| Spontaneous |
| Primary: no apparent underlying lung disease |
| Secondary: clinically apparent underlying disease (e.g. chronic obstructive pulmonary disease and cystic fibrosis) |
| Catamenial: in conjunction with menstruation |
| (Neonatal) |
| Traumatic |
| Iatrogenic: secondary to transthoracic and transbronchial biopsy, central venous catheterisation, pleural biopsy and thoracentesis |
| Non-iatrogenic: secondary to blunt or penetrating chest injury |