Composition of Air

Component	Symbol	Volume
Nitrogen	N2	78.084%	99.998%
Oxygen	O2	20.947%
Argon	Ar	0.934%
Carbon Dioxide	CO2	0.033%
Neon	Ne	18.2 parts per million
Helium	He	5.2 parts per million
Krypton	Kr	1.1 parts per million
Sulfur dioxide	SO2	1.0 parts per million
Methane	CH4	2.0 parts per million
Hydrogen	H2	0.5 parts per million
Nitrous Oxide	N2O	0.5 parts per million
Xenon	Xe	0.09 parts per million
Ozone	O3	0.07 parts per million
Nitrogen dioxide	NO2	0.02 parts per million
Iodine	I2	0.01 parts per million
Carbon monoxide	CO	trace
Ammonia	NH3	trace

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Human Respiration

The air that leaves a person's lungs during exhalation contains 14% oxygen and 4.4% carbon dioxide.

Atmospheres with oxygen concentrations below 19.5 percent can have adverse physiological effects, and atmospheres with less than 16 percent oxygen can become life threatening.