Ambient air is composed of 20.9% oxygen, 78.1% nitrogen, 0.03% carbon dioxide, and less than 1% other gases (Lide 1991). Data collected on nine nuclear-powered ballistic missile submarines indicate an average partial pressure of oxygen (PO₂, the product of the barometric pressure and the percentage of oxygen in the ambient atmosphere) of 148 millimeters of mercury (mmHg) and a range of 123-188 mmHg; data collected on 10 nuclear-powered attack submarines indicate an average PO₂ of 149 mmHg and a range of 118-180 mmHg (Hagar 2003). See Box 11-1 for descriptions of terms related to gas pressures and oxygen physiology.

Oxygen is a highly combustible gas that is necessary to sustain animal life. Excessive amounts of oxygen in the system (hyperoxia) can be detrimental to human health. Oxygen toxicity can occur with exposure to hyperoxic conditions at ambient pressure or in hyperbaric environments. Oxygen toxicity is characterized by pulmonary toxicity, neurotoxicity, and