In 9 out of 10 cases, pulmonary embolism (PE) begins as a blood clot in the deep veins of the leg (a condition known as deep vein thrombosis). The clot breaks free from the vein and travels through the bloodstream to the lungs, where it can block an artery.
Clots in the leg can form when blood flow is restricted and slows down. This can happen when you don't move around for long periods of time, such as after some types of surgeries, during a long trip in a car or on an airplane, or if you must stay in bed for an extended time Veins damaged from surgery or injured in other ways are more prone to blood clots.
Rarely, an air bubble, part of a tumor, or other tissue travels to the lungs and causes PE. Also, when a large bone in the body (such as the thigh bone) breaks, fat from the marrow inside the bone can travel through the blood to the lungs and cause PE.