What is pneumothorax?

Air around the lung: There is normally no air in the thin space between the rib cage and the lungs. Normally the negative pressure inside the ribcage keeps the lung inflated with the lung gliding against the cage. The lungs expand as the rib cage moves. If there is a hole in the lung or if there is a stab from the outside to inside of the rib cage, the pressure rises and the lung collapses - a pneumothorax.

Air in wrong place: Pneumothorax is an accumulation of air outside your lung but inside your chest. It is commonly described as a "collapsed lung" because the air in this space displaces your lung and prevents complete expansion of your lung.