A pneumothorax can arise spontaneously without underlying lung disease (spontaneous/primary pneumothorax).
Often there is underlying lung disease such as COPD (spontaneous/secondary pneumothorax).
Rib fractures and thoracic instrumentation (iatrogenic) are the other common causes.
This patient has very severe emphysema. The edge of multiple bullae are visible (curved lines).
The lung edge is also clearly seen on the right, but if there is a pneumothorax on one side don't forget to look at the other side. This patient has bilateral pneumothoraces (asterisks).