When we breathe in an environment with increased atmospheric pressure, our lungs absorb more nitrogen into the bloodstream than usual. Because our bodies don’t use, or metabolise, the nitrogen it remains in the system until it can be expelled when we breathe out.
Divers experience increasing pressure the deeper they reach and, conversely, the pressure reduces and returns to a normal level as they ascend. If excess nitrogen remains in the system after the pressure has returned to normal, it can form into bubbles in the blood or body tissues.
These bubbles are the cause of Decompression Illness, and the reason why divers are taught to ascend gradually. Ascending gradually gives the body a chance to expel as much of the excess nitrogen as possible.
Effects differ from person to person but varying predisposing factors are thought to heavily contribute to DCI even with correct procedures. Breath holding and air trapping in the lungs during ascent or decompression may also lead to DCI as well as some heart defects.