Air is 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.