It can be fatal if it cuts off the blood supply to a large part of the lung. Pulmonary emboli strike about 6 million Americans each year. About 2 out of every 100 cases are fatal.
Most blood clots start as small clots originating in the legs that come loose.
The clot travels upwards in the veins, through the heart, and goes into the blood vessels in the lungs. The blood vessels in the lungs get smaller and smaller and the clot finally stops where the blood vessel is so small that the clot can not get through it. It obstructs the flow of blood at that point and causes an embolus. This embolus cuts off the blood supply to a part of the lung. Depending on where the clot stops and how large the clot is, the part of the lung which has lost its blood supply will no longer function.