The mixture of gases that surrounds the Earth. The composition of the earth's atmosphere varies with altitude, but at sea level contains approximately 78% nitrogen and 21% oxygen by volume. An oxygen-deficient atmosphere contains less than 19.5% oxygen by volume. An oxygen-enriched atmosphere contains more than 23.5% oxygen by volume. An atmosphere that is immediately dangerous to life or health (IDLH) poses an immediate threat to life, would cause, irreversible adverse health effects, or would impair an individual's ability to escape from a dangerous atmosphere. The amount of watervapor in air tends to vary depending on the temperature and atmospheric pressure. Dry air at standard atmospheric pressure has the following composition: Gas: Nitrogen N2, Percent by Volume %V: 78.03; Gas: Oxygen O2, %V: 20.99; Gas: Argon Ar, %V: 0.94; Gas: Carbon Dioxide CO2, %V: 0.033; Gas: Neon Ne, %V: 0.0015; Gas: Helium He, %V: 0.000524; Gas: Krypton Kr, %V: 0.00014; Gas: Xenon Xe, %V: 0.000006; Standard atmospheric pressure is defined as one atmosphere = 14.7 pounds per square inch?(psi) = 760?torr. A list of pressure units and an on-line conversion calculator is available on this site. Cylinders of compressed air are often used in industrial settings and laboratories. Be certain you are familiar with cylinder and compressed gas safety, including the proper use of gas regulators. Oxygen-enriched atmospheres present exceptionally dangerous fire hazards (See:link below). Oxygen-deficient atmospheres may result in unconsciousness, coma or death (See:link below). MSDS Relevance: Air can appear in many contexts on a material safety data sheet: (1) Air-purifying respirators or other breathing devices may be specified for use with the chemical or by emergency responders. (2) Pyrophoric chemicals spontaneously inflame in air. (3) Flammable or explosive limits are defined using a percent volume in air. (4) In cases of inhalation of a hazardous chemical, the victim should be removed to fresh air. (5) Special engineering controls such as fume hoods or local exhaust ventilation may be indicated. In addition, many chemicals have high vapor pressures, which can result in an oxygen-deficient atmosphere, a flammable atmosphere, or narcosis. Never store cylinders of air or oxygen with flammable materials or gases. See also: Asphyxiant, Pressure Units, Ventilation