Lung Disease & Respiratory Health Center

Having a blood clot in the deep vein of your leg and having a previous pulmonary embolism are the two greatest risk factors for pulmonary embolism.

For more information on risk factors for blood clots in the legs, see the topic Deep Vein Thrombosis.

Risk factors for developing clots include having slowed blood flow, abnormal clotting, and a blood vessel injury.

Slowed blood flow

When blood does not circulate normally, clots are more likely to develop. Reduced circulation may result from:

• Long-term bed rest, such as if you are confined to bed after an operation, injury, or serious illness.
• Traveling and sitting for a long time, especially when traveling long distances by airplane.
• Leg paralysis. When you use your muscles, the muscles contract, and that squeezes the blood vessels in and around the muscles. The squeezing helps the blood move back toward the heart. Paralysis can reduce circulation because the muscles can't contract.

Abnormal clotting

Some people have blood that clots too easily or too quickly. People with this problem are more likely to form larger clots that can break loose and travel to the lungs. Conditions that may cause increased clotting include:

• Inherited factors. Some people have an inherited tendency to develop blood clots that can lead to pulmonary embolism.
• Cancer.
• Heart failure.
• Serious burns.
• Severe infections.
• Use of birth control pills or other medicines that contain estrogen or estrogen-like hormones, such as those some women take for hormone replacement therapy during menopause.
• Heavy smoking.

Injury to the blood vessel wall

Blood is more likely to clot in veins and arteries shortly after they are injured. Injury to a vein can be caused by:

• Recent surgery that involved the legs, hips, belly, or brain.
• A tube (catheter) placed in a large vein of the body (central venous catheter).

Other risk factors include:

• Pregnancy. A woman's risk for developing blood clots increases both during pregnancy and shortly after delivery.
• Age. As people get older (especially older than age 70), they are more likely to develop blood clots.
• Weight. Being overweight increases the risk for developing clots.
• Not taking anticoagulant medicine as prescribed.