Page 136 - Fundamentals of Air Pollution

106 7. Effects on Human Health and Welfare

help remove particles from the upper airways. The tracheobronchial walls
have fiber cilia which sweep the mucous fluid upward, transporting parti-
cles to the top of the trachea, where they are swallowed. This mechanism
is often referred to as the mucociliary escalator. In the pulmonary region of
the respiratory system, foreign particles can move across the epithelial
lining of the alveolar sac to the lymph or blood systems, or they may be
engulfed by scavenger cells called alveolar macrophages. The macrophages
can move to the mucociliary escalator for removal.
For gases, solubility controls removal from the airstream. Highly soluble
gases such as SO 2 are absorbed in the upper airways, whereas less soluble
gases such as NO 2 and O 3 may penetrate to the pulmonary region. Irritant
gases are thought to stimulate neuroreceptors in the respiratory walls and
cause a variety of responses, including sneezing, coughing, bronchocon-
striction, and rapid, shallow breathing. The dissolved gas may be eliminated
by biochemical processes or may diffuse to the circulatory system.

IV. IMPACT OF AIR POLLUTION ON HUMANS

The impact of air pollution on human beings has been the major force
motivating efforts to control it. Most persons do not have the luxury of
choosing the air they breathe. Working adults can make some choices in
the selection of their occupation and the place where they live and work,
but children and the nonworking elderly often cannot. The receptor popula-
tion in an urban location includes a wide spectrum of demographic traits
with respect to age, sex, and health status. Within this group, certain
sensitive subpopulations have been identified: (1) very young children,
whose respiratory and circulatory systems are still undergoing maturation;
(2) the elderly, whose respiratory and circulatory systems function poorly;
and (3) persons who have preexisting diseases such as asthma, emphysema,
and heart disease. These subpopulations have been found to exhibit more
adverse responses from exposure to air pollutants than the general popula-
tion (10).
Air pollution principally affects the respiratory, circulatory, and olfactory
systems. The respiratory system is the principal route of entry for air pollut-
ants, some of which may alter the function of the lungs.
Health effects data come from three types of studies: clinical, epidemio-
logical, and toxicological. Clinical and epidemiological studies focus on
human subjects, whereas toxicological studies are conducted on animals
or simpler cellular systems. Ethical considerations limit human exposure
to low levels of air pollutants which do not have irreversible effects. Table
7-1 lists the advantages and disadvantages of each type of experimental
information.