How harmful is a pulmonary embolism?

Depends on size: If the clot goes to many areas of the lungs or to main arteries and block them as the blood comes out of the heart, there can be no flow of blood and can be fatal, in term of areas, the clot also blocks flow and prevents oxygenation of blood. So large clots can obstruct flow and can be fatal.

Pulmonary embolism: A pulmonary embolism can be deadly (30% of patients of patients who are untreated). You can have severe respiratory compromise needing mechanical ventilation, cardiac compromise leading to shock and needing resuscitation. It is one of the top severe forms of chest pain that is continually screened for in the Emergency Room (Image by: James Heilman, , MD)