Page 135 - Fundamentals of Air Pollution

III. The Human Respiratory System 105

the solubility of the gaseous molecules in the linings of the different regions
of the respiratory system. The aerodynamic properties of particles are re-
lated to their size, shape, and density. The behavior of a chain type or fiber
may also be dependent on its orientation to the direction of flow. The
deposition of particles in different regions of the respiratory system depends
on their size. The nasal openings permit very large dust particles to enter
the nasal region, along with much finer airborne particulate matter. Particles
in the atmosphere can range from less than 0.01 /urn to more than 50 fim
in diameter.
The relationship between the aerodynamic size of particles and the re-
gions where they are deposited is shown in Fig. 7-2 (9). Larger particles
are deposited in the nasal region by impaction on the hairs of the nose or
at the bends of the nasal passages. Smaller particles pass through the nasal
region and are deposited in the tracheobronchial and pulmonary regions.
Particles are removed by impacts with the walls of the bronchi when they
are unable to follow the gaseous streamline flow through subsequent bifur-
cations of the bronchial tree. As the airflow decreases near the terminal
bronchi, the smallest particles are removed by Brownian motion, which
pushes them to the alveolar membrane.

B, Removal of Deposited Particles from the Respiratory System
The respiratory system has several mechanisms for removing deposited
particles (8). The walls of the nasal and tracheobronchial regions are coated
with a mucous fluid. Nose blowing, sneezing, coughing, and swallowing

Fig. 7-2. Particle deposition as a function of particle diameter in various regions of the
lung. The nasopharyngeal region consists of the nose and throat; the tracheobronchial region
consists of the windpipe and large airways; and the pulmonary region consists of the small
bronchi and the alveolar sacs. Source: Task Group on Lung Dynamics, Health Phys. 12, 173
(1966).