Free air in the chest outside the lung. It can occur spontaneously (out of the blue, with or without underlying lung disease), following a fractured rib, chest surgery, or be deliberately induced in order to collapse the lung. A small pneumothorax without underlying lung disease may resolve on its own. A larger pneumothorax or one associated with underlying lung disease often requires aspiration of the free air and/or placement of a chest tube to evacuate the air and allow lungs to re-expand.