A pneumothorax (noo-mo-THOR-acks) is a collapsed lung due to air accumulating inside the chest cavity. Pneumothorax occurs when air leaks into the space between the lungs and chest wall which pushes on the outside of the lung and makes it collapse. In most cases, only a portion of the lung collapses.

A pneumothorax can be caused by a blunt or penetrating chest injury, certain medical procedures involving the lungs or due to underlying lung disease. Sometimes, pneumothorax occurs for no obvious reason. When the lung collapses, it causes sudden chest pain and shortness of breath.