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Home > The ABC of air > Atmosphere

A layer of gases

The atmosphere is a layer of gases surrounding the Earth.

Air is mainly composed of :

  • nitrogen (78 %)
  • oxygen (21 %)
  • other gases often referred to as trace gases (argon, carbon dioxide, hydrogen, helium, krypton, xenon…)
  • and water vapor

The atmosphere is usually estimated with an altitude of 1 000 km.  But the first 10 km contain roughly 90 % of its air composition. Thus, the air we breathe is a very thin layer in comparison with Earth’s size (radius about 6 370 km). 


On compare souvent la Terre à une pomme et l'atmosphère à l'épaisseur de sa peau.

Different layers

Earth’s atmosphere is divided in different layers, each with specific characteristics such as temperature, composition or pressure…


LA LOI SUR L'AIR ("Air Law")  (article L220-2 of the Environmental Code)

Atmospheric pollution is defined as being the introduction by man, directly or indirectly, into the atmosphere and closed spaces,of substances having detrimental consequences likely : 

  • to put human health in danger
  • to damage biological resources and ecosystems
  • to influence climate change
  • to damage material goods
  • to result in odour nuisance