spaces, procedures were developed in which the subject inhaled a fixed volume of aerosol and held his breath at known lung volumes for a series of times prior to exhalation. With appropriate correcting, the persistence of aerosol could be measured as a function of time of breath holding. The distance that particles must be moved during breath holding by sedimentation or diffusion to reach a wall of the respiratory tract is determined by the size of the air spaces; the time required to reach the wall also depends on this distance. The persistence of aerosol, therefore, provides an index of the size of the air spaces containing aerosol during breath holding.
Palmes ED. Measurement of Pulmonary Air Spaces Using Aerosols. Arch Intern Med. 1973;131(1):76–79. doi:10.1001/archinte.1973.00320070072007
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