Emphysema is a long-term, progressive disease of the lungs that primarily causes shortness of breath. Subcutaneous emphysema is a condition when gas or air is present in the subcutaneous layer of the skin. In people with emphysema, the tissues necessary to support the physical shape and function of the lungs are destroyed. It is included in a group of diseases called chronic obstructive pulmonary disease or COPD (pulmonary refers to the lungs). Emphysema is called an obstructive lung disease because the destruction of lung tissue around smaller sacs, called alveoli, makes these air sacs unable to hold their functional shape upon exhalation. Emphysema is most often caused by tobacco smoking and long-term exposure to air pollution. The term 'emphysema' is derived from the Greek emphysan meaning 'inflate' - itself composed of en, meaning 'in', and physan, meaning 'breath, blast'.