A pulmonary embolism is caused when a blood clot that begins in the legs or another part of the body breaks loose and travels to the lung where it causes a blockage of blood flow and oxygen to the lung tissue. Blood clots that occur in a vein that is located near the surface of the skin are not usually problematic. But when a blood clot occurs deep in veins, it may lead to a pulmonary embolism. People who have had a blood clot in their arm or leg are at an increased risk of developing a pulmonary embolism. Other rare factors that may cause an artery blockage include air bubbles, a tumor, amniotic fluid, and fat that is released into the bloodstream when there is a broken bone.