List seven causes of pneumomediastinum.

1. Asthma
2. Surgery (postoperative complication)
3. Traumatic tracheobronchial rupture
4. Abrupt changes in intrathoracic pressure, such as those caused by vomiting, coughing, strenuous exercise, or parturition
5. Ruptured esophagus
6. Barotrauma
7. Smoking cocaine (i.e., crack)