Causes : It may be caused by :
1. Penetrating injury, e.g. compound fracture of rib.
2. Rupture of the lung over a diseased area, e.g. emphysema.
3. Therapeutic introduction of air to rest the lung.
Haemothorax : It is the presence of blood in the pleural cavity.
It may be caused by :
1. Penetrating chest injury involving blood vessels.
Pleural effusion (excess fluid in the pleural cavity) : It may be caused by :
1. Inflammation, usually due to infection.
2. Malignant tumour involving the pleura.