Pulmonary embolism

What causes a pulmonary embolism?

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

In many cases, the clot occurs because of a change such as pregnancy or recent surgery. But in about 40% of cases, your health care professional may be unable to find the cause of a pulmonary embolism. It can happen for no obvious reason.

This can be called ‘unprovoked’.

What increases my risk of getting a pulmonary embolism?

When 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 a lot. This can be:

  • 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 6 hours

Around half of all people with a pulmonary embolism get it while they’re in hospital.

Less commonly, you might have a condition that causes your blood to clot more easily than normal, such as cancer, or cancer treatments such as chemotherapy and radiotherapy.

Other factors that increase your risk of developing a pulmonary embolism include:

  • being overweight
  • pregnancy – your risk is increased for up to 6 weeks after giving birth
  • smoking
  • taking 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.

Next: How is a pulmonary embolism diagnosed? >

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Last medically reviewed: March 2018. Due for review: March 2021

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.