Pulmonary embolism is caused by a blocked artery in the lungs. The most common cause of such a blockage is a blood clot that forms in a deep vein in the leg and travels to the lungs, where it becomes lodged in a smaller lung artery.
Almost all blood clots that cause pulmonary embolism are formed in the deep leg veins. Clots also can form in the deep veins of the arms or pelvis.
Sometimes blood clots develop in surface veins. But these clots rarely lead to pulmonary embolism.
In rare cases, pulmonary embolism may be caused by other substances, including:
- Small masses of infectious material.
- Fat, which can be released into the bloodstream after some types of bone fractures, surgery, trauma, or severe burns.
- Air bubbles or substances that get into the blood from trauma, surgery, or medical procedures.
- Tumors caused by rapidly growing cancer cells.