Air is a mixture of gases and aerosols that composes the atmosphere surrounding Earth. The primary gases of air include nitrogen (78%) and oxygen (21%). Trace gases and aerosols make up the remaining 1% of air. The trace gases include the noble gases argon, neon, helium, krypton and xenon; hydrogen; and the greenhouse gases. The aerosols are solid or liquid particles having diameters in the region of 0.001 to 10 microns (millionth of a metre), and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles, which may be natural or man-made.
Earth maintains an atmosphere through its gravitational pull. Consequently, most air is found in the lowest 10 kilometres of the atmosphere. Experienced mountain climbers are aware of how thin the air becomes, and may carry oxygen tanks to assist breathing at high altitudes. Within the lower atmosphere, however, air remains remarkably uniform in composition, as a result of efficient recycling processes and turbulent mixing in the atmosphere.