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