The most hazardous slice of a scuba dive often sits in the last few meters of ascent, not at the maximum depth. In that narrow band, ambient pressure drops fast relative to depth change, and gas inside the body reacts in ways that physics makes unforgiving.
According to Boyle’s law, a small vertical rise near the surface produces a large relative expansion of gas in the lungs and air spaces. If a diver holds their breath or ascends too quickly, that expansion can cause pulmonary barotrauma and arterial gas embolism, even from shallow water. At the same time, dissolved nitrogen begins to leave tissues and blood; if the ascent rate exceeds what controlled decompression allows, bubbles can form and trigger decompression sickness, a problem of gas kinetics rather than simple oxygen lack.
Because most of the pressure drop occurs close to the surface, the gradient driving nitrogen off‑gassing and gas expansion peaks in those final meters. Safety stops, slow ascent rates, and continuous exhalation are designed to manage this steep pressure curve. The deepest moment of a dive can feel dramatic, but the physics of gas volume and tissue saturation reserve the sharpest biological stress for the way back up.
