What Gases Make Up Air?

Concentrations of various gases in air  

Gases that make up outdoor air: this article provides a table of the components in outdoor air - a list of the various gases found in air by percentage.
The table gives the percent of each gas found in typical outdoor air, includes citations of authoritative sources, and discusses variations in the data. These gas percentages are a useful reference for comparison when making both outdoor and indoor site measurements and studies of air quality and air contaminants.

What are Typical Concentrations of all of the Gases in Air

Outdoor air is commonly used as a baseline for comparison with indoor air quality. Provided that the outdoor air location sampled is not close to a particular source of contamination such as particulates or chemicals from a nearby trash burning or industrial facility, or such as gases from an adjacent garage or highway this is a reasonable approach. The table below provides the components of indoor air in typical percentages of make-up.

Component Gases of Outdoor Air
Gas In order of percentage of outdoor air	Outdoor Air Percent	Symbol, Comments
Nitrogen	78.084%	N2, one of four chief components of air
Oxygen	20.947%	O2, second of four chief components OXYGEN - O2
Argon	0.934%	Ar, third of four chief components, an inert gas
Carbon dioxide	0.33%	CO2, fourth of chief components of outdoor air, together these four 99.998% of air We often compare indoor CO2 with outdoor CO2 as a measure of the level of fresh air provided to the indoor environment. A healthy human breathing outdoor air exhales a mixture with about 6% less oxygen and much higher CO2, in a breath mixture of about 14% O2 and 4.4% CO2 but as exhaled breath is mixing with a much greater volume of indoor air, the normal indoor CO2 will not normally approach that 4% level.
Trace Gases detectable in outdoor air
Neon	18.2 ppm (parts per million)	Ne
Helium	5.2 ppm	He
Methane	2.0 ppm	CH4, See METHANE GAS HAZARDS
Krypton	1.1 ppm	Kr
Sulfur dioxide	1.0 ppm	SO2,,
Hydrogen	0.5 ppm	H2
Nitrous Oxide	0.5 ppm	N2O
Xenon	0.09 ppm	Xe
Ozone	0.07 ppm	O3,
Nitrogen dioxide	0.02 ppm	NO2
Iodine gas	0.01 ppm	I2
Ammonia	very low trace levels	NH3
Carbon monoxide	very low trace levels	CO, Typical IAQ screening should not detect in indoor air.
Notes: 1. Moeckel, W.E.; Weston, K.C., "Composition and Thermodynamic Properties of Air in Chemical Equilibrium", Technical Report NACA-TN-4265, Lewis Flight Propulsion Lab., Cleveland, 1 April 1958, updated 25 February 2008, Website citation: www.osti.gov, U.S. DOE 2. Bender, M. L., T. Sowers, J.‐M. Barnola, and J. Chappellaz (1994), Changes in the O2/N2 ratio of the atmosphere during recent decades reflected in the composition of air in the firn at Vostok Station, Antarctica, Geophys. Res. Lett., 21(3), 189–192, doi:10.1029/93GL03548. 3. A.P. Jones, "Indoor Air Quality and Health", School of Environmental Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, UK, Atmospheric Environment Volume 33, Issue 28, December 1999, Pages 4535–4564