Off-Gassing

The release of dissolved inert gas from body tissues as pressure drops during ascent and after the dive; the reverse of on-gassing during descent.

Off-gassing is the release of dissolved inert gas, mostly nitrogen, from your tissues back into the blood and out through the lungs as ambient pressure falls. Its mirror image is on-gassing: during descent and bottom time, Henry's law drives nitrogen from the breathing gas into solution in your body, with each tissue absorbing at its own speed. The moment you begin ascending, tissues whose gas load exceeds the surrounding pressure reverse direction and start giving that nitrogen back. A dive is, in gas terms, an on-gassing phase followed by a long off-gassing phase.

The whole craft of decompression is managing off-gassing so it happens in solution rather than as bubbles. Ascend slowly, no faster than 9 to 18 m per minute depending on agency, and the pressure gradient stays gentle enough for the lungs to wash nitrogen out gradually. Ascend too fast and the gas comes out of solution in the tissues themselves, which is the mechanism of decompression sickness. Safety stops and decompression stops exist purely to buy off-gassing time at controlled depths, and fast tissues unload quickly while slow tissues keep releasing gas for many hours after you surface.

That lingering tail is why off-gassing shapes the whole dive day: residual nitrogen carries into repetitive dives, surface intervals are off-gassing time, and flying after diving guidelines exist because cabin altitude accelerates bubble formation in a body still desaturating. Your dive computer is, at heart, an off-gassing bookkeeper, and understanding the concept explains conservative habits like a slow final ascent from the safety stop, since the last few meters carry the steepest proportional pressure change of the whole dive.