Earth's atmosphere is a layer of gases surrounding the
planet Earth and retained by the Earth's gravity. It contains roughly 78% nitrogen, (normally inert except upon electrolysis by lightning and in certain biochemical processes of nitrogen fixation),
21.12% oxygen, 0.93% argon, 0.04% carbon dioxide, and trace amounts of other gases, in addition to about 3% water vapor. This mixture of gases is commonly known as air. The atmosphere protects life on
Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
There is no definite boundary between the atmosphere and outer space. It slowly becomes
thinner and fades away into space. Three quarters of the atmosphere's mass is within 11 km of the planetary surface. In the United States, persons who travel above an altitude of 80.5 km (50.0 miles)are
designated as astronauts. An altitude of 120 km (75 miles) marks the boundary where atmospheric effects become noticeable during re-entry. The Karman line, at 100 km (62 miles), is also frequently used
as the boundary between atmosphere and outer space. >>more...
Air pollution is a chemical, physical (e.g. particulate
matter), or biological agent that modifies the natural characteristics of the atmosphere. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health as well
as to the earth's ecosystems.
Worldwide air pollution is responsible for large numbers of deaths and cases of respiratory disease. Enforced air quality standards, like the Clean Air Act in the
United States, have reduced the presence of some pollutants. While major stationary sources are often identified with air pollution, the greatest source of emissions are actually mobile sources,
principally the automobile. Gases such as carbon dioxide have been shown to contribute to global warming and have recently been identified as pollutants.
The World Health Organization estimates that 4.6
million people die each year from causes directly attributable to air pollution. Many of these mortalities are attributable to indoor air pollution. Worldwide more deaths per year are linked to air
pollution than to automobile accidents. Research published in 2005 suggests that 310,000 Europeans die from air pollution annually. Direct causes of air pollution related deaths include aggravated
asthma, bronchitis, emphysema, lung and heart diseases, and respiratory allergies. The US EPA estimates that a proposed set of changes in diesel engine technology (Tier 2) could result in 12,000 fewer
premature mortalities, 15,000 fewer heart attacks, 6,000 fewer emergency room visits by children with asthma, and 8,900 fewer respiratory-related hospital admissions each year in the United States. >>more...