What causes a pulmonary embolism?

A pulmonary embolism happens when a blood vessel in your lungs becomes blocked. Most of the time, this blockage is caused by a blood clot and happens suddenly.

Usually, a pulmonary embolism is caused by a blood clot travelling up from one of the deep veins in your body, usually in the leg. This kind of blood clot is called a deep vein thrombosis (DVT).

In some cases, the blood clot occurs because of a change in your physical condition, such as pregnancy or recent surgery. In about 20-30% of cases, your health care professional may be unable to find the cause of a pulmonary embolism. This can be called an ‘unprovoked’ pulmonary embolism.

If you’ve been inactive for a long time, your chance of getting a pulmonary embolism increases. This is because when you’re inactive, blood tends to collect in the lower parts of your body, particularly in your lower legs.

This isn’t usually a problem because when you start to move, your blood flow increases, and blood begins to move more evenly round your body. But if you’re immobile for a long time, the flow of blood around your body can slow and clot, causing a DVT that can break off and cause a pulmonary embolism.

This can happen:

• after an operation or a serious limb injury
• after long periods of bed rest
• during a long-haul flight or a long train or car journey lasting more than 4 hours

Around half of all people with a pulmonary embolism get it during a long stay in hospital.

Read more about what you can do to prevent getting a pulmonary embolism.

Some people are at a higher risk of blood clotting which can cause a pulmonary embolism. This includes people who:

• are overweight
• are pregnant – your risk is increased for up to six weeks after giving birth
• smoke
• take some forms of hormone-based contraception or hormone replacement therapy (HRT)

Your chances of developing a blood clot are very small if you’re taking the contraceptive pill or HRT, and your health care professional will consider your individual risk before they prescribe them.

Less commonly, you might have a condition that causes your blood to clot more easily than normal, such as cancer. Cancer treatments such as chemotherapy and radiotherapy can also increase the chances of blood clots.

Routine testing for genetic risk of blood clots is not recommended. But in certain circumstances if you have a confirmed diagnosis of pulmonary embolism, your doctor may recommend a blood test to look for inherited conditions which increase the risk of clots. They may be more likely to suggest this if there is a family history of pulmonary embolism or deep vein thrombosis (DVT) and if you are due to stop anticoagulation treatment.

Read next: Diagnosing pulmonary embolism