A pneumothorax is collection of air in the pleural space between the lung and the chest wall. Symptoms typically include sudden onset of sharp, one-sided chest pain and shortness of breath. In a minority of cases, a one-way valve is formed by an area of damaged tissue, in which case the air pressure in the space between chest wall and lungs can be higher; this has been historically referred to as a tension pneumothorax, although its existence among spontaneous episodes is a matter of debate. This can cause a steadily worsening oxygen shortage and low blood pressure, leading to a potentially-fatal obstructive shock unless reversed. Very rarely, both lungs may be affected by a pneumothorax. It is often called a "collapsed lung", although that term may also refer to atelectasis.