The small air sacs of the lungs (alveoli) become stretched and torn
The esophagus rips or tears
The intestine tears, causing air to leak into the chest cavity

(See also Overview of Pleural and Mediastinal Disorders.)
Most often, people have chest pain below the breastbone. The pain may be severe.
When doctors do a physical examination, they may hear a crunching or crackling noise when they listen to the chest through a stethoscope. A chest x-ray is done to confirm the diagnosis.
Usually, treatment is not necessary. However, the problem that caused pneumomediastinum may require treatment.