A pulmonary embolism is a blockage in an artery in the lung. It is usually due to a blood clot that has traveled to the lung from another part of the body, usually the leg. This type of blood clot is called a deep vein thrombosis (DVT). A pulmonary embolism is a serious, possibly life-threatening condition that can block blood flow to the lungs. This decreases blood oxygen levels and can lead to organ damage. A pulmonary embolism requires emergency medical treatment. Follow the links below to find WebMD's comprehensive coverage about how a pulmonary embolism develops, what it looks like, how to treat it, and much more.