What Is Pulmonary Embolism?

A pulmonary embolism (PULL-mun-ary EM-bo-lizm), or PE, is a sudden blockage in a lung artery, usually due to a blood clot that traveled to the lung from a vein in the leg. A clot that forms in one part of the body and travels in the bloodstream to another part of the body is called an embolus.

PE is a serious condition that can cause:
Permanent damage to part of your lung from lack of blood flow to lung tissue 
Low oxygen levels in your blood 
Damage to other organs in your body from not getting enough oxygen 

If the blood clot is large, or if there are many clots, PE can cause death. 
Overview

In most cases, PE is a complication of a condition called deep vein thrombosis (DVT). In DVT, blood clots form in the deep veins of the body—most often in the legs. These clots can break free, travel through the bloodstream to the lungs, and block an artery.

This is unlike clots in the veins close the skin's surface, which remain in place and do not cause PE.
Outlook 

At least 100,000 cases of PE occur each year in the United States. PE is the third most common cause of death in hospitalized patients. If left untreated, about 30 percent of patients with PE will die. Most of those who die do so within the first few hours of the event.