Partial Pressure

The share of total pressure exerted by one gas in a mixture, the number that governs oxygen toxicity, narcosis, and decompression in diving.

Partial pressure is the pressure a single gas contributes within a mixture, calculated by multiplying its fraction by the ambient pressure. Air is about 21 percent oxygen, so at the surface (1 ATA) the oxygen partial pressure, PO2, is 0.21 ATA. Take the same air to 30 m (4 ATA) and PO2 rises to 0.84 while the nitrogen partial pressure climbs to roughly 3.16 ATA. Your body does not care about percentages; it responds to partial pressures.

This is the number that draws diving's hard limits. Oxygen becomes toxic to the central nervous system at high PO2, so recreational nitrox training caps exposure at 1.4 ATA, with 1.6 reserved for contingency or resting decompression, which puts the maximum operating depth of EAN32 at 33.7 m. Elevated nitrogen partial pressure drives both narcosis at depth and the tissue loading that decompression models track.

Technical divers plan entire dives in partial pressures, choosing helium-based mixes to keep PO2 and PN2 inside chosen windows at the target depth. Recreational nitrox divers meet the same idea the first time they analyze a tank and set their computer.