A pneumothorax is a collapsed lung. A pneumothorax can occur in a diver who fails to exhale completely during ascent.

The sequence of events includes:

• Gas expands in the lungs as the surrounding pressure decreases during ascent.
• The lung becomes over-inflated.
• The small air sacs in the lung rupture.

When air escapes outside the lungs, the air collects in the space between the outside of the lung and the inside the chest cavity.