What is the most common cause of spontaneous pneumothorax?

Small blebs: Single or multiple small emphysematous blebs( appear like little blisters, or clusters of grapes ) ionesco or more pop like a ballon and air collects in the pleural space= pneumothorax.

Not definite: "the exact pathogenesis of the spontaneous occurrence of a communication between the alveolar spaces and the pleura remains unknown. Most authors believe that spontaneous rupture of a subpleural bleb, or of a bulla, is always the cause of psp..." http://goo.Gl/frgdy.