Causes of Pneumomediastinum
Pneumomediastinum, also known as mediastinal emphysema, is a condition in which air is present in the mediastinum (the space in the chest between the two lungs). This can be caused by a traumatic injury or in association with pneumothorax or other diseases. The mediastinum may fill with air due to:

injury to the neck or chest
surgery on the neck, chest, or abdomen
asthma or other conditions that cause forceful coughing
chest infections and lung diseases, such as COPD and interstitial lung disease
difficult childbirth
excessive vomiting
inhalation of toxic fumes
intense exercise
rapid changes in air pressure while diving
use of recreational drugs, such as cocaine and methamphetamine
use of a ventilator

The Valsalva maneuver may also cause a pneumomediastinum. This maneuver involves forcefully exhaling against a closed airway. The Valsalva maneuver is commonly done to pop the ears.

What are the risk factors?
Factors that increase the likelihood of a pneumomediastinum include:

Age: Babies and children may be at greater risk of the condition than adults because their chest tissues are less stiff, resulting in an easier path for air movement.

Sex: Approximately 76 percent of cases affect males, especially young males.

Lung health: People with lung diseases, including asthma, bronchiectasis, cystic fibrosis, COPD, interstitial lung disease, and cysts, are at higher risk of the condition.