If air enters the pleural space, the lung will collapse. This is called a pneumothorax. If the chest wall is penetrated, which may occur as a result of an injury, air can enter the pleural space from the outside. Air can also enter from the inside, from the lung itself, if the lung is torn or ruptured. One of the most common causes of spontaneous non-traumatic pneumothorax is a pulmonary bleb. This is a weakness and out-pouching of the lung tissue, which can rupture. This introduces air into the pleural space.