Subcutaneous emphysema is defined as the accumulation of gas in the subcutaneous tissue as a subcutaneous emphysema. When the skin of the subcutaneous emphysema is pressed with the hand, the gas can be felt to move within the subcutaneous tissue, and a twisting sensation or grip of snow can occur. The cause is usually a pneumothorax secondary to a fracture of the sternum or ribs, and then the pneumothorax enters the subcutaneous tissue through the damaged area. There are three ways to enter the tissues. If the first type, pneumothorax, is accompanied by damage to the wall pleura, the gas in the chest cavity can enter the tissue under the chest wall directly through the damaged area. The second kind, trachea, bronchus, and esophagus rupture, and then air enters the mediastinum through the rupture of the tube, then passes through the suprasternal recess, and then diffuses to the subcutaneous tissues of the neck, face, and chest. The third, air, outside air enters the chest wall, body surface, and wound directly into the tissues. These are the causes of subcutaneous emphysema and how it arises.