Air Pollution
Air Pollution is addition of harmful
substances to the atmosphere resulting in
damage to the environment, human
health, and quality of life. One of many forms
of pollution, air pollution
occurs inside homes, schools, and offices; in
cities; across continents; and
even globally. Air pollution makes people sick,
it causes breathing problems
and promotes cancer, and it harms plants, animals,
and the ecosystems in
which they live. Some air pollutants return to earth in
the form of acid rain
and snow, which corrode statues and buildings, damage
crops and forests, and
make lakes and streams unsuitable for fish and other
plant and animal life.
Pollution is changing the earth's atmosphere so that it
lets in more harmful
radiation from the sun. At the same time, our polluted
atmosphere is becoming
a better insulator, preventing heat from escaping back
into space and leading
to a rise in global average temperatures. Scientists
predict that the
temperature increase, referred to as global warming, will
affect world food
supply, alter sea level, make weather more extreme, and
increase the spread
of tropical disease. Most air pollution comes from one human
activity:
burning fossil fuels, natural gas, coal, and oil to power
industrial
processes and motor vehicles. Among the harmful chemical compounds
this burning
puts into the atmosphere are carbon dioxide, carbon monoxide,
nitrogen oxides,
sulfur dioxide, and tiny solid particles including lead from
gasoline additives
called particulates. Between 1900 and 1970, motor vehicle
use rapidly expanded,
and emissions of nitrogen oxides, some of the most
damaging pollutants in
vehicle exhaust, increased 690 percent. When fuels are
incompletely burned,
various chemicals called volatile organic chemicals also
enter the air.
Pollutants also come from other sources. For instance,
decomposing garbage in
landfills and solid waste disposal sites emits methane
gas, and many household
products give off Volatile organic chemicals. Some of
these pollutants also come
from natural sources. For example, forest fires
emit particulates and Volatile
organic chemicals into the atmosphere.
Ultrafine dust particles, dislodged by
soil erosion when water and weather
loosen layers of soil, increase airborne
particulate levels. Volcanoes spew
out sulfur dioxide and large amounts of
pulverized lava rock known as
volcanic ash. A big volcanic eruption can darken
the sky over a wide region
and affect the earth's entire atmosphere. The 1991
eruption of Mount
Pinatoubo in the Philippines, for example, dumped enough
volcanic ash into
the upper atmosphere to lower global temperatures for the next
two years.
Unlike pollutants from human activity, however, naturally
occurring
pollutants tend to remain in the atmosphere for a short time and do
not lead to
permanent atmospheric change. Once in the atmosphere, pollutants
often undergo
chemical reactions that produce additional harmful compounds.
Air pollution is
subject to weather patterns that can trap it in valleys or
blow it across the
globe to damage pristine environments far from the
original sources. Local and
regional pollution take place in the lowest layer
of the atmosphere, the
troposphere, which extends from the earth's surface to
about ten miles . The
troposphere is the region in which most weather occurs.
If the load of
pollutants added to the troposphere were equally distributed,
the pollutants
would be spread over vast areas and the air pollution might
almost escape our
notice. Pollution sources tend to be concentrated, however,
especially in
cities. In the weather phenomenon known as thermal inversion, a
layer of cooler
air is trapped near the ground by a layer of warmer air
above. When this occurs,
normal air mixing almost ceases and pollutants are
trapped in the lower layer.
Local topography, or the shape of the land,
can worsen this effect, an area
ringed by mountains, for example, can become
a pollution trap. Smog is intense
local pollution usually trapped by a
thermal inversion. Before the age of the
automobile, most smog came from
burning coal and was so severe that in
19th-century London, street lights
were turned on by noon because soot and smog
darkened the midday sky. Burning
gasoline in motor vehicles is the main source
of smog in most regions today.
Powered by sunlight, oxides of nitrogen and
volatile organic compounds react
in the atmosphere to produce photochemical
smog. Smog contains ozone, a form
of oxygen gas made up of molecules with three
oxygen atoms rather than the
normal two. Ozone in the lower atmosphere is a
poison; it damages vegetation,
kills trees, irritates lung tissues, and attacks
rubber. Environmental
officials measure ozone to determine the severity of smog.
When the ozone
level is high, other pollutants, including carbon monoxide, are
usually
present at high levels as well. In the presence of atmospheric
moisture,
sulfur dioxide and oxides of nitrogen turn into droplets of pure
acid floating
in smog. These airborne acids are bad for the lungs and attack
anything made of
limestone, marble, or metal. In cities around the world,
smog acids are eroding
precious artifacts, including the Parthenon temple in
Athens, Greece, and the
Taj Mahal in Agra, India. Oxides of nitrogen and
sulfur dioxide pollute places
far from the points where they are released
into the air. Carried by winds in
the troposphere, they can reach distant
regions where they descend in acid form,
usually as rain or snow. Such acid
precipitation can burn the leaves of plants
and make lakes too acidic to
support fish and other living things. Because of
acidification, sensitive
species such as the popular brook trout can no longer
survive in many lakes
and streams in the eastern United States. Smog spoils
views and makes outdoor
activity unpleasant. For the very young, the very old,
and people who suffer
from asthma or heart disease, the effects of smog are even
worse: It may
cause headaches or dizziness and can cause breathing difficulties.
In
extreme cases, smog can lead to mass illness and death, mainly from
carbon
monoxide poisoning. In 1948 in the steel-mill town of Donora,
Pennsylvania,
intense local smog killed nineteen people. In 1952 in London
over 3000 people
died in one of the notorious smog events known as London
Fogs; in 1962 another
700 Londoners died. With stronger pollution
controls and less reliance on coal
for heat, today's chronic smog is rarely
so obviously deadly. However, under
adverse weather conditions, accidental
releases of toxic substances can be
equally disastrous. The worst such
accident occurred in 1984 in Bhopal, India,
when methyl isocyanate released
from an American-owned factory during a thermal
inversion caused at least
3300 deaths. Air pollution can expand beyond a
regional area to cause global
effects. The stratosphere is the layer of the
atmosphere between ten miles
and thirty miles above sea level. It is rich in
ozone, the same molecule that
acts as a pollutant when found at lower levels of
the atmosphere in urban
smog. Up at the stratospheric level, however, ozone
forms a protective layer
that serves a vital function: it absorbs the wavelength
of solar radiation
known as ultraviolet-B (UV-B). UV-B damages deoxyribonucleic
acid (DNA), the
genetic molecule found in every living cell, increasing the risk
of such
problems as cancer in humans. Because of its protective function, the
ozone
layer is essential to life on earth. Several pollutants attack the
ozone
layer. Chief among them is the class of chemicals known as
chlorofluorocarbons
(CFCs), used as refrigerants (notably in air
conditioners), as agents in several
manufacturing processes, and formerly as
propellants in spray cans. CFC
molecules are virtually indestructible until
they reach the stratosphere. Here,
intense ultraviolet radiation breaks the
CFC molecules apart, releasing the
chlorine atoms they contain. These
chlorine atoms begin reacting with ozone,
breaking it down into ordinary
oxygen molecules that do not absorb UV-B. The
chlorine acts as a catalyst
that is, it takes part in several chemical
reactions, yet at the end emerges
unchanged and able to react again. A single
chlorine atom can destroy up to
100,000 ozone molecules in the stratosphere.
Other pollutants, including
nitrous oxide from fertilizers and the pesticide
methyl bromide, also attack
atmospheric ozone. Scientists are finding that under
this assault the
protective ozone layer in the stratosphere is thinning. In the
Antarctic
region, it vanishes almost entirely for a few weeks every year.
Although
CFC use has been greatly reduced in recent years, CFC molecules
already
released into the lower atmosphere will be making their way to the
stratosphere
for decades, and further ozone loss is expected. As a result,
experts anticipate
an increase in skin cancers, more cataracts (clouding of
the lens of the eye),
and reduced yields of some food crops. Humans are
bringing about another
global-scale change in the atmosphere: the increase in
what are called
greenhouse gases. Like glass in a greenhouse, these gases
admit the sun's light
but tend to reflect back downward the heat that is
radiated from the ground
below, trapping heat in the earth's atmosphere. This
process is known as the
greenhouse effect. Carbon dioxide is the most
significant of these gases; there
is 25 percent more carbon dioxide in the
atmosphere today than there was a
century ago, the result of our burning coal
and fuels derived from oil. Methane,
nitrous oxide, and CFCs are greenhouse
gases as well. Scientists predict that
increases in these gases in the
atmosphere will make the earth a warmer place.
They expect a global rise
in average temperature somewhere between 1.0º and 3.5º
C (1.8º and 6.3º
F) in the next century. Average temperatures have in fact
been rising, and
the years from 1987 to 1997 were the warmest ten years on
record. Most
scientists are reluctant to say that global warming has actually
begun
because climate naturally varies from year to year and decade to decade,
and
it takes many years of records to be sure of a fundamental change. There
is
little disagreement, though, that global warming is on its way. Global
warming
will have different effects in different regions. A warmed world is
expected to
have more extreme weather, with more rain during wet periods,
longer droughts,
and more powerful storms. Although the effects of future
climate change are
unknown, some predict that exaggerated weather conditions
may translate into
better agricultural yields in areas such as the western
United States, where
temperature and rainfall are expected to increase, while
dramatic decreases in
rainfall may lead to severe drought and plunging
agricultural yields in parts of
Africa, for example. Warmer temperatures
are expected to partially melt the
polar ice caps, leading to a projected sea
level rise of twenty inches by the
year 2050. A sea level rise of this
magnitude would flood coastal cities, force
people to abandon low-lying
islands, and completely inundate coastal wetlands.
If sea levels rise at
projected rates, the Florida Everglades will be completely
under water in
less than 50 years. Diseases like malaria, which at present are
primarily
found in the tropics, may become more common in the regions of the
globe
between the tropics and the polar regions, called the temperate zones.
For
many of the world's plant species, and for animal species that are not
easily
able to shift their territories as their habitat grows warmer, climate
change
may bring extinction. Pollution is perhaps most harmful at an often
unrecognized
site, inside the homes and buildings where we spend most of our
time. Indoor
pollutants include tobacco smoke; radon, an invisible
radioactive gas that
enters homes from the ground in some regions; and
chemicals released from
synthetic carpets and furniture, pesticides, and
household cleaners. When
disturbed, asbestos, a nonflammable material once
commonly used in insulation,
sheds airborne fibers that can produce a lung
disease called asbestosis.
Pollutants may accumulate to reach much higher
levels than they do outside,
where natural air currents disperse them. Indoor
air levels of many pollutants
may be 2 to 5 times, and occasionally more than
100 times, higher than outdoor
levels. These levels of indoor air pollutants
are especially harmful because
people spend as much as 90 percent of their
time living, working, and playing
indoors. Inefficient or improperly vented
heaters are particularly dangerous. In
the United States, the serious effort
against local and regional air pollution
began with the Clean Air Act of
1970, which was amended in 1977 and 1990. This
law requires that the air
contain no more than specified levels of particulate
matter, lead, carbon
monoxide, sulfur dioxide, nitrogen oxides, volatile organic
compounds, ozone,
and various toxic substances. To avoid the mere shifting of
pollution from
dirty areas to clean ones, stricter standards apply where the air
is
comparatively clean. In national parks, for instance, the air is supposed
to
remain as clean as it was when the law was passed. The act sets deadlines
by
which standards must be met. The Environmental Protection Agency (EPA) is
in
charge of refining and enforcing these standards, but the day-to-day work
of
fighting pollution falls to the state governments and to local air
pollution
control districts. Some states, notably California, have imposed
tougher air
pollution standards of their own. In an effort to enforce
pollution standards,
pollution control authorities measure both the amounts
of pollutants present in
the atmosphere and the amounts entering it from
certain sources. The usual
approach is to sample the open, or ambient, air
and test it for the presence of
specified pollutants. The amount of each
pollutant is counted in parts per
million or, in some cases, milligrams or
micrograms per cubic meter. To learn
how much pollution is coming from
specific sources, measurements are also taken
at industrial smokestacks and
automobile tailpipes. Pollution is controlled in
two ways: with
end-of-the-pipe devices that capture pollutants already created,
and by
limiting the quantity of pollutants produced in the first
place.
End-of-the-pipe devices include catalytic converters in
automobiles and various
kinds of filters and scrubbers in industrial plants.
In a catalytic converter,
exhaust gases pass over small beads coated with
metals that promote reactions
changing harmful substances into less harmful
ones. When end-of-the-pipe devices
first began to be used, they dramatically
reduced pollution at a relatively low
cost. As air pollution standards become
stricter, it becomes more and more
expensive to further clean the air. In
order to lower pollution overall,
industrial polluters are sometimes allowed
to make cooperative deals. For
instance, a power company may fulfill its
pollution control requirements by
investing in pollution control at another
plant or factory, where more effective
pollution control can be accomplished
at a lower cost. End-of-the-pipe controls,
however sophisticated, can only do
so much. As pollution efforts evolve, keeping
the air clean will depend much
more on preventing pollution than on curing it.
Gasoline, for instance,
has been reformulated several times to achieve cleaner
burning. Various
manufacturing processes have been redesigned so that less waste
is produced.
Car manufacturers are experimenting with automobiles that run on
electricity
or on cleaner-burning fuels. Buildings are being designed to take
advantage
of sun in winter and shade and breezes in summer to reduce the need
for
artificial heating and cooling, which are usually powered by the burning
of
fossil fuels. The choices people make in their daily lives can have
a
significant impact on the state of the air. Using public transportation
instead
of driving, for instance, reduces pollution by limiting the number
of
pollution-emitting automobiles on the road. During periods of
particularly
intense smog, pollution control authorities often urge people to
avoid trips by
car. To encourage transit use during bad-air periods,
authorities in Paris,
France, make bus and subway travel temporarily
free. Indoor pollution control
must be accomplished building by building or
even room by room. Proper
ventilation mimics natural outdoor air currents,
reducing levels of indoor air
pollutants by continually circulating fresh
air. After improving ventilation,
the most effective single step is probably
banning smoking in public rooms.
Where asbestos has been used in
insulation, it can be removed or sealed behind
sheathes so that it won't be
shredded and get into the air. Sealing foundations
and installing special
pipes and pumps can prevent radon from seeping into
buildings. On the global
scale, pollution control standards are the result of
complex negotiations
among nations. Typically, developed countries, having
already gone through a
period of rapid and dirty industrialization, are ready to
demand cleaner
technologies. Less developed nations, hoping for rapid economic
growth, are
less enthusiastic about pollution controls. They seek lenient
deadlines and
financial help from developed countries to make the expensive
changes
necessary to reduce pollutant emissions in their industrial
processes.
Nonetheless, several important international accords have been
reached. In 1988,
the United States and 24 other nations agreed in the
Long-Range Transboundary
Air Pollution Agreement to hold their production
of nitrogen oxides, a key
contributor to acid rain, to current levels. In the
Montreal Protocol, adopted
in 1987 and strengthened in 1990 and 1992, most
nations agreed to stop or reduce
the manufacture of CFCs. In 1992 the United
Nations Framework Convention on
Climate Change negotiated a treaty
outlining cooperative efforts to curb global
warming. The treaty, which took
effect in March 1994, has been legally accepted
by 160 of the 165
participating countries. In December 1997 at the Third
Conference of the
United Nations Framework Convention on Climate Change in
Japan, more than
160 nations formally adopted the Kyoto Protocol. This agreement
calls for
industrialized nations to reduce their emissions of greenhouse gases
to
levels 5 percent below 1990 emission levels between 2008 and 2012. The
United
States, which releases more greenhouse gases than any other
nation, has
traditionally been slow to support such strong measures. The U.S.
Senate may be
reluctant to ratify the Kyoto Protocol because it does not
require developing
countries, such as China and India, to meet similar
emissions goals. All these
antipollution measures have helped stem the
increase of global pollution
emission levels. Between 1970, when the Clean
Air Act was passed, and 1995,
total emissions of the major air pollutants in
the United States decreased by
nearly 30 percent. During the same 25-year
period, the U.S. population increased
28 percent and vehicle miles
traveled increased 116 percent. Air pollution
control is a race between the
reduction of pollution from each source, such as a
factory or a car, and the
rapid multiplication of sources. Smog in American
cities is expected to
increase again as the number of cars and miles driven
continues to rise.
Meanwhile, developing countries are building up their own
industries, and
their citizens are buying cars as soon as they can afford them.
Ominous
changes continue in the global atmosphere. New efforts to control
air
pollution will be necessary as long as these trends continue.