What Causes Blood Clots in Lungs (Pulmonary Embolism)?
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Medically reviewed by Brian Bezack, DOMedically reviewed by Brian Bezack, DO
A pulmonary embolism (PE) occurs when a blood clot (embolus) develops in a deep vein in your leg, breaks free, and travels through the bloodstream to a pulmonary artery (blood vessel) in your lung. Several causes ranging from underlying health conditions, vein injuries, and high estrogen levels can increase your risk of a blood clot in your lungs.
The pulmonary arteries carry low-oxygen blood from your heart to your lungs, where the blood picks up oxygen. The heart then pumps this oxygen-rich blood to the rest of the body's tissues and organs.
When a pulmonary embolism blocks blood flow and disrupts the normal oxygenation process, it can lead to symptoms like shortness of breath, chest pain, and rapid heart rate. Left untreated, PE can cause damage to the body's vital organs due to lack of oxygen.
Thrombus (Blood Clot)
A thrombus, or blood clot, is the leading cause of pulmonary embolism (PE). A thrombus typically forms in a deep vein in your lower leg, pelvis, thigh, or arm—a condition known as deep vein thrombosis (DVT). A pulmonary embolism occurs when a blood clot breaks away from the leg (or arm), travels through the bloodstream, and lodges in one of the lungs' arteries.
Several factors can contribute to your risk of developing DVT, including:
Injury to a vein: Damage to a vein in the leg or arm from a severe muscle injury, bone fracture, or major surgery
Slow blood flow: Low physical activity levels, bed rest (e.g., after surgery), and sitting for a long time with crossed legs—which can slow blood flow
Elevated estrogen levels: Higher than normal estrogen levels caused by pregnancy, birth control pills, or hormone replacement therapy (HRT)
Chronic conditions: Heart disease, lung disease, cancer and cancer treatments, and inflammatory bowel disease (IBD)
Air Embolism
An air embolism occurs when one or more air bubbles enter the bloodstream and travel to the lungs, blocking a pulmonary artery. Air can enter the bloodstream during medical procedures such as:
Intravenous (IV) catheter: Placement or removal of an IV catheter, which might allow air to flow through veins
Surgery: For example, cardiac or brain surgery
Endoscopy: Uses a thin, flexible, lighted tube with a camera (endoscope) to examine the inside of the body
Angiography: Uses injected dye to view the inside of arteries and blood vessels
Needle biopsy of the lungs: Taking a small tissue sample with a long, very thin needle and sending it to a laboratory for evaluation
You might develop an air embolism as a result of traumatic injury to the chest, such as blunt force trauma (e.g., direct blows to the chest) and penetrative injuries (e.g., gunshot wounds).
These types of embolisms can also occur in deep-sea divers who ascend from the ocean too quickly or do not sufficiently exhale as their lungs fill with air when rising to the surface after a dive.
Fat Embolism
A fat embolism occurs when a droplet of fat or a bone marrow particle enters the bloodstream and travels to an artery in the lungs. Common causes of fat embolisms include:
Bone fractures in a long bone, such as in the arms or legs
Liposuction procedures
Orthopedic surgeries, such as hip or knee replacements, spinal surgery, and surgery on the hands, wrists, feet, or ankles
Bone marrow necrosis (damage and destruction of bone marrow cells), which can occur with sickle cell disease or cancer treatments such as chemotherapy or radiation therapy
Amniotic Fluid Embolism
Amniotic fluid embolism (AFE) is a rare, life-threatening complication that affects pregnant people before, during, or immediately following labor and childbirth. AFE occurs when fetal cells or amniotic fluid (the fluid that surrounds an unborn baby) enter the pregnant parent's bloodstream and cause respiratory distress, cardiac arrest, or excessive bleeding.
In the past, experts believed that the amniotic fluid or fetal cells would block a pulmonary artery, similar to a blood clot that causes a pulmonary embolism. However, recent research suggests that AFE is an overactive immune response to exposure to the amniotic fluid or fetal cells. More research is needed to determine exactly what causes AFE.
Tumor Embolism
A tumor embolism occurs when cancer cells break away from a tumor and travel through the bloodstream to a pulmonary artery, where they can multiply, form a tumor, and block blood flow. Tumor embolisms are a rare complication of certain types of adenocarcinomas— cancers that form in tissues that line certain organs. The following cancers may increase your risk of a blood clot in the lungs:
Kidney cancer
Stomach cancer
Colon cancer
Septic Embolism
A septic embolism occurs when a piece of infected tissue from another body area or infectious microorganisms (such as viruses and bacteria) break off the original site of infection and travel through the bloodstream to a pulmonary artery. These embolisms can also occur when a blood clot containing infectious microorganisms breaks from a deep vein in your legs and travels to the lungs.
Intravenous (IV) drug use, right-sided infective endocarditis (a type of heart infection), and septic thrombophlebitis (an infected blood clot) are all factors that can increase your risk of this type of embolism.
Foreign Body Embolism
A foreign body embolism (FBE) occurs when small particles of nonbiological material enter the bloodstream. An FBE can migrate to a pulmonary artery, blocking blood flow. Foreign body embolism is rare and can develop in people who use intravenous drugs and inject inorganic substances, such as talc or elementary mercury.
Though rare, an FBE can also occur when material from tools healthcare providers use during medical procedures (e.g., polymer coating from coil embolizations) enter the bloodstream.
Is Pulmonary Embolism Hereditary?
Having a family history of pulmonary embolism can increase your risk of having a blood clot yourself. Studies show that some gene mutations (changes) that play a role in blood clotting are associated with a higher risk of developing blood clots, which can ultimately lead to PE. Inheriting these gene mutations (changes) may alter the balance of clotting factors in your blood, making clots more likely to form.
Genetic mutations for blood clots are known as inherited thrombophilia. The following genetic mutations are associated with an increased risk of abnormal blood clotting and blood clot formation:
Factor V Leiden: Mutations in the F5 gene affect your body's ability to form blood clots in response to an injury.
Antithrombin deficiency: Mutations in the SERPINC1 gene cause low levels of antithrombin protein, which controls blood clotting.
Protein C deficiency: Mutations in the PROC gene cause insufficient production of protein C, which controls blood clotting.
Protein S deficiency: Mutations in the PROS1 gene, which controls the production of protein S, can cause a lack of proteins that help promote blood clot formation.
Genetics alone doesn't guarantee that you'll experience pulmonary embolism. However, genetic mutations alongside other risk factors (such as high estrogen levels or an injury to the vein) may increase your risk of developing a blood clot in the lungs.
Who Gets a Pulmonary Embolism?
Anyone can develop a pulmonary embolism, but some people are more likely to experience the condition than others. Consider the following non-modifiable risk factors:
Age: The risk of PE increases with age. PE is more common in people ages 40 and up, partly due to changes in blood composition and vein function as you age.
Sex: People assigned female at birth have higher risks of blood clots in the lungs due to higher levels of estrogen due to menstruation, pregnancy, and taking contraception.
Ethnicity: Black people have higher rates of pulmonary embolism and PE-related deaths compared to other racial backgrounds. Systemic disparities in healthcare access and racial bias from providers might play a role. More cultural competency and inclusivity in healthcare remain necessary to lower the risk of PE-related deaths in Black communities.
For more information on support groups, education and advocacy efforts, and general resources about pulmonary embolism, please visit the National Blood Clot Alliance's services website here.
Related: How Do Social Determinants Play a Role in Your Health and Well-Being?
Risk Factors
Several environmental and lifestyle factors can also increase your risk of developing a blood clot in your lungs.
Medical History
Some conditions are associated with a higher risk of developing deep vein thrombosis and pulmonary embolism, including:
Heart disease
Cancer
Varicose veins
Varicose veins
Chronic inflammatory diseases, such as lupus
Prior history of DVT or PE
Recent surgery
Geography
Surprisingly, geographic location and weather patterns can also influence the risk of pulmonary embolism.
Studies show that rates of pulmonary embolism and PE mortality rates increase during the winter months. Researchers suggest a combination of lower activity levels, cold temperatures affecting blood flow, and higher incidences of respiratory infections may play a role in the increased PE and PE-related mortality rates in the winter.
Living in an area with high pollution rates is also associated with a higher risk of pulmonary embolism.
Lifestyle
The following lifestyle factors may also affect your likelihood of developing a blood clot in your lungs. For example, sitting for long periods (such as during a flight or car ride), bed rest, and smoking can all increase your risk of PE. Healthcare providers recommend getting light to moderate activity daily and limiting your tobacco intake to reduce these risks.
Medications
Certain medications can affect your body's blood clotting processes and increase the risk of pulmonary embolism, including:
Oral contraceptives (birth control pills)
Hormone replacement therapy (HRT)
Antipsychotic medications such as Abilify (Aripiprazole), Risperdal (risperidone), and Haldol (haloperidol)
A Quick Review
A pulmonary embolism (PE) occurs when a blood clot travels to your lungs and blocks a pulmonary artery (blood vessel). Most PEs develop when a blood clot originating in the legs breaks away and travels through the bloodstream before getting lodged into a pulmonary artery.
While blood clots are the leading cause, other embolisms, including air bubbles, foreign bodies, fat, and cancer cells, can also cause PE. People with a family history of DVT or PE also have an increased risk of PE.
Other risk factors include medical conditions like heart and lung disease, recent surgery or hospitalization, lifestyle habits like smoking, and taking medications like oral contraceptives and hormone replacement therapy.
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