Pneumothorax

A pneumothorax is an abnormal collection of air or gas in the pleural space that separates the lung from the chest wall. Like pleural effusion (liquid buildup in that space), pneumothorax may interfere with normal breathing.

Below: a chest x-ray showing a large right-sided spontaneous pneumothorax (on the left in the image) where the absence of lung markings indicate that there is free air in the chest. An arrow indicates the edge of the collapsed lung.