pneumothorax (nōōmōthôr´ăks), collapse of a lung with escape of air into the pleural cavity between the lung and the chest wall. The cause may be traumatic (e.g., gunshot or stab wound), spontaneous (rupture due to disease or localized weakness of the lung lining), or environmental (extreme change in atmospheric pressure). The only symptom may be a sudden pain in the chest. Physical and radiological examination reveals characteristic signs of lung collapse. Simple pneumothorax of only one lung generally requires only rest; the break in the pleura usually heals quickly after collapse of the lung has taken place. In tension pneumothorax (where there is high intrapleural pressure), or if both lungs are collapsed, it is mandatory to remove the air from the pleural cavity immediately. An artificial pneumothorax is one deliberately induced, as in the treatment of tuberculosis of the lung before modern drugs became available, or in the diagnosis of lung disease.