Pneumomediastinum is air in the space between the lungs, in the middle of the chest (the mediastinum).

Pneumomediastinum is uncommon. It occurs when air leaks from any part of the lung or airways into the mediastinum. Most often, one of the small air sacs (alveoli) ruptures and leaks air.

The condition can be caused by a traumatic injury or by disease. Increased pressure within the lungs or airways can rupture the air sacs or airways, allowing air to escape into surrounding structures. Such pressure can be caused by excessive coughing, sneezing, vomiting, or repeated bearing down to increase abdominal pressure (such as pushing during childbirth or a bowel movement).

It may also happen following:

• Accidental tearing of the trachea (windpipe)
• Rapid ascents in altitude, SCUBA diving
• Use of a breathing machine
• Using inhaled recreational drugs such as crack cocaine

Pneumomediastinum also can occur in association with pneumothorax or other diseases.