Pulmonary embolism (PE) is a sudden and acute condition caused by a blockage of the pulmonary arteries—the blood vessels that carry blood from the right side of the heart to the lungs—by a blood clot, air, or fat. It is among the leading causes of life-threatening illnesses. It most commonly occurs due to a blood clot blocking the arteries. Less frequently, it can be caused by air or fat emboli.