The Size of Fibrous Aerosols

Abstract

The deposition of fibrous aerosols in the human respiratory tract is an important subject in view of the health effects associated with the inhalation of the fibers. The estimation of the regional deposition of compact aerosols uses aerodynamic equivalent diameter as the size parameter. In the case of fibers, the four deposition mechanisms, impaction, interception, sedimentation, and diffusion, are not unique functions of one size parameter, in that each deposition mechanism involves the aspect ratio of the fibers.

In this article, a simple, single relationship between the fiber diameter and aspect ratio is sought for the characterization of fibers in terms of deposition in the respiratory tract. The comparisons of deposition predictions are made by using the equations that describe deposition compact particles in the extra-thoracic, tracheobronchial, and pulmonary regions of the human respiratory tract in conjunction with different fiber size definitions including equivalent aerodynamic, impaction, and volume diameters. The results obtained from these models were compared to Harris' model for the deposition of fibers. The results suggested significant differences in the deposition patterns at each region with each model. The performance of each of these models was also investigated with two pairs of synthetically generated diameter and aspect ratio distributions. These results were not encouraging. The comparisons of predicted deposition with Lippmann's health-related criteria suggested that none of the available models are useful paradigms for health-related criteria. Thus it is concluded that it is not possible to have a simple, one-parameter descriptor for fibrous aerosols with what is hitherto theoretically available.