Usually occurring in otherwise-healthy adults between ages 20 and 40, spontaneous pneumothorax may be caused by air leakage from ruptured congenital blebs adjacent to the visceral pleural surface, near the apex of the lung. It may also result from an emphysematous bulla that ruptures during exercise or coughing or from tubercular, pneumocystic, or malignant lesions that erode into the pleural space. Spontaneous pneumothorax may also occur with interstitial lung disease, such as eosinophilic granuloma or lymphangiomyomatosis.

Traumatic pneumothorax may result from insertion of a central venous line, thoracic surgery, or a penetrating chest injury, such as a gunshot or knife wound, or it may follow a transbronchial biopsy. It may also occur during thoracentesis or a closed pleural biopsy. When traumatic pneumothorax follows a penetrating chest injury, hemothorax (blood in the pleural space) may also occur.

With tension pneumothorax, positive pleural pressure develops because of any of the causes of traumatic pneumothorax. When air enters the pleural space through a tear in lung tissue and is unable to leave by the same vent, each inspiration traps air in the pleural space, resulting in positive pleural pressure. This in turn causes collapse of the ipsilateral lung and marked impairment of venous return, which can severely
compromise cardiac output and may cause a mediastinal shift. Decreased filling of the great veins of the chest results in diminished cardiac output and lowered blood pressure.