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.