32 AIR POLLUTANT EFFECTS
- Tracheobronchial—bronchi down to terminal
bronchiole - Pulmonary—respiratory bronchiole, alveoli ducts,
and alveoli
The trachea divides into left and right bronchi, which divide
many times into smaller and smaller tubes down to the respira-
tory bronchioles. These feed about 65,000 lobules, each con-
taining approximately 5,000 thin-walled air sacs called alveoli.
Thus, in an adult there are approximately 300 million alveoli
whose thin walls, totaling 70 m^2 in area, contain hundreds of
miles of tiny capillaries. Oxygen is added to and carbon diox-
ide is removed from the blood through the walls of these capil-
laries. The transfer of toxic chemicals into the blood also can
takes place in the alveoli.
Starting in the nose, where the air is conditioned for
proper temperature and humidity, the direction of airflow
is changed many times, thereby causing the impaction and
deposition of particles on the surfaces of the branching air-
ways. These surfaces contain hairlike ciliary cells whose
rapid, wavelike motion, over 15 times per second, carry
impacted particles on a mucus layer upward into the trachea
for subsequent ingestion.
The velocity of the airflow decreases from about
150 cm/ sec at the start to almost zero in the alveoli; the
smaller the particles, the greater the ease with which they turn
corners, thus escaping impaction to penetrate to the alveoli,
where they are collected via sedimentation. The larger par-
ticles and soluble gases will be trapped in the upper airways,
where tissues and their defense mechanisms can be damaged
reversibly or irreversibly depending upon the nature, inten-
sity, and duration of the attack.
The amounts of a water-soluble gas or suspended particles
that reach the pulmonary region are strongly dependent upon
their inhalation pathway into the body. When inhaled through
the nose instead of the mouth, they experience a number of
chances of removal by impaction. In the case of sulfur dioxide
this process is greatly enhanced by its very rapid solution in
the watery fluids on the surface of nasal tissues. The greater
tendency for mouth breathing combined with the greater
intake of air that accompanies increased exertions contraindi-
cates strenuous activity wherever pollutant levels are high.
Particle removal by deposition along the upper and lower
respiratory system is strongly dependent upon particle size.
Particles with an aerodynamic diameter above 10 m are
removed in the convoluted, moist passages of the nose and
tracheobronchial region. While almost all those below 2 m
reach the pulmonary region, intermediate sizes tend to dis-
tribute themselves along both regions. When the particles
are insoluble, they are removed in a few days from the upper
respiratory system by mucociliary action; however, those
that penetrate down farther can remain for many months or
even years. Removal of particles also occurs by phagocytosis
through the scavenging action of macrophages.
The size distribution of particles suspended in the atmo-
sphere exhibits a log-normal behavior. The distribution by
mass tends to separate into a fine and a coarse group depend-
ing principally upon whether they are formed by condensationof very small precursors, such as those produced in combus-
tion, or are produced from larger particles by mechanical
breakdown processes.OZONEOzone is a very reactive chemical that readily attacks other
molecules, including those in the tissues of the respiratory
system. Exertions that increase the need for oxygen will
increase air intake and allow ozone molecules to penetrate
and damage the sensitive areas of the lungs. Ozone can
aggravate asthma attacks by making individuals more sensi-
tive to allergens that promote the attacks and more suscep-
tible to respiratory infections. Lung tissue can be scarred by
continued exposure to ozone over the years. Researchers at
Johns Hopkins found that an increase of 10 ppb in weekly
ozone levels in cities whose average level was 26 ppb was
associated with a 0.52% daily increase in deaths the follow-
ing week. They calculated that a 10-ppb reduction in daily
ozone levels could save nearly 4,000 lives throughout the 95
urban communities included in the study. Out of 296 metro-
politan areas, 36 have significant upward trends in the crite-
ria pollutants; however, of these, only trends involving ozone
had values over the level of air-quality standards.
The presence of ozone and other photochemical pol-
lutants depends upon atmospheric conditions, notably tem-
perature, as experience shows that this type of pollution is
associated with warm temperatures. The precursors that are
affected by elevated temperatures are volatile organic com-
pounds (VOCs) and nitric oxide. Natural sources for these
compounds are a less important factor than the emissions
produced by human activities, but the long-range transport
of the precursors while atmospheric conditions are convert-
ing them to photochemical oxidants means that that there
is a possibility of picking up precursor material from nat-
ural sources en route. Control of this type of air pollutant
is focused on controlling emissions of VOCs and nitrogen
oxides. It should be noted that ambient concentrations of
the criteria pollutant nitrogen dioxide have been found to be
generally below the levels considered to be health-damaging;
therefore, efforts to control its presence in the atmosphere is
driven by the need to control ozone. The combustion of fuels
and other materials provides sufficient energy to cause the
nitrogen and oxygen in the air to react to form nitric oxide.
The slow air oxidation of nitric oxide to nitrogen dioxide
results in a mixture described as nitrogen oxides (NOx).
The chemical reactions involved in the formation of pho-
tochemical oxidants from these precursors is complex. The
basic reactions are:NO 2 hv NO O
O O 2 M O 3 M*where hv represents a photon and M and M* represent material
before and after absorbing energy from the ozone-formation
reaction. In the absence of other molecules capable ofC001_003_r03.indd 32 11/22/2005 11:06:03 AM