Collapsed lung is a term used to refer to pneumothorax, a potentially life-threatening condition.
A pneumothorax happens when air enters the space between the rib cage and a lung. The air causes part or all of the lung to collapse. It is then hard to breathe normally and your body gets less oxygen. A pneumothorax can suddenly occur for no clear reason in otherwise healthy young people, with or without strenuous activity. Middle-aged and older adults whose lungs have been damaged by asthma, chronic bronchitis, or emphysema may have a tension pneumothorax.