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Earth's atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and 0.1 percent other gases.
Animals live and need to breathe the oxygen in the atmosphere. In [[breathing]], the [[lung]]s put oxygen into the [[blood]], and send back [[carbon dioxide]] to the air. Plants need the carbon dioxide in the air to live. They give off the oxygen that we breathe. Without it
Air can be [[air pollution|polluted]] by some gases (such as [[carbon monoxide]], hydrocarbons, and nitrogen oxides), [[smoke]], and ash. This [[air pollution]] causes various problems including [[smog]], [[acid rain]] and [[global warming]]. It can damage people's [[health]] and the environment. There are debates about whether or not to act upon climate change, but soon enough the Earth will heat up
Since early times, air has been used to create [[technology]]. Ships moved with sails and windmills used the mechanical motion of air. Aircraft use [[propeller]]s to move air over a [[wing]], which allows them to [[flight|fly]]. [[Pneumatics]] use [[air pressure]] to move things. Since the late 1900s, air power is also used to generate electricity.
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Air is invisible: it cannot be seen by the eye, though a shimmering in hot air can be seen.<ref>{{cite web | url=https://www.sciencefocus.com/planet-earth/why-is-air-invisible/ | title=Why is air invisible? }}</ref>
Air is one of the 4 classical elements
== Main history ==
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At first it was mainly a [[hydrogen]] atmosphere. It has changed dramatically on several occasions—for example, the [[Great Oxygenation Event]] 2.4 [[1,000,000,000|billion]] years ago, greatly increased [[oxygen]] in the atmosphere from practically no oxygen to levels closer to present day. Humans have also contributed to significant changes in atmospheric composition through air pollution, especially since [[industrialisation]], leading to rapid environmental change such as ozone depletion and global warming.
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