Pneumatosis, also known as emphysema, is the abnormal presence of air or other gas within tissues. In the lungs, emphysema involves enlargement of the distal airspaces, and is a major feature of chronic obstructive pulmonary disease (COPD). Pneumoperitoneum (or peritoneal emphysema) is air or gas in the abdominal cavity, and is most commonly caused by a perforated abdominal organ. Pneumatosis is also a frequent result of surgery. Pneumarthrosis, the presence of air in a joint, is rarely a serious sign.