Page 134 - Fundamentals of Air Pollution

104 7. Effects on Human Health and Welfare

Fig. 7-1. Anatomy of the human respiratory system.

bifurcation of the trachea to form many branching pathways of increasingly
smaller diameter by which air moves to the pulmonary region. The trachea
branches into the right and left bronchi. Each bronchus divides and subdi-
vides at least 20 times; the smallest units, bronchioles, are located deep in
the lungs. The bronchioles end in about 3 million air sacs, the alveoli.
The behavior of particles and gases in the respiratory system is greatly
influenced by the region of the lung in which they are located (8). Air
passes through the upper region and is humidified and brought to body
temperature by gaining or losing heat. After the air is channeled through
the trachea to the first bronchi, the flow is divided at each subsequent
bronchial bifurcation until very little apparent flow is occurring within the
alveolar sacs. Mass transfer is controlled by molecular diffusion in this final
region. Because of Jthe very different flows in the various sections of the
respiratory region, particles suspended in air and gaseous air pollutants
are treated differently in the lung.

A. Particle and Gas Behavior in the Lung
Particle behavior in the lung is dependent on the aerodynamic characteris-
tics of particles in flow streams. In contrast, the major factor for gases is