Pneumothorax: A collapsed lung occurs when air escapes from the lung and fills up the space outside of the lung, inside the chest. It may be caused by a gunshot or knife wound to the chest, rib fracture, or certain medical procedures. In some cases, a collapsed lung occurs without any cause. This is called a spontaneous pneumothorax. A small area in the lung that is filled with air (bleb) can break open, sending air into the space around the lung.