Pulmonary embolism

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.

It’s a serious condition because it can prevent blood from reaching your lungs. Fast medical treatment can be lifesaving.

Most of the time, 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).

Often, we don’t know the cause of a pulmonary embolism. It can happen for no obvious reason.

Times to take extra care

When you’ve been inactive for a long time, your chance of getting a pulmonary embolism increases.

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. 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

Around half of all people who develop a pulmonary embolism do so 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 and 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 six 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 prescribing them.

Next: Symptoms of pulmonary embolism >

Download our pulmonary embolism PDF (266KB)

Last medically reviewed: March 2015. Due for review: March 2018

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.