Aerosol deposition and clearance in the human respiratory tract
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The deposition and short term clearance characteristics of monodisperse aerosol particles (size range /|' 5 - 13 μm diameter) in the human respiratory tract have been investigated in eighteen subjects. In order to improve on the accuracy of existing experimental techniques, a new type of apparatus has been constructed which accurately samples the aerosol to be inhaled by a subject and at the same time ensures a rigorous control of the physiological conditions of aerosol administration. Besides measuring the total respiratory tract deposition of the particles, their regional distribution has also been estimated, by means of indirect techniques, the accuracy of which partly depends on certain widely held assumptions. The total aerosol deposition results are lower than those of other workers, with one exception, in a limited region of comparison. The regional aerosol deposition results demonstrate the importance of the laryngeal/pharyngeal region in filtering out the larger particle sizes before they can enter the trachea. While the results of the present work would also ostensibly suggest that the largest particle sizes penetrate far more effectively into the respiratory zone than had hitherto been considered possible by most investigators the rapid declines observed in some independently obtained laryngeal/ tracheal clearance curves are inconsistent with this finding. Moreove an analysis of the present data using a simple filter model of the respiratory tract demonstrates that the respiratory zone aerosol deposition results are wholly inconsistent with the expected behaviour of large airborne particles in this zone. It is therefore concluded that the measured values of one-day retention at the largest particle sizes are being caused by an incomplete clearance of particles which initially deposited on the dead space airways. An interesting secondary phenomenon has also been investigated. The clearance curves exhibit distinct, non-random fluctuations. By means of single and double radiation detectors placed over the throat, it has been established that the fluctuations are not merely an artefact of the primary measuring detector but have their origin from somewhere below the trachea, not solely in the larynx itself. The possible biological and physical factors that may be implicated in the causation of the clearance pulses are discussed.