Damage to the airways can cause subcutaneous emphysema (air trapped in the subcutaneous tissue of the skin) in the abdomen, chest, neck, and head.[2] Subcutaneous emphysema, present in up to 85% of people with TBI,[14] is particularly indicative of the injury when it is only in the neck.[20] Air is trapped in the chest cavity outside the lungs (pneumothorax) in about 70% of TBI.[4][14] Especially strong evidence that TBI has occurred is failure of a pneumothorax to resolve even when a chest tube is placed to rid the chest cavity of the air; it shows that air is continually leaking into the chest cavity from the site of the tear.[18] Air can also be trapped in the mediastinum, the center of the chest cavity (pneumomediastinum).[4] If air escapes from a penetrating injury to the neck, a definite diagnosis of TBI can be made.[14] Hamman's sign, a sound of crackling that occurs in time with the heartbeat, may also accompany TBI.[11]