The pneumothorax is an abnormal collection of air or gas in the pleural space that separates the lung from the chest wall. Like pleural effusion where a large abnormal concentration of fluid (>100 mL) is liquid buildup in that space, pneumothorax may interfere with normal breathing. A medical term that it is used is the collapsed lung, although that term may also refer to atelectasis. There are two major types of pneumothorax; there is one that occurs without an apparent cause and in the absence of significant lung disease, while the so called; "secondary" pneumothorax occurs in the presence of existing lung pathology. In a minority of cases, the amount of air in the chest increases markedly when a one-way valve is formed by an area of damaged tissue, leading to a third type of pneumothorax, called "tensioned".