When a thrombus, or blood clot, suddenly moves from a vein in the leg to an artery the lung, it is called a pulmonary embolism. It is a life-threatening condition that requires immediate medical intervention. Pulmonary embolism can cause organ damage and death, according to WebMD.
Most blood clots that cause pulmonary embolism originate inside a vein deep within a leg muscle. Blood clots may produce no warning signs or symptoms until they break loose and travel to the heart, lungs or brain. Embolisms are a major cause of strokes and heart attacks, according to MedicineNet. Blood clots that occur near the surface of the skin are generally not harmful, says WebMD.