Primary spontaneousSmall tear in the outer lung allows air to escape, then trapped between a lung and the chest wall. Trapped air might clear by itself, by being absorbed into the bloodstream.
Secondary spontaneousDevelops as a complication (secondary event) of an existing lung disease, such as COPD.
InjuryDue to accident trauma, or during surgery. Usually requires surgical intervention.
TensionOccurs when air feeds into the pleural cavity from a bronchus, but is unable to return to the bronchus. The resultant increasing pressure, outside of the lung, can lead to collapse of that lung. Gives a monitor rhythm, but no pulse - similar to PEA.

It may be necessary to aspirate the trapped air, especially in a large pneumothorax, or when other respiratory problems are present. The common method of removing the air is by aspiration through a tube, which has a 3 way tap and syringe attached.