What typically causes pulmonary embolism?

Pulmonary embolism is primarily caused by blood clots that typically originate from the deep veins of the legs, a condition known as deep vein thrombosis (DVT). When these clots dislodge, they can travel through the bloodstream to the lungs, where they block the pulmonary arteries, leading to serious complications. This blockage can reduce oxygen levels in the blood and strain the right side of the heart, ultimately resulting in respiratory failure or even death if not treated promptly.

The other options, while relevant to lung issues, do not directly cause pulmonary embolism. For instance, infections in the lungs are associated with pneumonia or other respiratory illnesses, but they do not lead to the formation of emboli. Airborne pathogens can cause respiratory infections but are not linked to the creation of blood clots that lead to pulmonary embolism. Allergic reactions can provoke respiratory symptoms but again do not relate to embolism formation. Therefore, blood clots from deep veins are recognized as the primary and most significant cause of pulmonary embolism.