Collapsed lung is a term used to refer to pneumothorax, a potentially life-threatening condition.
A pneumothorax occurs when air enters the space between the two layers of the pleura. The air separates the two layers, and causes part or all of one of the lungs to collapse and lose the air that is normally inside the lung. 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.