Pulmonary embolism typically occurs when a blood clot blocks the pulmonary arteries. The primary reasons for this condition include damage to the inner lining of blood vessels, slowed blood flow (stasis), and an increased tendency for blood to clot.

Various factors elevate the risk of clot formation:
* Cardiovascular Diseases: Heart and vascular conditions can significantly increase the risk of blood clot formation.
* Prolonged Immobility: Situations like being bedridden or long journeys (e.g., plane or bus trips exceeding 4 hours) can lead to blood pooling and clotting in the leg veins.
* Occupational Risks and Varicose Veins: Professions requiring prolonged standing can lead to the formation of varicose veins in the legs. Varicose veins disrupt blood flow, thereby increasing the risk of clotting.
* Certain Medications: Some medications, such as birth control pills, can enhance the blood's clotting ability.
* Surgical Procedures: The risk of embolism is high after certain surgeries, particularly abdominal or leg operations, and procedures requiring prolonged general anesthesia. Additionally, cosmetic surgical procedures like liposuction can lead to fat emboli.
* Cancer and its Treatment: In cancer patients, especially those with lung cancer, there is an increased tendency for blood to clot. Cancer treatments, such as chemotherapy, can also raise the risk of clot formation.
* Genetic Predispositions: Genetic factors such as Protein C deficiency, Factor V Leiden mutation, and Antithrombin III deficiency can increase the risk of clot development regardless of an individual's age.
* Other Specific Conditions: Rapid ascents during deep-sea diving can cause nitrogen gas to form bubbles in blood vessels, leading to air or fat emboli (decompression sickness).
* Lifestyle Factors: Smoking and obesity are also significant factors that increase the risk of pulmonary embolism.