How bad is a collapsed lung?

Bad: Pneumothorax, hemothorax, chylothorax, hydropneumothorax and atelectasis all emcompass conditions where a lung is "collapsed." in atelectasis, the lung is not aerated because alveoli are not expanded. This is usually an issue internal to the airways. The rmainder are due to something outside the actual lung parenchyma - compressing the lung. All are bad, so talk to your doc.

Generally no bueno: Poorly/incomplete inflation of lung is non functioning lung. Collapse from air = pneumothorax. If pneumothorax creates enough pressure, can compress the heart. This decreases the blood return to the heart. Without adequate blood return, your heart does not have enough blood to pump out to your body. This can be rapidly fatal.