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Various industrial pollutants also may be present as gases or aerosols, such as chlorine (elemental or in compounds), fluorine compounds and elemental mercury vapor.

Water vapor accounts for roughly 0.25% of the atmosphere by mass.This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase.The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another.Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.The atmosphere has a mass of about 5.15 three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface.Filtered air includes trace amounts of many other chemical compounds.

Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, and volcanic ash.

The exosphere contains most of the satellites orbiting Earth.

The thermosphere is the second-highest layer of Earth's atmosphere.

In general, air pressure and density decrease with altitude in the atmosphere.

However, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the temperature section, below).

The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.