A pneumothorax (collapsed lung, dropped lung) is the entry of air into the pleural space (the space between the lungs and chest wall).
When air enters this area, the lung loses contact with the inside of the chest and “drops” down. As this happens, the lung is not able to expand as it usually does. A pneumothorax can be small and require no treatment or be a life-threatening emergency, based on the size of the air sac that is in the pleural space and how much pressure it places on the lung. If the air sac is large or places increased pressure on the lung, it makes it difficult to pull air in and out of the lungs when breathing.
Imran JB, Eastman AL. Pneumothorax. JAMA. 2017;318(10):974. doi:10.1001/jama.2017.10476