Humans must breathe oxygen to survive. Oxygen is essential for cellular respiration and energy production; without it, brain and heart tissue suffer damage within minutes. Ambient air provides about 21% oxygen, with nitrogen making up about 78% and trace amounts of other gases. In typical conditions, this mixture supports life and normal metabolism. If oxygen levels drop significantly, consciousness is lost and critical organ function fails; conversely, without adequate oxygen, survival is not possible for more than a short period.

Key points

• Essential gas: Oxygen (O2).

• Atmospheric composition: Approximately 21% O2, ~78% N2, and trace gases in ambient air.

• Time-sensitive risk: Prolonged oxygen deprivation leads to unconsciousness and potential death within minutes depending on exposure level.

• Special cases: Breathing pure oxygen or oxygen-enriched mixtures is used in certain medical, high-altitude, or diving contexts to prevent hypoxia, but these require appropriate equipment and safety measures.