Deep Vein Thrombosis (DVT): This is often the most common cause of unprovoked pulmonary embolism. The conditions that lead to DVT, such as immobility, surgery, or trauma, can increase the risk of clot formation, which can then break loose and travel to the lungs.

Atrial Fibrillation: This heart condition can lead to the formation of blood clots in the heart, which can then embolize to the lungs.
Pulmonary Embolism due to Hypercoagulable State: Conditions such as factor V Leiden, protein C or S deficiency, or antithrombin deficiency can increase the risk of blood clot formation.
Cancer: Certain types of cancer, such as pancreatic, lung, or ovarian cancer, can increase the risk of blood clots due to the production of pro-coagulant substances.

Pulmonary Embolism due to Septic Emboli: Infections such as endocarditis or septic thrombophlebitis can lead to the formation of septic emboli, which can be life-threatening if not promptly treated.
Fat Embolism Syndrome: This condition, often seen in patients with long bone fractures, can lead to the release of fat globules into the bloodstream, which can then embolize to the lungs.
Amniotic Fluid Embolism: This rare but life-threatening condition can occur during pregnancy or childbirth, where amniotic fluid enters the maternal circulation and triggers a severe reaction.

Paradoxical Embolism: This occurs when a blood clot passes from the right side of the heart to the left side through a patent foramen ovale or other cardiac defect, and then travels to the lungs.
Tumor Embolism: Rarely, tumor cells can break off from a primary site and embolize to the lungs, causing an obstruction.
Air Embolism: This can occur due to medical procedures, trauma, or other causes, where air enters the bloodstream and travels to the lungs.