A pulmonary embolism occurs when a blood clot (thrombus) travels from another part of the body, such as the legs or arms, to the lungs and blocks one or more pulmonary arteries. 

Common causes of pulmonary embolism include:

• Deep vein thrombosis (DVT): A blood clot in a deep vein, usually in the leg. 

Prolonged immobility: Sitting or lying still for long periods, such as during travel or surgery. Obesity: Excess weight increases the risk of DVT. Smoking: Nicotine damages blood vessels and increases clotting. Pregnancy: Hormonal changes and increased blood volume during pregnancy increase the risk of DVT. Certain medications: Birth control pills, hormone replacement therapy, and some chemotherapy drugs. Trauma: Injuries that damage blood vessels. Cancer: Some types of cancer can increase the risk of blood clots. Air embolism: When air enters the bloodstream, it can travel to the lungs. Fat embolism: When fat globules from a broken bone or other injury enter the bloodstream, they can travel to the lungs. 

It's important to note that in some cases, the cause of a pulmonary embolism may not be identified (idiopathic).