A pulmonary embolism happens when a blood clot or other material gets stuck in an artery of the lungs, blocking blood flow. About 90% of the time, the clot originally forms in the deep veins of the legs and then breaks free, traveling through the heart and into the pulmonary arteries. The causes range from temporary situations like surgery or long flights to inherited blood disorders and chronic conditions like cancer.
How Blood Clots Form and Travel to the Lungs
The story of most pulmonary embolisms begins in the legs. A clot forms in one of the deep veins, most commonly in the calf, then the thigh, and less often in the iliac veins near the pelvis. At some point the clot, or a piece of it, detaches from the vessel wall. It flows upward through larger and larger veins, passes through the right side of the heart, and lodges in one of the pulmonary arteries. Once it blocks blood flow there, the affected part of the lung can no longer exchange oxygen properly, and the heart has to work harder to push blood past the obstruction.
Three broad conditions set the stage for clot formation. Doctors have grouped these into slow blood flow (stasis), damage to blood vessel walls, and blood that clots too easily (hypercoagulability). Most people who develop a pulmonary embolism have at least one of these factors, and many have two or three overlapping at once.
Slow Blood Flow and Immobility
When blood moves sluggishly through veins, clotting proteins and platelets accumulate near the vessel walls instead of being swept along. This is why prolonged immobility is one of the most common triggers. Being bedridden after surgery, hospitalized for illness, or wearing a cast on a leg can all create enough stasis for a clot to form over days or weeks.
Travel is a well-known risk factor, though the absolute risk for any single trip is low. Flights or car rides lasting more than four hours are the usual threshold. For flights under four hours, the risk of a symptomatic clot is negligible. For longer flights, studies estimate roughly 1 clot event per 4,600 to 6,000 person-flights. Among travelers on journeys over eight hours who already have some baseline risk factors, the incidence rises to about 0.5%. Simple measures like walking the aisle, flexing your calves, and choosing an aisle seat help keep blood moving during long trips.
Conditions that alter blood flow inside the body matter too. Atrial fibrillation, certain heart valve problems, and weakened heart muscle can all slow circulation enough to promote clotting, even if you’re otherwise active.
Blood Vessel Damage
Healthy blood vessels have a smooth inner lining that actively prevents clotting. This lining releases molecules that keep platelets from sticking and clotting proteins from activating. When it gets damaged, those protective signals disappear, and the exposed tissue underneath triggers clot formation.
Vessel damage can come from surgery, trauma, or an IV catheter. It can also develop slowly from chronic conditions. Smoking, long-standing high blood pressure, and high cholesterol all gradually wear down the inner lining of veins and arteries. Once the surface is disrupted, blood flow becomes turbulent at that spot, which pushes even more platelets and clotting factors toward the damaged area.
Hypercoagulable States
Some people’s blood simply clots more easily than it should, either because of genetics or an acquired condition. This is the “hypercoagulability” piece of the puzzle, and it can quietly raise your risk for years before a clot ever forms.
Inherited Clotting Disorders
Factor V Leiden is the most common inherited clotting mutation. Carrying one copy of this gene raises the lifetime risk of a blood clot about sevenfold. Carrying two copies, which is rare, raises it roughly twentyfold. Another mutation, prothrombin G20210A, also increases clotting tendency, though it’s less common. Many people with these mutations never know they have one until a clot occurs, sometimes triggered by a second risk factor like surgery or hormonal birth control.
Cancer
Cancer is one of the strongest acquired risk factors for pulmonary embolism. Tumor cells release molecules that directly activate the clotting system. They also overproduce an enzyme called heparanase, which breaks down the body’s natural blood-thinning compounds on vessel walls and triggers clot-promoting proteins. On top of that, a growing tumor can physically press on nearby blood vessels, narrowing them and slowing blood flow. Certain chemotherapy drugs further increase the risk. Platinum-based regimens, for example, raise the chance of a blood clot by about 67% compared to regimens without them.
Hormonal Factors
Estrogen-containing medications are a significant and modifiable risk factor. Combined oral contraceptives (the pill) raise the risk of blood clots two- to ninefold compared to nonusers, depending on the specific formulation. Pills with lower estrogen doses carry somewhat lower risk, and the type of progestin matters too. Older progestins like levonorgestrel carry a lower relative risk (about 2.4 times normal) than newer ones like desogestrel or gestodene (about 3.6 to 4.3 times normal). The contraceptive patch and vaginal ring carry even higher risks, roughly 6.5 to 8 times that of non-users in some studies.
Hormone replacement therapy (HRT) during and after menopause also raises clot risk, roughly doubling it compared to nonusers. The route of delivery makes a real difference here. Oral estrogen consistently raises risk, while transdermal estrogen (patches or gels) does not appear to increase it meaningfully. Women considering HRT who have other clotting risk factors may want to discuss the transdermal option specifically.
Other Medical and Lifestyle Risk Factors
Obesity and smoking both contribute, though their individual effects are more modest than some people assume. Current smokers have about a 23% higher risk of blood clots than people who have never smoked, and the risk stays slightly elevated even after quitting (about 10% higher for former smokers). Interestingly, when researchers adjust for body weight, the risk for current smokers climbs to about 30% higher, suggesting that obesity and smoking each contribute independently. Higher body weight increases pressure on the veins of the pelvis and legs, slows blood return to the heart, and raises levels of clotting factors in the blood.
Other notable risk factors include pregnancy and the postpartum period (when clotting factors naturally increase), major orthopedic surgery on the hip or knee, spinal cord injury, and chronic inflammatory diseases. Age itself is a risk factor: clot risk rises steadily after 40 and climbs sharply after 60.
Non-Clot Causes
Not every pulmonary embolism is caused by a blood clot. In uncommon but serious situations, other materials can block the pulmonary arteries.
- Fat embolism: When a long bone (like the femur) fractures, fat from the bone marrow can enter the bloodstream and travel to the lungs. This also occasionally happens during orthopedic surgery, severe burns, or liposuction. Fat embolism syndrome involves lung, brain, and skin symptoms together, driven mainly by the inflammatory damage that fatty acids cause to blood vessel walls.
- Air embolism: Air bubbles can enter veins during trauma, surgery, diving accidents, or when a central IV line is inserted or removed incorrectly. A small amount of air is usually absorbed harmlessly, but a large bolus can obstruct blood flow in the lungs.
- Amniotic fluid embolism: During labor or shortly after delivery, amniotic fluid can enter the mother’s bloodstream. This triggers a severe immune-like reaction similar to anaphylaxis rather than a simple mechanical blockage. It is rare but life-threatening.
- Tumor embolism: Fragments of a cancer can break off and lodge in pulmonary arteries. This tends to happen with advanced cancers and often causes a gradual onset of breathlessness and, eventually, high blood pressure in the lungs.
Why Multiple Risk Factors Matter
Pulmonary embolism rarely results from a single cause acting alone. The typical scenario involves overlapping factors. A person with an undiagnosed Factor V Leiden mutation starts hormonal birth control, takes a long-haul flight, and develops a clot. Or a cancer patient undergoes surgery that requires several days of bed rest, combining hypercoagulability, vessel injury, and stasis all at once. Understanding your personal risk factors, especially the ones you can control or plan around, is the most practical takeaway. If you know you carry a clotting mutation, for instance, that information becomes relevant every time you face surgery, pregnancy, or a decision about hormonal medications.