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? >


We'll take good care of your personal info and you can update the way we contact you at any time - check out our privacy policy at blf.org.uk/privacy to find out more.

Download our pulmonary embolism PDF (269KB)

