A Theoretical Approach to the Deposition of Cancer-Inducing Asbestos Fibers in the Human Respiratory Tract

ABSTRACT

In the study presented here a stochastic model predicting the deposition of variably shaped asbestos fibers in the human respiratory tract is introduced. Deposition calculations are commonly based on the concept of the aerodynamic diameter. Besides Brownian diffusion, inertial impaction, and sedimentation, also interception representing the capture of particles at the carinal ridges of single airway bifurcations is considered as main deposition mechanism for the computation of regional and local deposition fractions. Concerning total deposition in the human respiratory tract, fibers with a cylindrical diameter, termed d_p, smaller than 0.1 µm exhibit lower deposition fractions than comparable spheres, whilst fibers with d_p greater than 0.1 µm show higher deposition fractions than spheres. The fiber aspect ratio β has only an insignificant influence on total deposition, i.e. total deposition fractions of fibers with β = 10 and β = 100 differ by 2 to 10 %. Regarding regional deposition, the fiber diameter represents a controlling factor insofar, as fibrous particles with d_p = 0.1 µm are preferably deposited in the bronchioles and alveoli, whereas fibers with d_p = 10 µm are exclusively accumulated in the extrathoracic region. Only deposition behavior of fibers with d_p = 1 µm is more complex, since valuable particle fractions deposit in all compartments of the lungs. Local (i.e. generation-by-generation) deposition of fibrous particles is characterized by a deposition peak at airway generation 19 in the case of fibers with d_p = 0.1 µm. The deposition maximum is subject to a continuous dislocation towards more proximal airway generations with increasing d_p. Therefore, particles with d_p = 10 μm are chiefly deposited in the first three bronchial airway generations. Differences of fiber deposition between sitting and light-work breathing conditions may be evaluated is insignificant in most cases. Only fibrous particles with d_p = 1 μm significantly change their deposition behavior with increasing inhalative flow rate in the way that proximal deposition is remarkably enhanced at the cost of bronchiolar and alveolar deposition. In general, any increase of the inhalative flow rate Q causes a successive dislocation of fiber deposition from distal to proximal compartments of the human respiratory tract. The results obtained from the theoretical approach lead to the conclusion that thin fibers with variable length tend to deposit in the pulmonary region of the lung, where they represent a remarkable hazard for mesothelioma. Thick fibers are preferentially accumulated in the proximal bronchi and therefore may induce bronchial lung cancer (adenocarcinoma).

Keywords:

Asbestos, lung deposition, Monte Carlo model, impaction, interception, particle trajectory, stochastic lung model, statistical weight method, aspect ratio, sitting breathing, light-work breathing, lung cancer.