For example, foreign body aspiration, in which someone inhales an object, can cause pneumomediastinum (and lead to subcutaneous emphysema) by puncturing the airways or by increasing the pressure in the affected lung(s) enough to cause them to burst.Subcutaneous emphysema of the chest wall is commonly among the first indications that barotrauma, damage caused by excessive pressure, has occurred; it suggests that the lung was subjected to significant barotrauma. Thus the phenomenon may occur in diving injuries.Trauma to parts of the respiratory system other than the lungs, such as rupture of a bronchial tube, may also cause subcutaneous emphysema. Air may travel upward to the neck from a pneumomediastinum that results from a bronchial rupture, or downward from a torn trachea or larynx into the soft tissues of the chest. It may also occur with fractures of the facial bones, neoplasms, during asthma attacks, when the Heimlich maneuver is used, and during  childbirth.Injury with pneumatic tools, those that are driven by air, is also known to cause subcutaneous emphysema, even in extremities (the arms and legs). It can also occur as a result of rupture of the esophagus; when it does, it is usually as a late sign. Medical treatment
Subcutaneous emphysema is a common result of certain types of surgery; for example it is not unusual in chest surgery. It may also occur from surgery around the esophagus, and is particularly likely in prolonged surgery. Other potential causes are positive pressure ventilation for any reason and by any technique, in which its occurrence is frequently unexpected.
A significant case of subcutaneous emphysema is easy to detect by touching the overlying skin; it feels like tissue paper or Rice Krispies. Touching the bubbles causes them to move and sometimes make a crackling noise. The air bubbles, which are painless and feel like small nodules to the touch, may burst when the skin above them is palpated. The tissues surrounding SCE are usually swollen. When large amounts of air leak into the tissues, the face can swell considerably. In cases of subcutaneous emphysema around the neck, there may be a feeling of fullness in the neck, and the sound of the voice may change. If SCE is particularly extreme around the neck and chest, the swelling can interfere with breathing. The air can travel to many parts of the body, including the abdomen and limbs, because there are no separations in the fatty tissue in the skin to prevent the air from moving. Causes
Trauma
Conditions that cause subcutaneous emphysema may result from both blunt and penetrating trauma; SCE is often the result of a stabbing or gunshot wound. Subcutaneous emphysema is often found in car accident victims because of the force of the crash. Chest trauma, a major cause of subcutaneous emphysema, can cause air to enter the skin of the chest wall from the neck or lung. When the pleural membranes are punctured, as occurs in penetrating trauma of the chest, air may travel from the lung to the muscles and subcutaneous tissue of the chest wall.